Pressure-Dependent Yields and Product Branching Ratios in the Broadband Photolysis of Chlorine Nitrate

Article abstract of DOI:10.1021/jp953612s

The photolysis of chlorine nitrate was studied using broadband flash photolysis coupled with long-path ultraviolet-visible absorption spectroscopy.Branching ratios for the Cl + NO3 and ClO + NO2 product channels were determined from time-dependent measurements of ClO and NO3 concentrations.Yields of the ClO and NO3 products displayed a dependence on the bath gas density and the spectral distribution of the photolysis pulse.Product yields decreased with increasing bath gas density regardless of the spectral distribution of the photolysis pulse; however, the decrease in product yield was much more pronounced when photolysis was limited to longer wavelengths.For photolysis in a quartz cell (λ > 200 nm) the yield decreased by a factor of 2 over the pressure 10-100 Torr.In a Pyrex cell (λ > 300 nm), the yield decreased by a factor of 50 over the same pressure range.When photolysis was limited to λ > 350 nm, the yield decreased by a factor of 250.Branching ratios for the photolysis channels ClO + NO2 (1a) and ClONO2 + hν -> Cl + NO3 (1b) were determined from the relative ClO and NO3 product yields at various pressures.Although the absolute product yield displayed a pressure dependence, the branching between the two channels was independent of pressure.The relative branching ratios (assuming negligible contributions from other channels) are 0.61 +/- 0.20 for channel 1a and 0.39 +/- 0.20 for channel 1b for photolysis with λ > 200 nm and 0.44 +/- 0.08 for channel 1a and 0.56 +/- 0.08 for channel 1b for photolysis with λ > 300 nm.The implications of these results for the chemistry of the lower stratosphere are discussed.

Full text of DOI:10.1021/jp953612s

J. Phys. Chem. 1996, 100, 10165-10178
10165
Pressure-Dependent Yields and Product Branching Ratios in the Broadband Photolysis of
Chlorine Nitrate
Scott L. Nickolaisen,† Stanley P. Sander,* and Randall R. Friedl
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak GroVe DriVe,
Pasadena, California 91109
ReceiVed: December 6, 1995; In Final Form: April 11, 1996X
The photolysis of chlorine nitrate was studied using broadband flash photolysis coupled with long-path
ultraviolet-visible absorption spectroscopy. Branching ratios for the Cl + NO3 and ClO + NO2 product
channels were determined from time-dependent measurements of ClO and NO3 concentrations. Yields of
the ClO and NO3 products displayed a dependence on the bath gas density and the spectral distribution of the
photolysis pulse. Product yields decreased with increasing bath gas density regardless of the spectral distribution
of the photolysis pulse; however, the decrease in product yield was much more pronounced when photolysis
was limited to longer wavelengths. For photolysis in a quartz cell (λ > 200 nm) the yield decreased by a
factor of 2 over the pressure 10-100 Torr. In a Pyrex cell (λ > 300 nm), the yield decreased by a factor of
50 over the same pressure range. When photolysis was limited to λ > 350 nm, the yield decreased by a
factor of 250. Branching ratios for the photolysis channels [ClONO2 + hν f ClO + NO2 (1a) and ClONO2
+ hν f Cl + NO3 (1b)] were determined from the relative ClO and NO3 product yields at various pressures.
Although the absolute product yield displayed a pressure dependence, the branching between the two channels
was independent of pressure. The relative branching ratios (assuming negligible contributions from other
channels) are 0.61 ( 0.20 for channel 1a and 0.39 ( 0.20 for channel 1b for photolysis with λ > 200 nm and
0.44 ( 0.08 for channel 1a and 0.56 ( 0.08 for channel 1b for photolysis with λ > 300 nm. The implications
of these results for the chemistry of the lower stratosphere are discussed.
I. Introduction
molecule. These values have served for many years as the basis
for recommendations for atmospheric modeling calculations. In
a recent set of experiments, Minton et al.8 photolyzed ClONO2
in a molecular beam and detected the products with time- and
angle-resolved mass spectrometry. They reported nearly equal
branching ratios for channels 1a and 1b for photolysis at 248
nm and a 1:2 ratio favoring channel 1b for photolysis at 193
and 308 nm. They looked for products resulting from reaction
1c but were unable to find any experimental evidence for this
product channel and placed an upper limit on this pathway of
4%.
The photolysis of chlorine nitrate (ClONO2) may proceed
through one of several bond-cleaving processes, the most likely
being
ClONO2 f ClO + NO2
f Cl + NO3
λthresh ) 1068 nm
λthresh ) 695 nm
(1a)
(1b)
f ClONO + O(3P) λthresh ) 516 nm
(1c)
Gran˜a et al. have recently performed ab initio calculations
on the ground and excited electronic states of ClONO2 using
configuration interaction methods.10 Vertical excitation energies
were calculated for several electronic states in the singlet and
triplet manifolds, and oscillator strengths were determined for
singlet-singlet transitions. This work has helped to identify
the electronic states involved in ClONO2 photodissociation and
has permitted the assignment of several of the bands observed
in the ultraviolet absorption spectrum.
where λthresh is the threshold wavelength for the indicated
channel. The quantum yields of the radical product channels,
the wavelength dependence of the dissociation rate, and the
possible effects of bath gas collisions are measurements that
provide information about the detailed dynamics of the dis-
sociation process. A number of previous studies have examined
the branching ratios among the various dissociation pathways.1-9
In 1983, Margitan reported that branching among the possible
product channels was dominated by reaction 1b, with 90% of
dissociation occurring via this process and the remaining 10%
proceeding via reaction 1c at 266 and 355 nm.6 He saw no
evidence of the ClO product from reaction 1a even though
dissociation via this channel cleaves the weakest bond in the
Recent interest in chlorine nitrate stems from its involvement
in the chemistry of the stratosphere. ClONO2 is produced by
the recombination reaction
ClO + NO2 + M f ClONO2 + M
and is an important temporary reservoir of inorganic chlorine
in the lower and middle stratosphere. The most significant loss
mechanism for ClONO2 is photodissociation, and the relative
photolysis quantum yields have implications for the production
and loss of ozone. While photolysis pathway 1a results in a
* Author to whom correspondence should be addressed.
† Permanent address: Department of Chemistry and Biochemistry,
California State University, Los Angeles, 5151 State University Drive,
Los Angeles, CA 90032.
X Abstract published in AdVance ACS Abstracts, May 15, 1996.
S0022-3654(95)03612-4 CCC: $12.00 © 1996 American Chemical Society

10166 J. Phys. Chem., Vol. 100, No. 24, 1996
Nickolaisen et al.
null cycle, photodissociation via pathway 1b results in a catalytic
cycle for ozone loss:
Cl2 in helium was slowly flowed over dried HgO supported on
4 mm glass beads. The reaction was carried out at a total
pressure of 250 Torr. Product Cl2O and unreacted Cl2 were
collected at 161 K in an ethanol/N2(l) trap. Cl2O was purified
by vacuum distillation at 195 K [isopropyl alcohol/CO2(s) slush]
until Cl2 was undetectable as observed by monitoring the UV
absorption band centered at 325 nm. N2O5 was produced from
the reaction of NO2 and O3. A slow flow of neat NO2 was
combined with O3 from an ozonizer and allowed to react in a
9 mm tube 21 cm in length. N2O5 was collected in an isopropyl
alcohol/CO2(s) trap. The flow of NO2 was adjusted so that the
reaction ran to completion within the length of the tube.
ClONO2 + hν f Cl + NO3
NO3 + hν f NO + O2
NO + O3 f NO2 + O2
Cl + O3 f ClO + O2
ClO + NO2 + M f ClONO2 + M
net: 2O3 f 3O2
Chlorine nitrate was produced by transferring under vacuum
a small amount of Cl2O into the container of N2O5. The
container was slowly warmed to 273 K, at which temperature
the reaction of Cl2O with N2O5 to form ClONO2 is complete.
This process of adding Cl2O to the N2O5/ClONO2 container was
repeated until only a small amount of N2O5 remained. By
leaving an excess of N2O5 relative to Cl2O, it was assured that
there would be no Cl2O impurity in the ClONO2 sample.
ClONO2 was separated from N2O5 by vacuum distillation to a
195 K bubbler. Finally, the chlorine nitrate sample was checked
for Cl2 impurity by monitoring the chlorine absorption band
centered at 325 nm. No perceptible impurities were detected.
This cycle is particularly important at the edges of the polar
winter vortex.
While the relative quantum yields are important for atmo-
spheric modeling, the absolute magnitude of the ClONO2
photodissociation rate is also important insofar as it determines
the partitioning of inorganic chlorine between ClONO2 and HCl
in the lower and middle stratosphere. The first-order photolysis
rate constant, JClONO , is given by
2
JClONO (z) )
Isolar(λ,z)σClONO (λ) Φ(λ) dλ
2
∫
2
Two reaction cells with identical geometry were used in this
study. One was constructed of quartz and the other from Pyrex
glass. The reaction cells consisted of a 30 mm i.d. × 90.5 cm
cylindrical tube surrounded by three annular jackets. The
outermost jacket was connected to a chiller/heater circulator to
allow temperature control of the reaction. The temperature of
the reactants was measured with a thermocouple located within
the reaction cell. Most data were collected at a temperature of
300 K with one set of measurements performed at 220 K. At
the lower temperatures employed, there was typically a 10-15
degree differential between the temperature within the reaction
cell and the temperature of the cooling jacket. The middle jacket
comprised the flash lamp. This jacket was connected to a high-
voltage power supply via tungsten electrodes and was filled with
30 Torr of xenon; 27.5 kV was discharged from a 2.4 µF
capacitor through the xenon gas to produce a broadband flash
of light 10-20 µs in duration. The innermost jacket was used
to spectrally filter the output of the xenon flash lamp by the
use of inorganic salt solutions. The use of different cell
materials also allowed spectral selection of the short wavelength
cutoff of the photolysis source. The Pyrex cell did not transmit
light at wavelengths shorter than 300 nm, but the quartz cell
transmitted light down to approximately 200 nm.
where Isolar(λ,z) is the solar flux at wavelength λ and altitude z,
ClONO (λ) is the absorption cross section, and Φ(λ) is the
σ
2
quantum yield for photodissociation into all radical channels.
Below 28 km, solar radiation is restricted primarily to wave-
lengths longer than about 290 nm. As a result of the rapidly
decreasing ClONO2 cross section and increasing solar flux at
wavelengths longer than this cutoff, the product IσΦ maximizes
at about 330 nm. However, laboratory measurements of
ClONO2 quantum yields usually take place at much shorter
wavelengths to take advantage of the larger absorption cross
sections. Atmospheric model calculations have adopted these
quantum yields under the assumption that they also apply in
the spectral region beyond 290 nm. This assumption may be
incorrect if, for example, excitation in the long wavelength tail
of ClONO2 results in intersystem crossing to the triplet manifold.
The formation of metastable triplet states could decrease the
effective value of JClONO if collisional de-excitation can compete
2
with dissociation. In the atmosphere, this would have the effect
of reducing HCl and increasing ClONO2 at the lower altitudes.
We have performed an extensive set of measurements on the
dissociation of chlorine nitrate to determine the product branch-
ing ratios and yields as functions of photolysis wavelength and
pressure. The technique employed a broadband flashlamp for
photolysis and long path UV-visible absorption to detect the
products. Section II describes the experimental apparatus and
data acquisition. Section III outlines the results including the
relative branching ratios into the different product channels as
a function of wavelength and the dependence of the product
yield on the total pressure within the reaction cell. Section IV
presents a mechanism for dissociation that qualitatively simu-
lates the essential features of the data. A brief discussion of
the implication for stratospheric chemistry is also presented.
Flash lamp spectral distributions were obtained by imaging
the scattered light exiting the reaction cell windows onto a 0.3
m monochromator that was equipped with a 1024 pixel diode
array detector. A 1200 line mm-1 grating blazed at 250 nm
was used for the spectral measurements, which resulted in a
spectral band-pass of approximately 60 nm. To obtain the
complete spectral distributions from 200 to 450 nm (the region
over which chlorine nitrate has measurable absorption cross
sections accessible in these experiments), it was necessary to
scan the monochromator in 50 nm steps, collect the scattered
light data, and splice the measured spectra together. Absolute
wavelength calibration was provided by a mercury pen ray lamp.
Spectral distributions for the unfiltered photolysis light in the
quartz and Pyrex cells are shown in Figure 1. Solutions of
chromium potassium sulfate [CrK(SO4)2‚2H2O, 100 g L-1 of
H2O] and ceric ammonium nitrate [Ce(NH4)2 (NO3)6, 5 g L-1
of H2O] were also used in the Pyrex cell filter jacket to provide
further spectral selection of the photolysis light. Spectral
distributions for these salt solutions are also shown in Figure
II. Experimental Section
The flash photolysis/long path UV-visible absorption ap-
paratus and basic techniques employed in these experiments
have been described in detail previously,11 and only those points
unique to this study will be emphasized. Chlorine nitrate was
synthesized by the reaction of Cl2O with N2O5. Cl2O was made
using the method of Cady12 in which a 10-20% mixture of

Broadband Photolysis of Chlorine Nitrate
J. Phys. Chem., Vol. 100, No. 24, 1996 10167
the cell between flash lamp pulses and ensuring that the cell
was filled only with the ClONO2/bath gas mixture and not
products from the previous flash lamp pulse, for each photolysis
cycle. The pressure of the reaction cell was measured with a
capacitance manometer and maintained at a preset value by
means of an automated control valve at the cell exit to the
vacuum pump.
The mechanism of chlorine nitrate photolysis was studied by
monitoring the ClO and NO3 products. ClO and NO3 were
monitored by UV-visible absorption spectroscopy in which the
collimated output of a 150 W xenon arc lamp was passed
through the reaction cell using White-type optics to achieve a
total optical path length of 724 cm. For those measurements
in which high temporal resolution was desired, the arc lamp
beam was directed to a 0.5 m monochromator equipped with a
photomultiplier tube (PMT). The slits of the single-wavelength
PMT spectrometer were 150 µm. For ClO detection, the
spectrometer was equipped with a 2400 line mm-1 grating
blazed at 300 nm. The spectrometer was tuned to the A2Π(ν′
) 12) r X2Π(ν′′ ) 0) vibrational band at 275.3 nm. The light
intensity was measured with a R166UH PMT (Hamamatsu).
The PMT signal was passed to an amplifier that had a variable
low-pass filter. The amplified signal was digitized and signal-
averaged using a 100 kHz digital oscilloscope. Typically, 100
scans were averaged for each data set. For the NO3 product, a
1200 line mm-1 grating blazed at 500 nm and a R955 PMT
(Hamamatsu) were used. The spectrometer was tuned to the A
r X transition at 662 nm. When NO3 was monitored at 662
nm, the first 200 µs of the data were lost due to saturation of
the photomultiplier from the photolysis flash. This limitation
was not encountered for the ClO photomultiplier data due to
the 300 nm cutoff of the Pyrex cell.
Figure 1. UV-visible absorption spectrum of ClONO2 from ref 14.
Bars at bottom of plot indicate spectral regions excited by different
cell/filter combinations. (Reprinted with permission from ref 14.
Copyright 1994 California Institue of Technology.)
ClO and NO3 were also monitored using a 0.3 m monochro-
mator equipped with a 1024 pixel diode array detector. This
detector required a minimum of 16.67 ms to read and store data
from all pixels of the array. The minimum time required to
scan the array could be shortened by reading only a subset of
the array pixels: for example, the time resolution was reduced
to 5 ms by reading only 270 consecutive pixels. For ClO
detection, a 2400 line mm-1 grating was used, which resulted
in a spectral band-pass of ∼30 nm. The grating was tuned to
280 nm, which lies in the middle of the A2Π r X2Π vibrational
progression. For NO3, a 1200 line mm-1 grating with a spectral
band-pass of ∼60 nm was tuned to 640 nm, where the prominent
A rX (0,0) and (1,0) vibrational transitions could be simulta-
neously detected. This spectrometer offered increased sensitiv-
ity for both ClO and NO3 over the single-wavelength spec-
trometer because of the multiplex advantage of using the diode
array detector, but sensitivity was gained at the expense of
temporal resolution. The sensitivity was increased over that of
the PMT spectrometer because each scan of the array produced
a portion of the absorption spectrum of ClO or NO3. The
experimental spectrum was then fit to a reference spectrum using
a least-squares routine. Reference spectra were created by
averaging 1000 scans of the diode array collected under
conditions in the reaction cell, which maximized the production
of either ClO or NO3. The reference spectrum was then
compared to the known cross section to calculate the concentra-
tion of the product species in the reference spectrum. The cross
sections for ClO were those of Sander and Friedl,11 and the
NO3 cross sections were taken from Sander.13 The fitting
routine directly determined the concentration of the species being
detected from the absorption spectrum recorded by the diode
array.
Figure 2. Spectral distributions of the xenon flashlamp with different
cell/filter solution combinations: (a) quartz cell; (b) Pyrex cell; (c) Pyrex
cell with CrK(SO4)2‚12H2O (100 g L-1) filter solution; (d) Pyrex cell
with Ce(NH4)2(NO3)6 (5 g L-1) filter solution.
2. The overlap of the spectral distributions from the four cell/
filter combinations with the ClONO2 absorption spectrum is
shown in Figure 1.
Chlorine nitrate was introduced into the reaction cell by
passing helium (99.999%) through a bubbler containing pure
ClONO2 liquid at 195 K [isopropyl alcohol/CO2(s) slush]. The
backing pressure of helium was set at 510 or 1020 Torr
depending on the desired ClONO2 concentration in the reaction
cell. Bath gas was added to the ClONO2/helium mixture prior
to entrance into the reaction cell to allow complete mixing of
all gases. ClONO2 concentrations were determined using
calibrated mass flow meters, and flow rates of each gas were
adjusted to maintain the desired total pressure and ClONO2
concentration. The accuracy of this was confirmed by compar-
ing ClONO2 concentrations from flow meter measurements and
from UV spectrophotometry in the reaction cell using the OMA
over the spectral range 215-250 nm. The [ClONO2] value
determined directly from the absorption spectrum was always
within 5% of the value determined from the flow meter settings.
The residence time of gases flowing through the cell was 20 s
with the flash lamp firing once every 30 s, thus entirely flushing
Temporal information on the formation and decay of the
product signals measured with the diode array spectrometer was

10168 J. Phys. Chem., Vol. 100, No. 24, 1996
Nickolaisen et al.
TABLE 1: Summary of Experimental Conditions
species
detected
[ClONO2]
pressure
(Torr)
detectiona
scheme
no. of
runs
(1014 molecules cm-3
)
bath gas
cell
ClO
NO3
NO3
ClO
NO3
ClONO2
ClO
ClO
NO3
NO3
NO3
NO3
ClONO2
ClONO2
NO3
NO3
NO3
1.2-3.2
0.5-3.3
1.6-8.1
1.3-8.0
1.3-6.5
3.2
0.51-1.0
2.0
0.25-10
0.25-4.0
0.25-10
1.0
1.0
1.0
0.75-1.0
2.0
0.50-2.5
0.75-4.0
0.75-2.0
1.0-5.0
He
He
He
He
He
He
N2
N2
N2
N2
N2
N2
N2
N2
O2
O2
air
air
air
air
3.9-75
3.9-600
10-75
5.4-75
5.4-75
10-75
5.0-50
13-25
3.0-400
3.0-50
5.0-400
3.2-50
3.2-20
3.2-100
5.0-200
10-50
3.0-250
2.4-400
3.2-100
3.2-100
OMA
OMA
PMT
PMT
PMT
PMT
OMA
PMT
OMA
PMT
OMA
OMA
PMT
PMT
OMA
PMT
OMA
OMA
OMA
OMA
Pyrex
Pyrex
Pyrex
quartz
quartz
quartz
Pyrex
Pyrex
Pyrex
Pyrex
quartz
19
42
51
23
40
4
15
4
110
18
71
7
Pyrex/cericb
Pyrex
6
quartz
Pyrex
Pyrex
Pyrex
11
17
4
43
49
21
NO3
NO3
NO3
quartz
Pyrex/cericb
Pyrex/CrK(SO4)2
c
19
a OMA, optical multichannel diode array spectrometer; PMT, photomultiplier tube single wavelength spectrometer. b Filter solution made of 5.0
g L-1 (NH4)2Ce(NO3)6 in water. c Filter solution made of 50 g L-1 CrK(SO4)2‚12H2O in water.
obtained by successive scans of the array. The array interface
electronics allowed for the acquisition, co-addition, and storage
in dedicated memory locations of up to 512 separate scans of
the array each time the flash lamp fired, with each scan
accounting for a 5-16.67 ms portion of the signal waveform
depending on the number of pixels read. Each of the 512 scans,
which consisted of the spectrum of the particular product species
being monitored, was fit to the appropriate reference spectrum
to determine the product concentration at the time following
the flash corresponding to that scan. Temporal signals were
obtained by averaging 100 photolysis flashes.
The two spectrometers described above provide complemen-
tary experimental information. The single-wavelength spec-
trometer has the advantage of temporal resolution. The
minimum sampling period with this instrument is 10 µs, which
Figure 3. NO3 temporal signals recorded with the OMA spectrometer
following photolysis in the Pyrex cell at total pressures of (a) 10.3, (b)
25.3, (c) 50.1, and (d) 100.1 Torr. Bath gas was N2, [ClONO2]0 ) 1 ×
1014 molecule cm-3, and T ) 298 K.
is sufficiently short to allow the study of the initial product
formation processes involved in this system. However, the
sensitivity of the spectrometer is not as high as with other
methods of detection, particularly when ClO is monitored. The
detection limits for the single-wavelength spectrometer are 4
× 1011 molecules cm-3 for ClO and 5 × 1010 molecules cm-3
for NO3. The array spectrometer offers the advantage of higher
sensitivity. The multiplex advantage of using up to 1024
channels with the capability of signal averaging for the
determination of a single temporal data point provides a
tremendous improvement in the signal-to-noise ratio. However,
it offers relatively poor temporal resolution, rendering this
technique unsuitable for monitoring the nascent photoproducts.
Detection limits are more than an order of magnitude less using
the diode array spectrometer relative to the single-wavelength
spectrometer with minimum detectable concentrations of 1 ×
1010 molecules cm-3 for ClO and 2.5 × 109 molecules cm-3
for NO3. The somewhat lower sensitivity for ClO is due to the
bath gas identity and density, the photolysis wavelength range,
and the temporal resolution of the data acquisition. The
experimental conditions for all data collected are summarized
in Table 1.
A. NO3 and ClO Product Yields. Product yields were
determined from the ClO and NO3 signal waveforms. Figure
3 is a plot of a typical NO3 signal as a function of time after
photolysis in the Pyrex cell with no additional spectral filtering.
Data were acquired with the diode array spectrometer with a
time resolution of 16.67 ms per spectrum. [ClONO2]0 was 1.0
× 1014 molecules cm-3 with N2 as the bath gas. Total pressure
ranged from 10 to 400 Torr, although only the data for 10, 25,
50, and 100 Torr are shown in the figure. Two features of the
data are apparent: the maximum NO3 concentration decreases
and the time required for the NO3 signal to maximize increases
with increasing pressure. While there is secondary formation
of NO3 on the time scale of the experiment due to the reaction
difference in absorption cross sections for ClO and NO3 (σClO
-
[275.3 nm] ) 6.7 × 10-18 cm2 molecule-1 Vs σNO3[662 nm] )
2.27 × 10-17 cm2 molecule-1) and to the decreased light
intensity of the arc lamp, which drops by a factor of about 2
between 662 and 275 nm.
Cl + ClONO2 f Cl2 + NO3
k298 ) 1.0 × 10-11 cm3 molecule-1 s-1 (ref 14) (2)
III. Results
this process cannot be responsible for the observed pressure
dependence of the NO3 yield because reaction 2 is itself
pressure-independent and [ClONO2] is held constant while the
Data were collected under a variety of experimental condi-
tions by changing the initial chlorine nitrate concentration, the

Broadband Photolysis of Chlorine Nitrate
J. Phys. Chem., Vol. 100, No. 24, 1996 10169
Figure 4. NO3 temporal signals recorded with the PMT spectrometer
following photolysis in the Pyrex cell at total pressures of (a) 10.2, (b)
15.2, (c) 25.1, and (d) 50.3 Torr. Bath gas was N2, [ClONO2]0 ) 2 ×
1014 molecule cm-3, and T ) 298 K.
Figure 6. ClO temporal signals recorded with the PMT spectrometer
following photolysis in the Pyrex cell at total pressures of (a) 12.8
([N2] ) 0.0 molecule cm-3), (b) 15.1 ([N2] ) 0.789 × 1017 molecule
cm-3), and (c) 20.1 Torr ([N2] ) 2.41 × 1017 molecule cm-3).
[ClONO2]0 ) 4 × 1014 molecule cm-3, and T ) 298 K.
Figure 5. ClO temporal signals recorded with the OMA spectrometer
following photolysis in the Pyrex cell at total pressures of (a) 5.1, (b)
10.1, (c) 15.1, (d) 24.9, and (e) 50.4 Torr. Bath gas was N2, [ClONO2]0
) 1 × 1014 molecule cm-3, and T ) 298 K.
Figure 7. NO3 temporal signals recorded with the OMA spectrometer
following photolysis in the quartz cell at total pressures of (a) 5, (b)
100, and (c) 400 Torr. Dashed lines are a fit of the data to a Stern-
Volmer type mechanism that also included product secondary chemistry.
Bath gas was N2, [ClONO2]0 ) 1 × 1014 molecule cm-3, and T ) 298
K.
pressure is varied. The effects of this reaction and variations
in [ClONO2] are discussed later.
To verify that the observed yield behavior was not due to
the limited temporal resolution of the OMA detection system,
data were also collected using the monochromator with PMT
detection. In this case, the temporal resolution of the data was
limited only by the 10 µs resolution of the digital oscilloscope.
Figure 4 shows photomultiplier data acquired with 50 µs of
temporal resolution. For this run [ClONO2]0 was 2 × 1014
molecules cm-3, and the pressure was varied from 10 to 50
Torr with N2 as the bath gas. The same qualitative trends are
observed as with the OMA detector; i.e., the maximum NO3
concentration decreases and the time required for the NO3 signal
to maximize increases with increasing pressure. The increase
in formation time with increasing pressure is particularly evident
with this higher time resolution. These data confirm the results
of the OMA spectrometer measured on a 5-20 ms time scale
and show that the decrease in the NO3 yield and the increase in
the NO3 formation time are due to pressure dependence in the
ClONO2 photodissociation and not secondary reactions. The
yields and formation times of NO3 obtained using the PMT and
OMA detectors cannot be quantitatively compared because of
the limited time resolution of the OMA and the differences in
the ClONO2 concentrations used in the two sets of experiments;
as discussed below, ClONO2 self-quenching significantly influ-
ences the observed pressure dependence.
reasons discussed under Experimental Section, particularly for
the photomultiplier data, but the same general features of
decreased yield and increased rise time are present. Of
particular importance is the ClO PMT data of Figure 6. This
shows a marked pressure dependence in the ClO yield even on
the shortest time scales. Additionally, unlike the NO3 product,
there is no secondary channel for the production of ClO (see
the reaction mechanism below), so the observed increase in the
formation time of ClO with increasing pressure cannot be due
to secondary chemical reactions.
Figure 7 shows typical NO3 data for photolysis in the quartz
cell collected with a temporal resolution of 16.67 ms. For this
particular set of data, the initial chlorine nitrate concentration
was 1.0 × 1014 molecule cm-3, and the pressure was varied
from 5 to 400 Torr. There is a pressure dependence exhibited
in the NO3 yield over this pressure range, but the decrease is
only a factor of about 2, whereas for similar conditions in the
Pyrex cell, the yield dropped by more than 2 orders of
magnitude. Also, the temporal behavior of the NO3 signals
observed under these conditions when photolysis is extended
to the shorter wavelengths transmitted by the quartz cell is
different from that shown by the Pyrex data in Figure 3 in that
the NO3 signal maximum occurs within the first 16.67 ms
immediately following the flash and does not display the
secondary rise in NO3 formation seen in Figure 3. Figure 8
shows NO3 data collected with higher temporal resolution (100
µs) using helium as the bath gas. The observed rise in the NO3
Figures 5 and 6 show signals recorded for the ClO product
with the diode array and photomultiplier detectors, respectively.
The quality of the signals is not as high as those of NO3 for the

10170 J. Phys. Chem., Vol. 100, No. 24, 1996
Nickolaisen et al.
Figure 8. NO3 temporal signals recorded with the PMT spectrometer
following photolysis in the quartz cell at total pressures of (a) 10.2,
(b) 25.1, (c) 50.3, and (d) 100.6 Torr. Bath gas was helium, [ClONO2]0
) 6.5 × 1014 molecule cm-3, and T ) 298 K.
Figure 10. Relative NO3 yield as a function of N2 density for photolysis
in the Pyrex cell. Solid symbols are from the PMT data; open symbols
are from the OMA data. [ClONO2]0 ) 1 × 1014 molecule cm-3, and T
) 298 K.
Figure 9. ClONO2 temporal signals measured with the PMT spec-
trometer at 225 nm following photolysis in the Pyrex cell. Absorbance
is defined as -log10(I/I0). Total pressures are (a) 3.2 ([N2] ) 0.0
molecule cm-3), (b) 5.1 ([N2] ) 0.616 × 1017 molecule cm-3), and (c)
15.1 Torr ([N2] ) 3.80 × 1017 molecule cm-3). [ClONO2]0 ) 1 × 1014
molecule cm-3, and T ) 298 K.
Figure 11. Comparison of NO3 and ClO product yields as a function
of N2 density for photolysis in the Pyrex cell. Solid symbols are for X
) NO3; open symbols are for X ) ClO. Initial chlorine nitrate
concentrations are (b) 2.5 × 1013 molecule cm-3, (9) 5.0 × 1013
molecule cm-3, (2) 7.5 × 1013 molecule cm-3, and (1) 1.0 × 1014
molecule cm-3
.
signal is due solely to reaction 2 and can be fit with a single
bimolecular rate coefficient of (8 ( 2) × 10-12 cm3 molecule-1
s-1. This is in reasonable agreement with the literature value
Figure 10 shows a comparison of the NO3 yield collected in
the Pyrex cell with the photomultiplier and diode array
spectrometers. Product yield is defined as the maximum product
concentration normalized by the initial chlorine nitrate concen-
tration. Single-wavelength data were collected from 3.2 to 50.2
Torr, and diode array data were collected from 3.2 to 400 Torr.
Data were normalized to a value of 1 for the lowest pressure.
Data from the two detection schemes result in the same pressure
dependence, indicating that the observed trends in the yield are
not an artifact of one of the detection methods employed. Figure
11 provides a comparison of the NO3 and ClO product yields.
Again, the pressure dependences of the data are very
similarsdifferences in absolute yield are expected due to
unequal branching ratios into each product channel. In addition,
the similarity in the functional form of the pressure dependences
of ClO and NO3 is an indication that secondary photolysis of
the nascent products or secondary chemistry is not creating
artifacts which may be mistaken as a pressure dependence.
Because the duration of the flashlamp is relatively long and
NO3 absorbs strongly in the visible region of the spectrum, it
is possible that the nascent NO3 product undergoes secondary
photodissociation to form NO + O2 and NO2 + O. The ClO
cross sections at wavelengths longer than the 300 nm cutoff of
the Pyrex cell are very small, so secondary photodissociation
will be insignificant.
of 1 × 10-11 cm3 molecule-1 s-1 14
.
This rise occurs on a time
scale much shorter than the temporal resolution of the diode
array detector and is therefore not seen in the data of Figure 7.
However, the rise of the NO3 signal observed for photolysis in
the quartz cell is independent of pressure and, as expected from
reaction 2, the time constant for NO3 formation changes only
as the initial concentration of chlorine nitrate changes.
Figure 9 displays the results of an additional set of measure-
ments taken in the Pyrex cell in which the photomultiplier
spectrometer was tuned to 225 nm to monitor the removal of
chlorine nitrate. The initial [ClONO2] was 1.0 × 1014 molecules
cm-3, and data were collected at pressures from 3.2 to 15.1
Torr with N2 as the bath gas. Because the output of the arc
lamp is quite weak at this wavelength, it was necessary to
average 300 flashes to achieve an acceptable signal-to-noise
ratio. The weak signals also limited the pressure range over
which meaningful data could be collected. However, the trend
in the recorded signals for chlorine nitrate removal is the same
as that observed for the NO3 and ClO product formation,
namely, that the magnitude of chlorine nitrate removal decreases
with increasing pressure. Although the data are not of sufficient
quality over a range of pressures necessary to produce quantita-
tive results, the qualitative features of the data confirm the results
obtained when either the ClO or NO3 product formation was
monitored.
The NO3 yield as a function of air density and photolysis
wavelength is shown in Figure 12 with the data presented in

Broadband Photolysis of Chlorine Nitrate
J. Phys. Chem., Vol. 100, No. 24, 1996 10171
TABLE 2: NO3 Yields for Quartz Cell, Pyrex Cell, Pyrex
Cell with Chromium Potassium Sulfate Filter, and Pyrex
Cell with Ceric Ammonium Nitrate Filtera
air
[NO3]max
(1012
standard
deviation (1012
molecules
density (1018
molecules
cell
configuration
molecules
cm-3
)
cm-3
)
cm-3
)
quartz
0.000
0.000
0.057
0.084
0.224
0.242
0.389
0.409
0.713
0.737
1.515
1.549
3.138
3.161
0.000
0.021
0.046
0.062
0.086
0.115
0.224
0.253
0.273
0.386
0.412
0.435
0.707
0.734
0.762
1.522
1.543
1.575
3.143
3.164
3.241
6.387
0.000
0.039
0.057
0.063
0.089
0.119
0.139
0.166
0.222
0.223
0.248
0.283
0.386
0.386
0.412
0.605
0.706
0.713
0.734
1.419
3.039
3.139
0.000
0.056
0.072
0.090
0.142
0.173
0.224
0.236
0.340
0.383
0.564
0.653
0.715
1.361
1.471
1.517
2.992
3.081
3.148
4.699
6.050
5.680
3.842
4.384
2.980
3.867
2.584
3.559
2.441
3.518
2.411
3.164
2.176
1.047
0.817
0.598
1.092
1.007
0.651
0.567
0.480
0.385
0.272
0.165
0.190
0.109
0.0073
0.0040
0.0037
0.0118
0.0046
0.0079
0.0038
0.0095
0.0051
0.0096
0.0063
0.0087
0.0083
0.0103
0.0050
0.0053
0.0134
0.0061
0.0070
0.0069
0.0067
0.0048
0.0089
0.0056
0.0065
0.0069
0.0061
0.0058
0.0083
0.0053
0.0074
0.0055
0.0033
0.0077
0.0058
0.0043
0.0098
0.0030
0.0076
0.0043
0.0058
0.0022
0.0055
0.0046
0.0056
0.0047
0.0067
0.0029
0.0032
0.0078
0.0059
0.0049
0.0043
0.0054
0.0050
0.0018
0.0018
0.0037
0.0050
0.0069
0.0057
0.0038
0.0075
0.0033
0.0109
0.0056
0.0024
0.0028
0.0061
0.0019
0.0047
0.0045
0.0017
0.0040
0.0092
0.0027
0.0025
Figure 12. NO3 product yield as a function of air density for the four
cell/filter solution combinations: (O) unfiltered quartz cell; (3) unfiltered
Pyrex cell; (]) Pyrex cell with CrK(SO4)2‚12H2O filter solution; (4)
Pyrex cell with Ce(NH4)2(NO3)6 filter solution. Solid lines are empirical
fits to each data set and are provided as a visual guide.
Pyrex
0.0808
0.0822
0.0425
0.0429
0.0227
0.0253
0.0207
0.0184
0.0232
2.810
Pyrex/
CrK(SO4)2‚12H2O
0.312
0.374
Figure 13. Inverse NO3 product yield as a function of N2 density and
initial chlorine nitrate concentration. Initial concentrations are (b) 2.5
× 1013 molecule cm-3, (O) 5.0 × 1013 molecule cm-3, (9) 7.5 × 1013
molecule cm-3, (0) 7.5 × 1013 molecule cm-3 (220 K), (2) 1.0 × 1014
molecule cm-3, (4) 2.0 × 1014 molecule cm-3, ([) 3.0 × 1014 molecule
0.404
0.170
0.0981
0.135
0.0883
0.0541
0.0493
0.0340
0.0485
0.0220
0.0191
0.0276
0.0168
0.0184
0.0140
0.0074
0.0064
0.0057
0.0064
3.677
cm-3, (]) 4.0 × 1014 molecule cm-3, (1) 7.0 × 1014 molecule cm-3
,
and (3) 1.0 × 1015 molecule cm-3. Temperature was 298 K unless
otherwise indicated.
Table 2. The four data sets displayed correspond to four
different photolysis configurations of the reaction cell, namely,
the quartz cell, the Pyrex cell, the Pyrex cell with a chromium
potassium sulfate filter, and the Pyrex cell with a ceric
ammonium nitrate filter. Flashlamp spectral distributions for
each configuration are shown in Figure 2. The x-axis of Figure
12 covers the range from 0 to about 100 Torr of total pressure.
The solid lines in the plot are empirical fits to the data and are
intended only as a guide to the eye. In each case, the product
yield decreases with increasing pressure. In the quartz cell, the
decrease is about a factor of 2; the data from Pyrex cell
photolysis decrease by about 60; the data for the filter solutions
decrease by approximately 200 and 350, respectively, for the
chromium and ceric solutions. Figure 12 indicates that as the
short wavelength cutoff for photolysis is extended to the red,
the pressure dependence of the product yield becomes more
pronounced.
Pyrex/
(NH4)2Ce(NO3)6
filter
0.445
0.187
0.124
0.119
0.0764
0.0564
0.111
0.0497
0.0383
0.108
0.0361
0.0255
0.0417
0.0210
0.0180
0.0378
0.0127
0.0174
The inverse NO3 yield is displayed in Stern-Volmer form
in Figure 13 for a series of measurements in which the initial
chlorine nitrate concentration and bath gas density were
systematically varied. The data are summarized in Table 3.
[ClONO2]0 ranged from 2.5 × 1013 to 1.0 × 1015 molecules
cm-3, and the pressure ranged from 5 to 400 Torr. Included is
one set of measurements acquired at a temperature of 220 K.
a Chlorine nitrate concentration for all data presented was 1.0 × 1014
molecule cm-3
.

10172 J. Phys. Chem., Vol. 100, No. 24, 1996
Nickolaisen et al.
TABLE 3: Inverse NO3 Yield, ONO , as a Function of Initial Chlorine Nitrate Concentration and N2 Bath Gas Density for
3
Photolysis in the Pyrexa
[ClONO2]0
N2 density
temp
(K)
[ClONO2]0
N2 density
temp
(K)
(1014 molecules cm-3
)
(1018 molecules cm-3
)
φNO
(1014 molecules cm-3
)
(1018 molecules cm-3
)
φNO
3
3
0.25
0.073
0.137
0.217
0.299
0.794
1.608
2.417
3.215
0.046
0.112
0.193
0.273
0.438
0.597
0.765
1.579
2.396
3.192
6.440
9.693
12.92
0.164
0.248
0.407
0.575
0.733
1.544
2.366
3.168
6.409
9.655
12.89
0.148
0.365
0.585
1.017
2.154
3.233
4.333
0.230
0.714
1.522
2.342
3.137
6.387
9.626
12.86
24.0
28.6
33.8
298
2.00
3.00
4.00
0.120
0.608
1.423
2.230
3.041
6.281
9.525
12.76
0.181
0.512
1.313
2.128
2.945
6.185
9.420
12.66
0.080
0.407
0.486
1.218
2.039
2.843
6.075
9.320
12.55
0.353
0.489
0.651
0.810
1.624
2.440
3.244
4.864
6.481
0.519
0.651
0.807
1.637
2.437
3.241
4.861
6.491
35.8
199.2
586.3
992.6
1300
2422
3393
3400
51.2
250.2
748.7
1226
1630
2703
3206
3479
65.5
139.0
280.0
953.5
1538
1866
2719
3091
3270
141.5
205.4
565.4
717.9
1515
1839
2137
2540
2895
564.8
785.4
977.5
1894
2254
3240
3167
3239
298
298
298
39.2
168.0
306.5
489.6
958.7
21.6
23.4
29.5
35.2
88.7
132.1
157.4
385.8
614.4
679.8
1313
1779
2660
28.0
24.2
55.1
137.6
138.0
325.8
539.0
694.4
1440
2284
2918
0.50
298
0.75
298
7.00
298
298
0.75
1.00
47.1
220
298
54.4
132.2
233.6
538.6
777.6
10.0
1066
35.8
179.4
456.4
748.5
980.4
1828
2990
3321
a φNO is defined as [ClONO2]0/[NO3]max
.
3
For all chlorine nitrate concentrations, the yield displays a
pressure dependence. However, for [ClONO2]0 e 1.0 × 1014
molecules cm-3, the functional form of the pressure dependence
is independent of the initial chlorine nitrate concentration. For
higher [ClONO2]0, the drop in the yield with increasing pressure
becomes more extreme as the amount of chlorine nitrate is
increased. This is indicative of a dissociative mechanism in
which self-quenching of the excited electronic state by the parent
molecule is important.
B. Product Branching Ratios. The direct measurement of
absolute quantum yields was not possible in this experiment,
but by measuring both the NO3 and ClO product yields under
identical experimental conditions, we could determine the
relative branching ratios for these two product channels. For
these calculations, the relative branching into the ClO + NO2
channel was defined as
of the ClO and NO3 temporal signals. The coefficient 0.5 for
[NO3]max was used because the Cl + NO3 product channel
produces two NO3 molecules due to the fast reaction of Cl with
ClONO2 (reaction 2). It was further assumed that the photolysis
channel leading to O(3P) + ClONO (reaction 1c) is negligible
compared to the ClO + NO2 and Cl + NO3 channels. These
assumptions are based on the results of Nelson et al.,8 which
placed an upper limit to the quantum yield for O(3P) formation
of 4% at 193 and 248 nm. Hence, the relative branching ratio
for the Cl + NO3 channel is given as unity minus the branching
into the ClO + NO2 channel.
Branching ratios measured at several values of [ClONO2]0
and total pressure are presented in Table 4. Although the
absolute product yield for each channel exhibits a pressure
dependence, the dependence is the same for both channels, so
the resulting branching ratios are pressure-independent. Since
both the ClO and NO3 signals are dependent on the initial
chlorine nitrate concentration and the total pressure, the data
of Table 4 contain only the subset of measurements in which
the ClO and NO3 signals were collected under identical
experimental conditions. Relative branching ratios for pho-
[ClO]max
rel branching ratio ≡
[ClO]max + 0.5[NO3]max
where [ClO]max and [NO3]max were determined from the maxima

Broadband Photolysis of Chlorine Nitrate
J. Phys. Chem., Vol. 100, No. 24, 1996 10173
TABLE 4: ClO and NO3 Product Branching Ratios
pressure
(Torr)
[ClO] (1012
rel ClO
branching ratio
[NO3] (1012
rel NO3
branching ratio
molecules cm-3
)
molecules cm-3
)
Pyrex Cell Photolysis
[ClONO2] ) 1.2 × 1014
3.9
5.0
10.0
25.2
50.4
75.0
0.684
0.655
0.681
0.576
0.342
0.207
0.448
0.407
0.399
0.376
0.402
0.417
1.685
1.906
2.052
1.911
1.018
0.578
0.552
0.593
0.601
0.624
0.598
0.583
[ClONO2] ) 2.5 × 1014
10.0
25.2
50.2
75.1
1.643
1.319
0.664
0.357
0.477
0.419
0.447
0.483
3.596
3.653
1.643
0.765
0.523
0.581
0.553
0.517
[ClONO2] ) 3.2 × 1014
10.0
25.4
50.0
2.335
2.142
0.889
0.372
0.472
0.459
0.472
0.487
5.327
5.265
1.818
0.649
0.528
0.541
0.528
0.513
75.1
average
ClO channel ) 0.44 ( 0.08
NO3 channel ) 0.56 ( 0.08
Quartz Cell Photolysis
[ClONO2] ) 1.3 × 1014
5.5
10.3
25.2
50.3
75.3
4.789
4.697
3.728
3.153
2.745
0.628
0.598
0.585
0.638
0.663
5.683
0.372
0.402
0.415
0.632
0.337
6.316
5.289
3.573
2.787
[ClONO2] ) 2.5 × 1014
5.4
10.1
25.3
50.0
75.0
7.477
8.131
6.618
5.637
5.202
0.579
0.591
0.591
0.642
0.658
10.86
11.24
9.157
6.285
5.415
0.421
0.409
0.409
0.358
0.342
[ClONO2] ) 3.2 × 1014
10.0
25.3
50.1
11.91
8.345
6.009
5.225
0.594
0.590
0.578
0.604
16.28
11.59
8.762
6.845
0.406
0.410
0.422
0.396
75.3
average
ClO channel ) 0.61 ( 0.07
NO3 channel ) 0.39 ( 0.07
tolysis in the Pyrex cell are 0.44 ( 0.08 for the ClO + NO2
channel and 0.56 ( 0.08 for the Cl + NO3 channel, where the
error bounds are reported as 2σ and include the uncertainty
introduced by the possible 4% contribution of the O + ClONO
channel. When photolysis is extended to shorter wavelengths
transmitted by the quartz cell, the relative branching ratios are
0.61 ( 0.07 for the ClO + NO2 channel and 0.39 ( 0.07 for
the Cl + NO3 channel. A complication arises if there is
significant unimolecular decomposition of excited NO3 in the
quartz cell photolysis as implied by the results of Nelson et
al.8 at 193 nm and to a lesser extent at 248 nm. Under these
conditions the branching ratio for Cl + NO3 will be underes-
timated and the branching ratio for ClO + NO2 overestimated
(see Discussion). To account for this possible effect, the
experimental uncertainties in the relative branching ratios for
photolysis in the quartz cell have been increased to give values
of 0.61 ( 0.20 for the ClO + NO2 channel and 0.39 ( 0.20 for
the Cl + NO3 channel.
secondary processes to changes in the ClONO2 concentration
and buffer gas pressure. The reaction mechanism and rate
coefficients for secondary reactions in this system are given in
Table 5. The uncertainties in the rate coefficients are greatest
for the reactions involving NO3 with ClO and Cl (reactions 5
and 6); however, these reaction will not have a pressure-
dependent effect on the product yieldsthey only affect the rate
of product removal.
The simulations of the ClO and NO3 time dependences are
in reasonable agreement with the quartz cell datasthe differ-
ences stem from uncertainties in the rate coefficients for
reactions involving NO3 with Cl and ClO, the reactions of
ClONO, and some of the recombination reactions. Simulations
invoking only secondary chemistry, however, fail to replicate
the essential features of the data collected for photolysis in the
Pyrex cell, namely, the decrease in product yield and the increase
in the formation time with increasing pressure. Simulations of
this mechanism over a range of pressures corresponding to the
experimental data indicate that the rate of product formation
becomes larger at higher pressure with no substantial decrease
in the ClO product yield and there is also a small pressure
dependence in the NO3 product yield due to removal by the
termolecular recombination with NO2 (reaction 8). This is the
expected result for a mechanism involving termolecular pro-
cesses. The measured NO3 time dependence following Pyrex
cell photolysis has the opposite behavior, however; the formation
time increases and the yield decreases with increasing pressure.
IV. Discussion
A. Pressure-Dependent Product Yields and Formation
Rates. Photodecomposition Mechanism. The pressure depend-
ences of the product yields for NO3 and ClO formation and
ClONO2 disappearance for photolysis in the Pyrex cell cannot
be explained by secondary reactions of the primary products.
To establish this, we have examined the sensitivity of known

10174 J. Phys. Chem., Vol. 100, No. 24, 1996
Nickolaisen et al.
TABLE 5: Secondary Reactions Involving ClONO2
Photolysis Products
reaction
no.
rate coefficient
reaction
(cm3 molecule-1 s-1
)
ref
ClONO2 + hν f ClO + NO2
1a
1b
ΦPyrex ) 0.44,
a
Φquartz ) 0.61
f Cl + NO3
ΦPyrex ) 0.56,
a
Φquartz ) 0.39
f ClONO + O
Cl + ClONO2 f Cl2 + NO3
O + ClONO2 f ClO + NO3
ClO + NO2 + M f ClONO2 + M
ClO + NO3 f OClO + NO2
Cl + NO3 f ClO + NO2
Cl + NO2 + M f ClONO + M
NO2 + NO3 + M f N2O5 + M
Cl + ClONO f Cl2 + NO2
ClO + ClO + M f Cl2O2 + M
ClO + ClO f Cl2 + O2
f ClOO + Cl
1c
2
3
4
5
6
7
8
9
10a
10b
10c
10d
11
12
13a
13b
14
15
16
Φ e 0.04
8
k298 ) 1.0 × 10-11
k298 ) 2.0 × 10-13
k298 ) 1.8 × 10-31 c
k298 ) 4.7 × 10-13
k298 ) 2.4 × 10-11
k298 ) 1.3 × 10-30 c
k298 ) 2.2 × 10-30 c
k298 ≈ 2.0 × 10-11
14
14
14
14
14
14
14
b
k298,N2 ) 2.0 × 10-32 c 15
Figure 14. Simulations of the NO3 data for photolysis in the unfiltered
Pyrex cell using a mechanism that includes quenching to and dissocia-
tion from a metastable intermediate as well as the product secondary
chemistry outlined in Table 5. Total pressures are (a) 5, (b) 10, (c) 25,
and (d) 100 Torr.
k298 ) 4.9 × 10-15
k298 ) 8.0 × 10-15
k298 ) 3.5 × 10-15
k298 ) 3.0 × 10-18
k298 ) 1.0 × 10-10
k298 ) 2.3 × 10-10
k298 ) 1.2 × 10-11
k298 ) 5.8 × 10-11
k298 ) 1.1 × 10-12
k298 ) 2.7 × 10-33 c
15
15
15
15
14
14
14
14
14
14
f OClO + Cl
Cl2O2 + M f 2ClO + M
Cl + Cl2O2 f Cl2 + ClOO
Cl + ClOO f Cl2 + O2
f 2ClO
To more closely approximate the NO3 yield and temporal
behavior for photolysis in the Pyrex cell, it was necessary to
modify the above mechanism to include a second long-lived
Cl + OClO f 2ClO
ClOO + M f Cl + O2 + M
Cl + O2 + M f ClOO + M
‡
intermediate state (ClONO2 ):
a This work. b Estimated from rate constants for Cl + ClONO2 and
Cl + ClNO. c Units of cm6 molecule-2 s-1
.
†
ClONO2
9
hν8 ClONO2 Jex excitation
To explain the pressure dependence of the product yield data,
it is necessary to invoke a mechanism analogous to Stern-
Volmer quenching in which a metastable electronic state is
formed following the initial excitation. This state may dissociate
to form products or undergo collisional quenching to the ground
electronic state:
ClONO2† f products k†
dissociation
diss
ClONO2†9M8 ClONO2 k† quenching to intermediate
‡
q
ClONO2‡ f products k‡
dissociation of intermediate
quenching of intermediate
hν8 ClONO2
Jex excitation
diss
†
ClONO2
9
ClONO2‡ 9M8 ClONO2 k‡
q
ClONO2† f products
M8 ClONO2
kdiss dissociation
Figure 14 shows simulations of the NO3 temporal signals of
Figure 4 using the modified mechanism which also includes
the secondary reactions given in Table 5. The simulations
qualitatively describe the essential features of the data, namely,
a decrease in yield and an increase in product formation time
with increasing bath gas density. It was not possible, however,
to obtain a satisfactory fit to observed NO3 temporal profiles
over the entire pressure range. One reason for this may be the
complexity introduced by using a broadband flashlamp as a
photolysis source. Under our experimental conditions, the initial
absorption step prepares a large ensemble of excited states
possibly in different electronic levels. The interactions between
these states is not well understood, and it is therefore difficult
to quantify the effects. In addition, the modified mechanism
may not be a complete description of the dynamics of chlorine
nitrate photolysis under our experimental conditions. In mea-
surements performed using a ferric chloride solution in the filter
jacket, the short wavelength cutoff was extended to ap-
proximately 400 nm, limiting the excitation to only the very
weakest transitions in the absorption spectrum. The NO3
product exhibited two distinct maxima separated in time by
about 1.0 s. This may be an indication of multiple intermediates
involved in the dissociation.
9
kq
quenching
This simple mechanism adequately describes the pressure
dependence of the product yield but incorrectly predicts that
the rate of product formation becomes faster with increasing
bath gas density. This mechanism coupled with the reaction
mechanism in Table 5 can, however, be used to interpret the
data collected for photolysis in the quartz cell in which product
formation always occurred promptly following the photolysis
pulse. The dashed lines in Figure 7 are the resulting fit of this
mechanism to the experimental data. The simulated data were
produced by simultaneously fitting the measured signals at
pressures of 5, 100, and 400 Torr to this mechanism using a
least-squares routine (FACSIMILE). Six parameters were
varied in the fitting process including [ClONO2]†, kdiss, and kq.
Three rate coefficientssk5, k6, and k8swere also varied in the
fitting process to allow enough flexibility in the least-squares
routine to locate the deepest local minimum on the ø2 hyper-
surface. The final values for these three rate coefficients were
within the experimental uncertainty of the accepted values. The
rate constants for dissociation and quenching resulting from the
fitting routine were kd ) 1.9 × 107 s-1 and kq ) 2.6 × 10-12
cm3 molecule-1 s-1. Whether these values represent a true
minimum on the fitting surface is uncertainsvarying the initial
guesses for each value by a factor of approximately 5 produced
similar numerical results, but we did not perform an exhaustive
search of the parameter space of initial values to verify that
this was indeed the surface minimum.
Recent ab initio calculations by Gran˜a et al.10 determined
the vertical excitation energies for transitions from the chlorine
nitrate ground state to excited electronic states in the singlet
and triplet manifolds. They also determined oscillator strengths
for the singletfsinglet transitions and used these results to
interpret the chlorine nitrate UV-visible absorption spectrum.

Broadband Photolysis of Chlorine Nitrate
J. Phys. Chem., Vol. 100, No. 24, 1996 10175
electronic state. Population of this state is likely to occur
through a rapid spin-orbit radiationless transition which is
known to be rapid in systems involving heavy atoms such as
chlorine. This is consistent with our observation that photolysis
of chlorine nitrate at longer wavelengths proceeds via a
mechanism that involves a long-lived intermediate state.
The photodecomposition mechanism discussed above does
not include reactions of O(3P) because this species is not
expected to play a major role, especially for photolysis in the
Pyrex cell. Some O(3P) is expected to form due to the
photolysis channel leading to O(3P) + ClONO (<4%)8 and also
from the decomposition of excited NO3. Nelson et al.8 estimate
that 17% and 85% of the NO3 decompose in the photolysis of
ClONO2 at 248 and 193 nm, respectively. The NO3 that
decomposes will affect our relative yield measurements in two
ways. First, the NO3 channel will be underestimated due to
the loss of NO3 arising from unimolecular decomposition. There
is no easy way to quantify this effect. We do not expect a large
correction, however, because the flashlamp intensity drops
rapidly at wavelengths shorter than 248 nm, at which ClONO2
photolysis apparently leads to efficient decomposition of nascent
NO3.17 Second, O(3P), which is the primary product of NO3
decomposition, will influence the observed temporal depend-
ences of ClO and NO3 through secondary reactions. The first
such reaction is O + ClO f Cl + O2, for which k298 ) 3.8 ×
10-11 cm3 molecule-1 s-1. This reaction will affect both the
yield and time dependence of ClO. Simulations which include
a quantum yield of 10% for O(3P) formation indicate that the
effect on the ClO signal lies within the noise when photomul-
tiplier detection is used so that no conclusion can be drawn
concerning the extent of branching into the O atom channel
from the ClO data. Second, O atoms react with ClONO2 with
Figure 15. Vertical excitation energies of the excited electronic states
of chlorine nitrate adapted from ref 10.
They assigned the large feature centered at 190 nm to the 41A′
r 11A′ transition (π* r π excitation) and the peak at 215 nm
to the 31A′′ r 11A′ transition (π* r n excitation). The weak
absorption band at 370 nm was not assigned because it lies
approximately 1 eV below the lowest calculated excited singlet
state. The temperature dependence of the absorption spectrum
measured by Burkholder et al.16 suggests that this low-energy
band derives from a transition from the ground electronic state.
The bands at 215 and 370 nm were the only absorption features
that were not reduced in relative intensity with decreasing
temperature. It is likely that the 370 nm band arises from a
spin forbidden transition from the ground state to the lowest
lying triplet state. The position of this band corresponds very
well with the vertical excitation energy calculated by Gran˜a et
al. for this process, and the small cross section indicates that
the transition is forbidden.
With the information provided by the ab initio results of
Gran˜a et al., the electronic states involved in the mechanism
for the broadband photolysis of chlorine nitrate can be tentatively
assigned. An energy diagram of the excited chlorine nitrate
states is shown in Figure 15. Photolysis in the unfiltered quartz
cell is dominated by excitation to the 31A′′ state, which rapidly
dissociates to products. Photolysis in the unfiltered Pyrex cell
with a short wavelength cutoff at 300 nm restricts the number
of excited states that are energetically accessible. The uncer-
tainty in the calculated vertical excitation energies is ∼0.2 eV.
The 11A′′ state is accessible in our experiments if the calculated
energy of this state is overestimated by 0.23 eV, which is
consistent with the stated uncertainties of the computational
results. The only other states energetically accessible are in
the triplet manifold, with the 13A′′ state clearly within the energy
range of the photolyzing light, and the 13A′′ state perhaps
accessible if the calculated energy is high by 0.17 eV. As stated
above, to qualitatively simulate the data, it was necessary to
invoke the participation of two excited electronic states.
Therefore, we believe that the initial excitation is dominated
by the transition to the 11A′′ state with the possibility that a
small fraction of the absorbed light will directly populate the
triplet manifold. The 11A′′ state undergoes competition between
dissociation to products and intersystem crossing to the triplet
manifold. From their calculations and by analogy to nitric acid,
Gran˜a et al. determined that the lowest lying triplet state is
metastable in that the 13A′′ state represents a local minimum
on the potential energy surface. Because the 13A′′ state is
metastable, it is expected to have a long lifetime relative to
dissociation to products or collisional quenching to the ground
a rate coefficient of k298 ) 2.0 × 10-13 cm3 molecule-1 s-1
,
but the reaction products have not been identified.14 If the
assumed products of this reaction are ClO and NO3, then this
reaction will result in a third process for NO3 production in
addition to ClONO2 photodissociation and the Cl + ClONO2
reaction. The O + ClONO2 reaction is slower under these
conditions than the other two processes, resulting in an increase
in the NO3 formation time. Simulations of this chemistry, which
include 10% branching into the O atom channel, indicate that
the time required for the NO3 signal to attain its maximum value
will increase by a factor of 5 relative to the formation time
excluding oxygen atom chemistry. Because the signal-to-noise
ratio of our apparatus is an order of magnitude larger for NO3
than for ClO, a difference of this size would have been observed
in the NO3 measurements. These simulations also showed that
the NO3 formation time would decrease and the product yield
would only slightly decrease with increasing pressure, counter
to the experimental observations.
Metastable Intermediate Detection. We have shown recently
that the broadband photolysis of dichlorine monoxide (Cl2O)
at wavelengths beyond the Pyrex cutoff (300 nm) results in
pressure-dependent ClO quantum yields.17 The similarities
between ClONO2 and Cl2O are striking in that both molecules
exhibit a decrease in the product yield and a decrease in the
rate of product formation for an increase in the total pressure.
The analysis for Cl2O is simpler because of the less complicated
secondary chemistry of the products (ClO and Cl). In our Cl2O
study, a transient absorption spectrum was detected following
Cl2O photolysis in the Pyrex cell. Vibrational progressions were
identified in this spectrum that were consistent with the transient
species being Cl2O. Ab initio calculations of band oscillator
strengths in the singlet and triplet manifolds suggested that the
observed spectrum could be assigned to a very intense 23A2 r
13B1 transition. This implied that a radiationless transition to

10176 J. Phys. Chem., Vol. 100, No. 24, 1996
Nickolaisen et al.
the triplet manifold in Cl2O is rapid and that the resulting triplet
states are sufficiently long-lived to undergo collisional relax-
ation, causing the observed pressure-dependent quantum yields.
the Pyrex tube, the yield decreases with pressure as observed
in the Pyrex cell. Thus, the quartz cell can be “converted” into
the Pyrex cell without changing the physical properties of the
discharge.
For the chlorine nitrate photolysis experiments, we attempted
to identify spectroscopic evidence for the existence of metastable
states, but for a number of reasons we were unable to observe
any transient absorption spectra in the 250-600 nm spectral
range. For example, the oscillator strengths for excitations from
the intermediate state to higher energy triplet states may be very
small. In the case of Cl2O, the transition from the lowest energy
triplet state, 13B1, to the 23A2 state has a calculated oscillator
strength greater than 0.1. Unfortunately, the ab initio techniques
employed by Gran˜a et al. did not allow for the computation of
oscillator strengths in the triplet manifold. Another reason for
the lack of experimental evidence of the intermediate state is
that its lifetime may be short relative to the 170 ms temporal
resolution of the detection scheme used to search for the
metastable absorption spectrum. Finally, spectral interferences
from ClONO2, ClO, or NO3 may have complicated the search.
Another possible explanation for the observed pressure-
dependent yields is the existence of a very efficient chain
mechanism that catalyzes the decomposition of ClONO2 and is
initiated by the photolysis flash and quenched with increasing
pressure. Such a catalytic mechanism is unlikely as atomic
chlorine is the only known chain carrier that reacts rapidly with
ClONO2, and this reaction terminates the chain.
C. Comparison with Previous Work. Product Yields. A
number of studies have examined the photolysis of chlorine
nitrate,1-9 with several being of particular relevance to the
current work. Margitan measured the product channel branching
ratios following photolysis at 266 and 354 nm using an atomic
fluorescence technique to detect the atomic chlorine product
and chemical conversion to detect ClO by reaction with NO to
form Cl.6 He reported quantum yields of Φ1b ) 0.90 ( 0.10
and Φ1c ) 0.10 ( 0.10 and saw no evidence for the ClO
product. Marinelli and Johnston detected the NO3 product at
662 nm using absorption spectroscopy following 248 nm
photolysis.5 They reported a quantum yield of 0.55 (+0.3/-
0.1) for channel 1b. Minton et al. and Nelson et al. used a
molecular beam apparatus to measure the Cl, ClO, and NO2
products by time- and angle-resolved mass spectrometry fol-
lowing photolysis at 193 and 248 nm.8 They reported relative
yields of 0.36 ( 0.08 for reaction 1a and 0.64 ( 0.08 for
reaction 1b for 193 nm photolysis and, for photolysis at 248
nm, relative yields of 0.46 ( 0.08 for reaction 1a and 0.54 (
0.08 for reaction 1b. They found no evidence for channel 1c
and placed an upper limit on the branching into this channel of
0.04. These results have recently been extended to 308 nm by
Moore et al.,18 who obtained relative yields of 0.33 ( 0.06 for
the ClO + NO2 channel and 0.67 ( 0.06 for the Cl + NO3
channel. The relative branching ratio for the Cl + NO3 channel
from this work for photolysis in the quartz cell is somewhat
smaller than those of Marinelli and Johnston and Minton et al.;
however, the broadband source used in our study makes a
comparison difficult. The results reported by Margitan lie well
outside the error bounds of these three studies, but a careful
examination of Margitan’s work fails to reveal a clear reason
for the discrepancy. Recent unpublished results by Burkholder
and co-workers19 at wavelengths between 193 and 351 nm and
by Orlando and co-workers20 at 248 and 308 nm confirm that
there is a substantial yield of ClO from ClONO2 photolysis over
the range 193-308 nm. Burkholder et al. also observe a
significant increase in the yield of O(3P) and a decrease in the
yield of NO3 with decreasing photolysis wavelength. This is
further evidence for spontaneous decomposition of highly
excited NO3.
B. Possible Experimental Artifacts. The possibility that
the observed pressure dependence of the photolysis product yield
was caused by experimental artifacts was thoroughly investi-
gated. The first, and perhaps most obvious, artifact would be
wall processes in which dissociation occurs at the cell walls,
and the temporal dependence of the signals is determined by
diffusion back into the viewing region. As the pressure in the
cell increases, the time required to diffuse to the walls and back
into the viewing region of the arc lamp would increase, thus
explaining the increased formation time of the product signals.
However, if reactions at the cell walls were responsible for the
formation of product, similar behavior should be observed
independent of the cell material (quartz Vs Pyrex) and of the
salt solution in the filter jacket (no filtering Vs ceric ammonium
nitrate Vs chromium potassium sulfate filters). Each of these
experimental configurations displayed marked differences in the
observed signals, thus arguing against wall processes. Ad-
ditionally, measurements were made in the Pyrex cell under
identical conditions of chlorine nitrate concentration and bath
gas pressure using a number of different bath gases. For the
cases in which either N2 or O2 was used, the diffusion rate
through the cell is approximately equal due to the similarities
in molecular weight and size, so the losses from wall effects
would be expected to by similar. However, a comparison of
the signals shows that the product formation time approximately
doubles when the bath gas is changed from O2 to N2. This is
additional evidence against a diffusion-controlled process such
as wall reactions.
Another possible artifact would be dark dissociative processes;
i.e., the high voltage discharge of the flashlamp creates a plasma
not only in the xenon jacket but also within the reaction cell
itself, which results in the dissociation of chlorine nitrate from
a mechanism not associated with the absorption of light. Such
a plasma would be highly dependent on the total pressure in
the cell and might produce the observed effect in the product
yields. Again, the difference between the cell configurations
argues against such a mechanism in that seemingly small
changes in cell properties (quartz to Pyrex) which have no effect
on the xenon discharge result in dramatic changes in the product
behavior. To provide further evidence against such an artifact,
a set of measurements was carried out in which, first, the NO3
product yield was determined at a number of pressures in the
quartz cell with no other filtering of the photolysis wavelengths.
Then a tube of thin Pyrex glass of the same length as the inner
reaction cell was inserted into the quartz cell, and the measure-
ments were repeated. The quartz cell data displayed the same
small pressure dependence seen in Figure 12, but by inserting
The difference in branching ratios between our measurements
performed in the quartz cell (λ > 200 nm) and the results of
Minton et al.8 for 193 nm photolysis may be due to decomposi-
tion of a small fraction of the nascent NO3 in our experiment
as discussed above. Another possibility is that there are
differences in the dynamics of the electronic states excited in
the initial absorption step due to different photolysis conditions.
Photolysis at 193 nm in the laser experiments is dominated by
a π* r π transition (41A′ r 11A′) localized on the NO2 moiety,
whereas photolysis in our quartz cell leads primarily to an π*
r n transition (31A′′ f 11A′) also localized on the NO2 moiety.10
The intramolecular couplings will be different for each case,
with the π* r n transition favoring dissociation of the weaker
O-N bond (∆H°0 ) 109 kJ mol-1).
Buffer Gas Quenching. The lifetimes of intermediate states
and the dependences of product yields on buffer gas pressure

Broadband Photolysis of Chlorine Nitrate
J. Phys. Chem., Vol. 100, No. 24, 1996 10177
in ClONO2 photolysis have not been systematically studied until
recently. The molecular beam photodissociation experiments
of Okumura et al. provide information concerning the lifetimes
of possible intermediate states.8,18 In these experiments, angular
distributions were measured by electron impact mass spectros-
copy in the recoil of ClONO2 photodissociation products at 193,
248, and 308 nm. At all three wavelengths, the observed
products were scattered anisotropically, indicating that the
lifetimes of the dissociating states were less than a rotational
period. Recent work by Burkholder et al.19 found no pressure
dependences for photolysis at 193, 248, 308, or 351 nm,
although at the latter wavelength, the extremely small chlorine
nitrate absorption cross sections and impurities were significant
complications; 308 nm is very near the cutoff of the Pyrex flash
photolysis cell used in these experiments. The results of
Okumura et al. and Burkholder et al. at this wavelength, which
imply prompt dissociation, are therefore not incompatible with
our results obtained using the Pyrex cell. Our results appear to
be inconsistent with the 351 nm results of Burkholder et al.
however. The main difference in experimental technique is the
use of a broadband photolysis source in our experiments.
Whether the excitation of an ensemble of states using broadband
photolysis plays a significant role in the dissociation dynamics
is uncertain, but may be one source for the differences in
observed product yields.
unity quantum yield, but when photolysis occurred at 366.0 or
435.8 nm, the quantum yield had a pressure dependence which
could be well fit with a simple Stern-Volmer mechanism.
Additionally, the magnitude of the effect varied from 366.0 to
435.8 nm with the pressure dependence most pronounced at
the longer wavelength.
D. Atmospheric Implications. The implications of these
results for the partitioning of inorganic chlorine in the lower
stratosphere are potentially significant. The product of solar
flux and ClONO2 cross section peaks at about 340 nm in the
lower stratosphere. Reduction of the photodissociation quantum
yields due to collision effects would be especially important
for excitation in the 290-450 nm spectral region and would
result in an underprediction of the ratio [ClONO2]/[HCl] by
atmospheric models. This will cause the models to overestimate
the rate of ozone catalytic destruction by the chain mechanism
which involves ClONO2 photolysis.23
Atmospheric models estimate J values at each altitude by
performing a wavelength integration of the product of absorption
cross section, dissociation quantum yield, and solar flux. Due
to the broadband nature of the photolysis pulse from the
flashlamp, it is not possible to retrieve wavelength-dependent
quantum yields from this experiment for use in model calcula-
tions. However, some qualitative observations can be made.
The wavelength distribution of the photolysis radiation from
the Pyrex cell mimics the solar spectral distribution because
both spectral distributions cut off at about 300 nm. As shown
in Figure 10, a direct application of the results of the Pyrex
ClONO2 Self-Quenching. A modified Stern-Volmer mech-
anism provides a reasonable explanation for the effect of the
initial chlorine nitrate concentration on the pressure dependence
of the quantum yield as shown in Figure 13. The quenching
efficiency for excited electronic states will be much greater for
chlorine nitrate than for the buffer gas because of the exact
overlap of energy levels and the increased number of vibrational
modes in ClONO2. At low [ClONO2], the collisional frequency
between excited and ground state chlorine nitrate is so small
that the enhanced quenching efficiency has no observable effect
relative to quenching by the bath gas, whereas at higher
[ClONO2], the collisional frequency increases and self-quench-
ing becomes significant as illustrated in Figure 13.
cell experiments would therefore imply a reduction in JClONO
2
of about a factor of 10 relative to the low-pressure limiting case
at an altitude of 20 km. This is very likely to be an overestimate
of the effect in the atmosphere, however, because the solar
spectrum extends to somewhat shorter wavelengths and the solar
flux increases more rapidly with wavelength than the Pyrex
flashlamp spectrum. These factors increase the weighting of
solar photolysis to shorter wavelengths than our experiment,
which will reduce the sensitivity of the photolysis rate toward
molecular collisions. A more precise determination will require
the measurement of the pressure dependence of the quantum
yield at several discrete wavelengths throughout the 290-450
nm spectral region.
Burrows et al. observed dependences of the quantum yields
for NO3 formation and ClONO2 removal on the [ClONO2]0 in
the steady-state photolysis of ClONO2 at 254 nm. To explain
these results, they invoked a mechanism involving a metastable
chlorine nitrate intermediate which could undergo either dis-
sociation to products or efficient self-quenching back to the
ground state. While the mechanism we have proposed to
explain the observed product yields from photodissociation is
similar to that of Burrows et al., our mechanism is applicable
only at wavelengths longer than 300 nm. The Burrows et al.
results imply a lifetime for the excited state that is longer than
about 10-4 s because their observed quantum yields decrease
for [ClONO2] > ∼1014 molecule cm-3. This is inconsistent
with the results of Nelson et al.,8 which give an excited state
lifetime less than 10-11 s at for 248 nm photoexcitation.
Burrows et al. did not examine the effects of buffer gas
quenching in their studies.
Since present stratospheric models do not consider the effect
of pressure on the ClONO2 J values, we would expect that these
models would tend to overpredict HCl and underpredict ClONO2
mixing ratios in the lower stratosphere. There are relatively
few simultaneous direct measurements of the partitioning of
inorganic chlorine in the lower stratosphere with which to
validate the models. The most rigorous test of photochemical
models comes from instruments on balloons, satellites, and
aircraft which directly measure vertical profiles of ClONO2,
HCl, and other species such as O3 that have a strong influence
on chlorine partitioning. One such data set is provided by the
Mark IV balloon solar interferometer of Toon et al. This
instrument records infrared atmospheric spectra at sunset and
sunrise during flights from a float altitude of 40 km near Ft.
Sumner, NM.24 These spectra are inverted to provide vertical
concentration profiles of ClONO2, HCl, and many other species
from the tropopause (12 km) to 40 km. Jaegle´ et al. have
compared measurements from balloon flights from 1990 to 1993
with results from the Caltech-JPL one-dimensional atmospheric
model using kinetic and photochemical parameters recom-
mended by DeMore et al.14 In particular, they found that their
model underpredicted the [ClONO2]/[HCl] measurements of
Toon et al. below 20 km. At these altitudes, where the
collisional quenching of ClONO2 metastable states would be
expected to play a role, it was found that the agreement would
Pressure-dependent photolysis quantum yields have been
observed in a number of gas phase studies involving other
molecules. Early work by Ayscough and Steacie on the
photolysis of hexafluoroacetone at 313 nm revealed a depen-
dence of both the CO and C2F6 quantum yields on the
hexafluoroacetone pressure.21 Their data were well explained
with a classic Stern-Volmer mechanism with a decomposition
to deactivation ratio of 1:60. In the steady state photolysis of
thiophosgene (SCCl2), Okabe also reported pressure effects in
the chlorine atom quantum yield.22 For photolysis at wave-
lengths of 253.7 nm and shorter, the chlorine atom exhibited

10178 J. Phys. Chem., Vol. 100, No. 24, 1996
Nickolaisen et al.
be improved if values of JClONO were reduced from their
there are major uncertainties in both models and atmospheric
field measurements, comparisons between predicted and ob-
served concentrations of HCl and ClONO2 in this altitude region
seem to be improved if the models employed pressure-dependent
ClONO2 quantum yields. Further laboratory measurements and
future in situ observations of ClONO2, HCl, and ClO will help
to better quantify the role of pressure dependence in the
photolysis of ClONO2 and possibly other species.
2
nominal values by a factor varying from 1 to 10 as the altitude
decreased from 20 to 12 km. Data sets from other atmospheric
instruments are consistent with this conclusion. Recently,
Dessler et al. have shown that standard chemistry underpredicts
[ClONO2]/[HCl] compared with global data at 20 km from the
Upper Atmosphere Research Satellite.25 Also, midlatitude in
situ measurements of [HCl] from the ER-2 aircraft between 15
and 20 km by Webster et al.26 are significantly smaller than
model predictions. However, there are no simultaneous mea-
surements of [ClONO2] with which to define the partitioning
of inorganic chlorine.
Acknowledgment. We thank the JPL Kinetics and Photo-
chemistry Group, R. Salawitch, C. Webster, Y. Yung, L. Jaegle´,
M. Okumura, T. Minton, and T. Moore, for many valuable
discussions and preprints of their work and D. Natzic for expert
technical assistance. This research was performed by the Jet
Propulsion Laboratory, California Institute of Technology, under
contract with the National Aeronautics and Space Administra-
tion.
There are other atmospheric measurements that do not support
a pressure dependence in JClONO
. Michelson et al. find
2
reasonably good agreement between their one-dimensional
model and ATMOS measurements of [ClONO2]/[HCl] in the
lower stratosphere using pressure-independent ClONO2 quantum
yields.27 Also, in situ measurements of [ClO] between 15 and
20 km from the ER-2 aircraft28 and from balloons29 tend to be
larger than predicted by standard models, whereas a decrease
References and Notes
(1) Smith, W. S.; Chou, C. C.; Rowland, F. S. Geophys. Res. Lett.
1977, 4, 517.
in JClONO that would be consistent with the measurements of
2
Toon et al. would result in a slight reduction in [ClO]. There
are also a number of other possible explanations for deviations
between the observed inorganic chlorine partitioning and model
predictions including heterogeneous reactions on volcanic
aerosols, missing reservoir(s) of inorganic chlorine, and mea-
surement errors. Remote sensors are currently the only instru-
ments capable of direct ClONO2 measurements. Because most
of the HCl and ClONO2 column density lies above 20 km,
retrievals below this altitude are subject to large uncertainties.
(2) Chang, J. S.; Barker, J. R.; Davenport, J. E.; Golden, D. M. Chem.
Phys. Lett. 1979, 60, 385.
(3) Adler-Golden, S. M.; Wiesenfeld, J. R. Chem. Phys. Lett. 1981,
82, 281.
(4) Knauth, K. D.; Schindler, R. N. Z. Naturforsch. 1982, 38A, 893.
(5) Marinelli, W. J.; Johnston, H. S. Chem. Phys. Lett. 1982, 93, 127.
(6) Margitan, J. J. J. Phys. Chem. 1983, 87, 674.
(7) Burrows, J. P.; Tyndall, G. S.; Moortgat, G. K. J. Phys. Chem.
1988, 92, 4340.
(8) Minton, T. K.; Nelson, C. M.; Moore, T. A.; Okumura, M. Science
1992, 258, 1342. Nelson, C. M.; Moore, R. A.; Okumura, M.; Minton, T.
K. Chem. Phys., submitted for publication.
V. Summary
(9) De Saxce, A.; Schriver, L. Chem. Phys. Lett. 1992, 199, 596.
(10) Gran˜a, A. M.; Lee, T. J.; Head-Gordon, M. J. Phys. Chem. 1995,
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(11) Sander, S. P.; Friedl, R. R. J. Phys. Chem. 1989, 93, 4764.
(12) Cady, G. H. Inorg. Syn. 1967, 5, 156.
The broadband photolysis of chlorine nitrate has been studied
in both the strongly allowed and weak tail regions of the
absorption spectrum. Time-resolved long path absorption
spectroscopy has shown that both the Cl + NO3 and ClO +
NO2 product channels are active. There is strong evidence for
dependences of the quantum yields for product formation and
ClONO2 disappearance on the ClONO2 and buffer gas densities.
The functional form of the product yield Vs bath gas density is
strongly dependent on the spectral distribution of the flashlamp.
As the photolyzing light is limited to wavelengths longer than
300 nm, the product yield decreases more rapidly with increasing
bath gas density. These results imply that, following excitation
beyond ∼320 nm, ClONO2 undergoes a radiationless transition
to a long-lived metastable electronic state induced by strong
spin-orbit coupling. Comparison with ab initio results suggest
that the metastable species is the lowest energy triplet state,
13A′′. These observations are consistent with a mechanism we
have proposed recently for the photodissociation of Cl2O. In
the latter study, broadband photolysis beyond 300 nm gives rise
to a metastable triplet state which is observable by UV
absorption spectroscopy.
(13) Sander, S. P. J. Phys. Chem. 1986, 90, 4135.
(14) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.;
Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina,
M. J. Chemical Kinetics and Photochemical Data for Use in Stratospheric
ModelingsEValuation Number 11; JPL Publication 94-26; Jet Propulsion
Laboratory, California Institute of Technology, Pasadena, CA, 1994.
(15) Nickolaisen, S. L.; Friedl, R. R.; Sander, S. P. J. Phys. Chem. 1994,
98, 155.
(16) Burkholder, J. B.; Talukdar, R. K.; Ravishankara, A. R. Geophys.
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