Photolysis of nitric acid at 308 nm in the absence and in the presence of water vapor

Article abstract of DOI:10.1021/acs.jpca.5b00951

We have re-examined the NO_x channels from the 308 nm gas-phase photolysis of nitric acid (HNO₃) by using excimer laser photolysis combined with cavity ring-down spectroscopy. The photolysis products were monitored in the 552-560 and 640-648 nm regions. Direct comparison of the photolysis product spectrum in the 640-648 nm region with literature vibronic band origins and line intensities in electronically excited NO₂ (NO₂) suggests that NO₂ is not formed from HNO₃ photolysis at 308 nm. A comparison of the photolysis product spectrum in the 552-560 nm region with a standard NO₂ spectrum indicates that ground-state NO₂ is a photolysis product. We have determined the NO₂ quantum yield from the 308 nm HNO₃ photolysis. We also investigated HNO₃ photolysis in the presence of water vapor. For equilibrated HNO₃/H₂O mixtures, we did not observe significant variation of product absorption around 552 nm with delay times between the firing of the photolysis and the probe lasers. Transient product absorption measurements at 342.0 and 343.5 nm (respective wavelengths where the peak and valley of HONO absorptions are located) are consistent with ground-state NO₂ being the predominant NO_x product from the 308 nm photolysis of a HNO₃/H₂O mixture. Atmospheric implications are also discussed.

Full text of DOI:10.1021/acs.jpca.5b00951

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Article
Photolysis of Nitric Acid at 308 nm in the
Absence and Presence of Water Vapor
Lei Zhu, Manuvesh Sangwan, Li Huang, Juan Du, and Liang T Chu
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b00951 • Publication Date (Web): 24 Apr 2015
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Photolysis of Nitric Acid at 308 nm in the Absence and Presence of Water Vapor
Lei Zhu,* Manuvesh Sangwan, Li Huang,† Juan Du, and Liang T. Chu
Wadsworth Center, New York State Department of Health
Department of Environmental Health Sciences, SUNYꢀAlbany
Albany, New York 12201
†Current address: California Air Resources Board, Sacramento, CA.
*
Corresponding author. Tel: 518ꢀ474ꢀ6846; fax: 518ꢀ473ꢀ2895; eꢀmail: lzhu@albany.edu
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Abstract
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We have reꢀexamined the NOx channels from the 308 nm gas phase photolysis of nitric
acid (HNO3), by using excimer laser photolysis combined with cavity ringꢀdown spectroscopy.
The photolysis products were monitored in the 552ꢀ560 nm and 640ꢀ648 nm regions. Direct
comparison of the photolysis product spectrum in the 640ꢀ648 nm region with literature vibronic
band origins and line intensities in electronicallyꢀexcited NO2 (NO2*) suggests NO2* is not
formed from the HNO3 photolysis at 308 nm. A comparison of the photolysis product spectrum
in the 552ꢀ560 nm region with standard NO2 spectrum indicates that ground state NO2 is a
photolysis product. We have determined the NO2 quantum yield from the 308 nm HNO3
photolysis. We also investigated the HNO3 photolysis in the presence of water vapor. For
equilibrated HNO3/H2O mixtures, we did not observe significant variation of product absorption
around 552 nm with delay times, between the firing of the photolysis and the probe lasers.
Transient product absorption measurements at 342.0 nm and 343.5 nm (respective wavelengths
where peak and valley of HONO absorptions are located) are consistent with ground state NO2
being the predominant NOx product from the 308 nm photolysis of HNO3/H2O mixture.
Atmospheric implication has been discussed.
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1. Introduction
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Field studies1,2 have shown that atmospheric daytime HONO levels greatly exceed those
calculated based upon photochemical stationary state involving OH, NO, and HONO. Various
photochemical HONO formation mechanisms have been proposed to explain such a discrepancy.
One proposed HONO source is the sunlight photolysis of nitric acid (HNO3) adsorbed on ground
surfaces.3ꢀ5 Observation of HONO formation after exposing glass manifold surfaces coated with
HNO3 and H2O with sunlight5 appears to support such a hypothesis. Reaction of H2O with
NO2*, generated by sunlight photolysis of adsorbed HNO3, has been postulated3 to be the cause
of HONO formation (NO2*+H2O = HONO+OH). Field experiments only monitor the endꢀ
products; a complex mixture of reactants in addition to HNO3 and water vapor, and complex
matrices of surface could play the role of photosensitizers in HONO formation. To
experimentally verify whether HONO can be formed from the proposed photolysis of HNO3 and
H2O only system by sunlight, as well as whether NO2* is a HNO3 photolysis product, we have
investigated the 308 nm gas phase photolysis of HNO3 in the absence and presence of water
vapor. Studying gas phase photolysis of HNO3 and HNO3/H2O mixture is necessary in order to
determine whether HONO formation is energetically feasible following sunlight photolysis of
HNO3 in the presence of water vapor.
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Gas phase photolysis of nitric acid in the actinic UV region (λ ≥ 290 nm) can occur
through the following pathways:
HNO3 + hν → OH + NO2
→ OH + NO2*
(λ ≤ 604 nm)
(λ ≤ 381 nm)
(λ ≤ 393 nm)
(1)
(2)
(3)
→ HONO + O(3P)
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where photochemical thresholds were calculated from the corresponding enthalpy changes6.
While the gas phase photolysis of HNO3 in the 190ꢀ250 nm region has been studied
extensively,7ꢀ26 only a few studies27ꢀ29 were conducted at around 300 nm. These few photolysis
studies mostly monitored the OH product. The OH quantum yield from the photolysis of HNO3
vapor at 308 nm was determined28 using laserꢀinduced fluorescence technique (LIF). An OH
quantum yield of 1.05±0.29 was obtained. August and coworkers27 used Dopplerꢀresolved LIF
to probe the scalar and vector properties of the OH(X) fragments generated from HNO3
photodissociation at 280 nm. Photodissociation at 280 nm was postulated to occur through a
vibronicallyꢀmediated transition to an electronic state of A” character. 70% of the excess energy
was found in the internal modes of NO2, and 30% was in the OH and NO2 translational mode.
Thermodynamically, the excess energy following 308 nm photolysis of HNO3 (Eqn. 1) is
estimated29 about 43.8 kcal/mol. Assuming the excess energy is partitioned in a similar fashion
for both 308 nm and 280 nm HNO3 photolysis, the internal energy of the NO2 chromophore from
308 nm photolysis can be up to 30.7 kcal/mol. In the visible spectral region, the lowest energy
vibronic transition for NO2 is between the electronicallyꢀexcited 12B2 state and the ground X2A1
state. The energy level between these two states is strongly dependent on the geometry of NO2,
in particular the bending angle. The calculated excess internal energy (30.7 kcal/mol) is only
slightly higher (about 3 kcal/mol) than the lowest energy between X2A1 and 12B2. Thus, in
principle, NO2 chromophore formed from the HNO3 photolysis at 308 nm could be either NO2,
NO2* or both. We previously conducted the only timeꢀresolved study of the NOx channel29
following the HNO3 photolysis at 308 nm; we suggested that OH+NO2* is a dominant HNO3
photolysis channel. The nature of the NOx product(s) formed from the HNO3 photolysis is
important because if NO2* is formed from the HNO3 photolysis at 308 nm, it may react with
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H2O to form HONO+OH; the proposed mechanism for atmospheric HONO formation from
adsorbed HNO3 photolysis may hold. On the other hand, if the ground state NO2 is formed from
the 308 nm HNO3 photolysis, the proposed mechanism for the HONO formation from sunlight
photolysis of nitric acid in the presence of H2O may not hold. Given that there is only one
published29 timeꢀresolved study for the NOx production channel from the HNO3 photolysis in the
actinic UV region and in light of some of our recent laboratory works, there is uncertainty in the
nature of the electronic states of NO2 in the NOx production channel from the 308 nm HNO3
photolysis. This motivated us to revisit the nature of the NOx production channel from the 308
nm HNO3 photolysis due to its importance in understanding HONO formation in the
troposphere.
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In this paper, we directly measured absorption spectra of the NOx product(s) in the 552ꢀ
560 nm and 640ꢀ648 nm region following 308 nm gas phase photolysis of HNO3 by using cavity
ringꢀdown spectroscopy32,33. Photolysis study was conducted in the absence and presence of
water vapor. Temporal absorption profile around 552 nm following HNO3 photolysis was
obtained. A survey study was also conducted in the near UV region where both HONO and NO2
exhibit absorption. Discussions have been made concerning whether HONO formation is
energetically feasible following actinic UV photolysis of HNO3/H2O only system.
2. Experimental
The experimental setup is briefly described here. Details can be found elsewhere.34ꢀ36
The output from a 308 nm excimer laser (Coherent; LPX 110i) is introduced into a stainlessꢀsteel
reaction cell, at a 15° angle with the main cell axis through a side arm. The probe beam from a
dye laser (Continuum, ND6000) pumped by a Nd:YAG laser (Continuum, Surelite II) is directed
along the main optical axis of the cell, which is vacuumꢀsealed with a pair of high reflectance
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cavity mirrors. Laser dyes used include Rhodamin 590, 4ꢀDicyanmethyleneꢀ2ꢀmethylꢀ6ꢀ(pꢀ
dimethylaminostyryl)ꢀ4Hꢀpyran (DCM), and Pyridine 1 (LDSꢀ698). The probe beam overlaps
geometrically with the photolysis beam at the center of the cavity. The photolysis/probe laser
overlap region can be conceptualized as a rectangular prism in which the prism's center coincides
with that of the cell; its width and height defined by those of the photolysis beam and its length
defined by (beam width)×(tan 15°)ꢀ1, where 15° is the crossing angle between the pump and the
probe laser beams. The length of the photolysis/probe laser overlap region is defined by (beam
width)×(sin 15°)ꢀ1. A 12 mm wide × 6 mm tall rectangular aperture was placed in the photolysis
beam path prior to the beam's entrance to the cell. For a 12 mm wide photolysis beam, the length
of the photolysis/probe laser overlap region is about 4.6 cm. After the probe beam enters the cell
through the front cavity mirror, its intensity decay inside the cavity is monitored with a
photomultiplier tube (PMT) placed behind the back cavity mirror. The PMT output is amplified,
digitized, and transferred to a computer. The decay curve is fitted to a singleꢀexponential decay
function, from which the ringꢀdown time constant (τ) and the total loss (Г) per roundꢀtrip pass
are calculated. Nitric acid and its photolysis product spectra were recorded by scanning the
wavelength of the probe beam in the 552ꢀ560 nm and 640ꢀ648 nm regions in the absence and
presence of the photolysis beam. Potential HONO product following 308 nm photolysis of
HNO3 in the presence of H2O was examined by probing absorptions at 342.0 nm and 343.5 nm
(corresponding to the peak and valley of HONO absorption), both with and without photolysis
pulses. A pulse/delay generator was used to vary the delay time between the firings of the
photolysis and the probe lasers.
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Highꢀpurity HNO3 was prepared by vacuum distillation of 3:2 mixture of sulfuric acid
(98%; Mallinckrodt Baker) and nitric acid (70%; Mallinckrodt Baker) at 273 K; the distilled
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HNO3 was collected into a trap cooled at ethanol/dry ice temperature (195 K).37 At least four
successive distillations were conducted to purify the HNO3 sample. Extra distillation of the
HNO3 sample was made on the day of the experiment to further reduce NO2 impurity. After the
purification, we subsequently transferred HNO3 into a clean bubbler and continued purge NO2 by
flowing N2 carrier gas through the bubbler (held in iceꢀwater bath) for about 20ꢀ25 min, followed
by pumping the bubbler (held in iceꢀwater bath) in the absence of N2 flow for about 10ꢀ15 min.
The NO2 impurity in the purified HNO3 vapor was determined to be about 0.02%, obtained by
measuring NO2 absorption in situ in the 552ꢀ560 nm region using cavity ringꢀdown spectroscopy.
Distilled water was drawn from a Barnstead nanopure ultrapure water system (Thermo Scientific,
USA); the distilled water was evacuated at room temperature for 30 min to remove dissolved air.
The HNO3 pressure inside the cell was monitored by an MKS Baratron capacitance
manometer (10 Torr full scale, measurement accuracy is about 10ꢀ4 Torr). The inner surfaces of
the cell and the stainless steel joints on the gas transport lines were treated with halocarbon wax
or grease (series 1500 or 25ꢀ5S; Halocarbon Products Corp.) to minimize decomposition of
HNO3 on surfaces. Although coating of the cell surfaces with halocarbon wax decreased the rate
of decomposition of HNO3, the NO2 impurity in HNO3 was found to increase with residence
time in the cell on the time scale of several minutes. For quantitative HNO3 photolysis product
study, an equivalent method of fast flow was used. In other words, HNO3 was first evacuated in
the bubbler; then it was introduced into the cell. The photolysis and background measurements
were carried out immediately after filling the fresh HNO3 sample (the background and the
photolysis measurements were done at time scale of less than 25 seconds in order to beat the
background NO2 increase), and afterward the HNO3 sample was pumped out. We repeated this
procedure for every experiment. Detailed methodology used in studying the photolysis of
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HNO3/H2O mixture is described in the Results and Discussion Section. All experiments were
carried out at an ambient temperature of 295 K.
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Results and Discussion
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3.1. Gas Phase Photolysis of HNO3 in the Absence of Water Vapor
3.1a. Nitric Acid and Photolysis Product Spectra in the 640-648 nm Region.
Shown in Figure 1 are nitric acid absorption spectra in the 640ꢀ648 nm region recorded
by cavity ringꢀdown spectroscopy, in the absence and presence of 308 nm photolysis pulses, and
the difference spectrum. Also illustrated in Fig. 1 is literature vibronic band origins and line
intensities38 in electronicallyꢀexcited NO2(2B2).
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0
640
642
644
646
648
Wavelength (nm)
Figure 1.
Cavity ringꢀdown absorption spectra of 1.2 Torr HNO3 in the 640ꢀ648 nm region,
in the absence (solid line) and presence (dashꢀdot line) of 308 nm photolysis. The
spectra were recorded at a wavelength interval of 0.1 nm. The difference
spectrum is shown as medium dash line. The bar graph shows literature38
vibronic band origins and line intensities in electronicallyꢀexited NO2 (2B2).
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A comparison of the difference spectrum with literature NO2* vibronic band origins and line
intensities in the same wavelength region shows that NO2* has different spectral pattern
compared with that of HNO3 photolysis product, suggesting NO2* is not formed from the HNO3
photolysis at 308 nm.
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3.1b. Nitric Acid and Photolysis Product Spectra in the 552-560 nm Region.
Our previous study29 shows that 308 nm HNO3 photolysis product displays absorption in
the 552ꢀ560 nm region. Both NO2 and NO2* have broad visible absorption;39,40 552ꢀ560 nm is at
the longerꢀwavelength tail of the ground state NO2 absorption spectrum.41ꢀ43 It is also a
wavelength range that NO2* starts to exhibit absorption.38,44ꢀ46 In our previous study, we
monitored the decay of product absorption at 552.57 nm, and obtained a decay time constant of
55 ꢀs. The measured decay time constant, which was within the range of the literature decay
lifetime of NO2* of 55ꢀ90 ꢀs, led us to suggest NO2*+OH is a predominant channel from the 308
nm photolysis of HNO3 (The groundꢀstate NO2 was monitored at 352 nm. The total absorption
of HNO3 and the NO2 impurity was on the order of thousand of ppm; transient groundꢀstate NO2
formation was not observed after the HNO3 photolysis). Yet, a direct comparison of the 308 nm
HNO3 photolysis product spectrum to that of literature NO2* in the 640ꢀ648 nm region in the
current study (Fig. 1) suggests that NO2* is not formed. So could the product spectrum in the
552ꢀ560 nm region come from the ground state NO2 generated by the HNO3 photolysis at 308
nm? Could NO2 impurity in the HNO3 sample interfere with the decay profile measurement at
552.57 nm in the previous study? Could the NO2 impurity in the sample be too high such that
NO2 formed from the 308 nm HNO3 photolysis was too small in comparison, which led to the
conclusion that the ground state NO2 was not formed in the previous study? To address these
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questions, we prepared a high purity HNO3 sample and have an independent method to measure
the NO2 impurity concentration in situ at the time of the experiment.
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To ensure the high purity of the HNO3 sample for the photolysis study, we always
performed a redistillation of the HNO3 sample on the day of the experiment in the current study.
In the previous study,29 we did not perform additional distillation on the day of the experiment.
We later found47 that additional distillation on the day of the experiment significantly reduces the
NO2 impurity in the HNO3 sample. Also, our previous reported NO2 impurity29 (< 0.05%) was
determined at the sample preparation and made at 448ꢀ452 nm. Knowing that NO2 impurity in
the sample varies with time and using absorption cross section data of NO2 in the 552ꢀ560 nm
region obtained in this study, we directly determined the NO2 impurity in the HNO3 sample
immediately before the photolysis experiment.
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Shown in Figure 2 are cavity ringꢀdown absorption spectra of 1.0 Torr HNO3 vapor in the
552ꢀ560 nm region in the absence (solid line) and presence (dashed line) of the 308 nm
photolysis. The plots showed that the absorbance was increased after HNO3 was photolyzed at
308 nm. The spectra were scanned from shorter wavelength to longer wavelength, and during
the spectral recording process (it took about 10 min to complete a spectral scan), the
decomposition of HNO3 contributed to the upward sloping of the recorded spectra. A close
examination shows that the peaks and valleys of both spectra are matched on each other,
suggesting that no new species were formed and the ground state NO2 is likely a product from
the HNO3 gas phase photolysis. Also plotted in Figure 2 is an absorption spectrum of a 1:98.3
mixture of NO2:N2 with NO2 partial pressure of 0.33 mTorr in the cell. A comparison of the
NO2 spectrum and that of the HNO3 photolysis product provides clear experimental evidence
that ground state NO2 is indeed a product from the HNO3 photolysis. To understand the cause of
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552.80 nm
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560
Wavelength (nm)
Figure 2.
Cavity ringꢀdown absorption spectra of 1.0 Torr HNO3 vapor in the 552ꢀ560 nm
region in the absence (solid line) and presence (dashed line) of photolysis at 308
nm. Both spectra were taken under closed cell conditions, and at a wavelength
interval of 0.1 nm. The difference spectrum (dotted line) for HNO3 with and
without photolysis is also included. Also shown for comparison is a cavity ringꢀ
down absorption spectrum (dashꢀdotꢀdot line) of 1:98.3 NO2:N2 mixture at an
NO2 partial pressure of 0.33 mTorr.
timeꢀresolved absorption at 552.57 nm from the HNO3 photolysis in our previous study,29 we
tuned the cavity ringꢀdown spectrometer to product absorption maximum at 552.80 nm (the
probe laser system used in previous study was replaced with a more accurate current probe laser
system; there is slight difference in wavelength reading, but it is within the uncertainty of the
instrument); transient roundꢀtrip absorptions from the HNO3 photolysis at 0.5, 1.0, and 2.0 Torr
pressures were measured as a function of delay time between the firing of the photolysis and the
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probe laser. We cannot decrease the HNO3 pressure to that much lower than 0.5 Torr as the
product absorption in the 552ꢀ560 nm region decreases with decreasing HNO3 pressure; we need
to have sufficient signalꢀtoꢀnoise ratio to prove the point. Illustrated in Figure 3 is a temporal
absorption profile from the photolysis of 1.0 Torr HNO3.
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Figure 3. Roundꢀtrip absorption at 552.80 nm as a function of pumpꢀprobe laser
delay time after 308 nm photolysis of 1.0 Torr HNO3. Each absorption value is
that from an accumulation of 10 photolysis pulses.
Consistency check at 15 ꢀs was performed at the beginning, in the middle, and at the end of the
time dependence experiments. Absorptions at 552.80 nm from photolyzing 0.5, 1.0, or 2.0 Torr
HNO3 are found to be independent of the photolysis/probe laser delay time over the 10ꢀ400 ꢀs
range, within the experimental error. The lack of temporal dependence of the product absorption
from the 308 nm photolysis of HNO3 in the 0.5ꢀ2.0 Torr range and the larger product absorption
in the 552ꢀ560 nm region compared to that in the 640ꢀ648 nm region are consistent with the
ground state NO2 (not the electronicallyꢀexcited NO2) being a product from the 308 nm
photolysis of HNO3.
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Reported decay lifetimes48ꢀ54 of NO2* with NO2 as a quencher are in the range of 30ꢀ100
ꢀs as the NO2 excitation wavelength is varied in the 398 to 600 nm range. The long lifetime of
NO2* is postulated55 to arise from the vibrational energy mismatch between the vibronicallyꢀ
excited NO2 (i.e., NO2*) and the ground state NO2; thus, the vibrationalꢀtoꢀvibrational (V→V)
energy transfer between NO2* and NO2 bath molecules is not that efficient. The quenching rate
constant of NO2* by NO2 was found48 to decrease with decreasing photon energy used to excite
NO2 into NO2*. The quenching rate constant of NO2* by HNO3 has not been previously
reported. In the presence of a quencher such as 0.5ꢀ2.0 Torr HNO3, the lifetime of the presumed
NO2* is expected to be shorter than the literature NO2* decay lifetime of 30ꢀ100 ꢀs. If NO2*
were indeed a predominant NOx product formed from the 308 nm HNO3 photolysis, we expect to
see absorption at 552.80 nm to show prompt increase immediately after photolysis, and then
decrease quickly with delay time at a short time scale in Fig. 3. But that is not the case in the
current study. So why did our previous study29 show timeꢀresolved absorption in the 552ꢀ560
nm region? Was such timeꢀdependence real or was it a spurious effect likely caused by the NO2
impurity in the previous HNO3 sample? In our earlier study,29 the temporal absorption profile
was determined by performing measurements at short photolysis/probe laser delay time first,
then intermediate delay time, and subsequently longer delay time. The NO2 impurity in the
HNO3 bubbler decreased with increasing pumpingꢀflow time, and the experiment was done from
shorter delay time to longer delay time. This is equivalent to sampling more NO2 absorption at
shorter delay time and less at longer delay time. The interference from the NO2 impurity could
give this coincidental agreement between the HNO3 photolysis product decay rate constant and
the literature reported decay rate constant of NO2*. As mentioned earlier in this section, our
previous study had a higher NO2 impurity in the HNO3 sample. In addition, no in situ
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measurement of the NO2 impurity level was made during the photolysis experiments in the
previous study. A constant NO2 impurity level in HNO3 was assumed in the slowꢀflow system in
the previous study, our current in situ measurement shows that the NO2 impurity level in HNO3
is not a constant; it decreases with time in the slowꢀflow system. We also concluded that it is
best to prepare the high purity HNO3 rather than to use this flow system to remove the NO2
impurity in HNO3. The later depends on the delicate balance between the impurity NO2
formation and removal rate. Given that the previous study had more NO2 impurity in the HNO3
sample, the impurity level could be variable and was not monitored in the experiment, the results
of the current study should supersede those from our previous study29.
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The ground state NO2 quantum yield from the HNO3 photolysis at 308 nm was obtained
from the ratio of the NO2 concentration generated in the photolysis/probe laser overlap region
per photolysis pulse to the absorbed photon density in the same region. Absorbed photolysis
photon density in the pump/probe laser overlap region was derived from the difference between
the transmitted photolysis beam energies entering (Ein) and leaving (Eout) that region, the
individual photon energy (hc/λ) at the photolysis wavelength (λ), and the volume (v) of the
overlap region. The photolysis beam energy entering or leaving the overlap region was
calculated from the incident photolysis beam energy entering the cell (E0), the HNO3 absorption
cross section (σHNO ) at 308 nm, the HNO3 concentration (nHNO3) in the cell, and the absorbing
3
path length, by applying Beer’s law:
Ein = E0 exp(ꢀσHNO nHNO l1)
(4)
(5)
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Eout = E0 exp(ꢀσHNO nHNO l2)
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where l1 represents the distance between the incident beam at the cell entrance and the initial
position of the overlap region, and l2 denotes the distance between the photolysis beam entrance
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and the end of the overlap region. E0 was determined by a Joulemeter placed in front of the cell.
The photolysis energy inside the cell was corrected for transmission loss at the front cell window
and for reflection of the photolysis beam from the rear cell window. The incident photolysis
fluence inside the cell was determined to be 0.04ꢀ0.05 J/cm2. In light of low photolysis fluence,
multiphoton excitation process is unlikely. We previously obtained29 an HNO3 absorption cross
section of (1.24±0.19)×10ꢀ21 cm2/molecule at 308 nm using cavity ringꢀdown spectroscopy.
The NO2 concentration generated from 308 nm photolysis of HNO3 per photolysis pulse
was acquired from measurements of the roundꢀtrip NO2 absorption at 552.80 nm averaged over
the number of photolysis pulses used. The NO2 concentration resulting from the HNO3
photolysis at a given HNO3 pressure can be obtained from the corresponding roundꢀtrip NO2
absorption per photolysis pulse, the length of the photolysis/probe laser overlap region (l), and
the absorption cross section of NO2, σNO2, using the equation [NO2] = Absorption/(2σNO2l). We
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directly measured NO2 absorption cross section at 552.80 nm, which is (1.34 ±0.05)×10ꢀ19
cm2/molecule. Using the HNO3 absorbed photon density and NO2 absorption determined in this
study, we obtain an NO2 quantum yield of 1.1±0.2 from the HNO3 photolysis at 308 nm,
independent of HNO3 pressure (~ Torr range).
3.2. 308 nm Gas Phase Photolysis of HNO3 in the Presence of Water Vapor
To understand the photolysis of the HNO3/H2O mixture at 308 nm, we first examined
what happened to the cavity loss at 552.80 nm when H2O was introduced into the cell. Cavity
loss increased significantly after the water vapor was introduced into an evacuated reaction cell,
previously exposed to HNO3. This phenomenon suggested that the water vapor likely dispersed
HNO3 or NO2 that was adsorbed on the cell wall. The backꢀfilled water vapor was then pumped
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away along with HNO3 that was adsorbed on the chamber wall. This process was repeated a
number of times to condition the cell wall.
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We noticed that it took much longer time for the water vapor to reach equilibrium
pressure inside the cell than that of HNO3. In the experiment, H2O was first introduced into the
cell to allow sufficient time to reach pressure equilibrium. During this time, HNO3 was pumped
in the bubbler. When H2O pressure reached the equilibrium inside the cell, the HNO3 vapor was
then admitted to the cell. The HNO3/H2O mixture background absorption was first determined,
followed by prompt measurement of absorption of the 308 nm photolyzed mixture. In this way,
the photolysis of the equilibrated HNO3/H2O mixture was studied. Presented in Figure 4 is a plot
of absorption at 552.80 nm as a function of photolysis/probe laser delay time, after 308 nm
photolysis of mixtures containing 1.0 Torr HNO3 and 0.2 Torr H2O. Figure 4 shows that
absorption at 552.80 nm is independent of delay time after the photolysis of the HNO3/H2O
mixtures, suggesting that the HNO3 photolysis product does not react with H2O. If NO2* were
the predominant product formed from the 308 nm photolysis of the HNO3 vapor, and if NO2*
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Figure 4. Roundꢀtrip absorption at 552.80 nm as a function of time after
308 nm photolysis of mixtures containing 1.0 Torr HNO3/0.2 Torr H2O.
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underwent physical and/or chemical quenching by H2O, we should expect absorption at 552.80
nm to decay with time. The lack of timeꢀdependent absorption from the photolysis of mixtures
containing 1.0 Torr HNO3/0.2 Torr H2O or 1.1 Torr HNO3/0.5 Torr H2O is consistent with
ground state NO2 being formed from the 308 nm photolysis of the HNO3/H2O mixture.
To investigate whether HONO was formed from the 308 nm photolysis of HNO3/H2O
mixture, we also tuned the wavelengths of the ringꢀdown probe beam to those at 342.0 nm and
343.5 nm, where literature56ꢀ60 peak and valley of HONO absorption are located. Brief survey
study showed that absorptions at 342.0 nm and 343.5 nm are comparable in size, but lack
temporal dependence following the 308 nm photolysis of HNO3/H2O mixtures. The result
suggests that HONO is not formed from the 308 nm photolysis of HNO3/H2O mixture. Since the
ground state NO2 is produced from the 308 nm photolysis of HNO3, it is not surprising we did
not observe HONO formation at < 1 ms after the 308 nm photolysis of the HNO3/H2O mixture.
The reaction of the ground state NO2 with H2O (2NO2+H2O+M = HNO3+HONO+M) occurs
slowly through heterogeneous process, with a rate constant61 of ~ 2.4×10ꢀ23 cm3moleculeꢀ1sꢀ1.
3.3. Comparison with Literature Studies and Atmospheric Implications.
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Both the photolysis product absorption spectrum and the lack of dependence of
absorption on photolysis/probe laser delay time in the 552ꢀ560 nm region indicate that the NOx
product produced from the 308 nm photolysis of HNO3 is that of NO2 in the ground electronic
state. Our observation is consistent with those18,27 from the HNO3 photodissociation dynamics
studies. On the basis of thermodynamic analysis and literature report7, one might argue for the
formation of some NO2* from the 308 nm photolysis of HNO3. But if we take the symmetry,
barrier, and adiabatic state correlation into the consideration, our experimental observation can
be fully explained. Ab initio calculation7 shows that the ground state and two lowest energy
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excited states of HNO3 are 1 1A’, 1 1A”, and 2 1A”. The energy difference7,62 between these two
electronicallyꢀexcited states and the ground state are 3.40 and 4.54 eV, respectively. Once HNO3
molecule absorbs a 308 nm photon, it can be excited into the first electronicallyꢀexcited 11A”
state due to the fact that the absorption band is broad and the vertical excitation energy is 3.73
eV. In terms of electronic structure, the transition essentially involves excitation of an electron
from nonꢀbonding NO2 molecular orbital to NO2 π* antibonding orbital in HNO3, with symmetry
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a’ → a” (Cs). This HNO3 11A” quantum state is correlated7,18 with X A1 in NO2, which is the
ground electronic state of the NO2 molecule. On the other hand, 308 nm light is not energetic
enough7,61 to access 2 1A” of HNO3 (4.54 eV); this 2 1A” state is correlated with 1 2B2, which is
the first electronicallyꢀexcited state of NO2. The correlation diagram is shown in Figure 5.
Figure 5. A sketch of the HNO3 photodissociation energy level diagram based upon reference 18.
1
Photodissociation of HNO3 at 2 A” is governed by both symmetry and barrier height. Despite
the broad HNO3 absorption, the energy barrier is the limiting step to form NO2*. This elucidates
well our experimental observation that only the ground state NO2 is observed from the 308 nm
photolysis of HNO3.
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Our experimental results are consistent with the ground state NO2 being the only NOx
product formed from the 308 nm HNO3 photolysis. Energetically, the electronicallyꢀexcited NO2
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1 B2 is accessible via HNO3 1 A”, but it is symmetryꢀforbidden (Fig. 5). We cannot totally
exclude a minor NO2* channel (e.g., < few percent) formed through a hypothetical, nonꢀ
adiabatic HNO3 photodissociation pathway. For example, if there were nonꢀadiabatic interaction
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between 1 A” and higher electronicallyꢀexcited state(s) of HNO3, or if the geometry of HNO3
(NO2 angle) were changed in a way that the symmetryꢀforbidden pathway became allowed, the
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HNO3 photolysis product might have some characteristics of electronicallyꢀexcited NO2 1 B2.
Nonetheless, we expect this hypothetical nonꢀadiabatic HNO3 photodissociation channel to be of
minor importance.
In this paper, we have reexamined the NOx product channel from the 308 nm photolysis
of HNO3 in the gas phase; we found ground state NO2 to be the experimentallyꢀobserved NOx
product from this photodissociation process, with a near unit quantum yield. The ground state
NO2 may also be expected to be the NOx product formed from adsorbed HNO3 photolysis in the
daytime troposphere. As the near UV absorption cross sections of surfaceꢀadsorbed HNO3 are
two to three orders of magnitude larger than47,63 those in the gas phase, photolysis of adsorbed
HNO3 molecules by sunlight is an efficient renoxification process and an important source of OH
radicals. However, photolysis of adsorbed HNO3 only system by actinic UV radiation is not
expected to be significant for tropospheric HONO formation, as the solar photon energy in the
troposphere is likely not energetic enough to form the HONO product.
Acknowledgments
Discussions with Professors J. Robert Huber, G. A. Segal, and John R. Barker are
acknowledged. We are grateful for the support provided by the National Science Foundation
under grant #AGSꢀ0969985.
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