Photodissociation of vibrationally excited pernitric acid: HO 2NO2 (2v1) + 390 nm

Article abstract of DOI:10.1021/jp040285s

Pernitric acid (HO₂NO₂) molecule vibrationally excited in the vicinity of its first OH stretching overtone (2v₁) is dissociated via electronic excitation using 390-nm light. The nascent energy distribution of the resulting OH fragments is probed by using laser-induced fluorescence at sub-Doppler resolution. We find that for the minor OH + NO ₃ channel, 55% of the ～19 600 cm^-1of available energy goes into relative translation of the two fragments and 45% into internal excitation. In addition, electronically excited NO₂ associated with the dominant HO₂ + NO₂ channel is also observed. The visible fluorescence from the excited NO₂ fragments is sufficiently intense to allow vibrational action spectra of HO₂NO₂ to be recorded with good signal-to-noise. As vibrational states in the vicinity of the 2v₁ level can also undergo unimolecular dissociation on the ground electronic surface, comparing relative integrated intensities of spectral features appearing in the vibrational state selected action spectra with their known total infrared absorption cross sections provides a means for estimating unimolecular dissociation quantum yields from these levels under collision-free conditions. The present results indicate that at 298 K the unimolecular dissociation quantum yield for the HO₂NO₂ (2v₁) state is ～30 ± 5%. Evidence is also presented for the generation of HOONO impurity in the standard synthesis route for HO₂NO₂.

Full text of DOI:10.1021/jp040285s

8134
J. Phys. Chem. A 2004, 108, 8134-8139
Photodissociation of Vibrationally Excited Pernitric Acid: HO2NO2 (2ν1) + 390 nm†
Jamie Matthews, Ramesh Sharma, and Amitabha Sinha*
Department of Chemistry and Biochemistry, UniVersity of California-San Diego, 9500 Gilman DriVe,
La Jolla, California 92093-0314
ReceiVed: April 8, 2004
Pernitric acid (HO
2ν ) is dissociated via electronic excitation using 390-nm light. The nascent energy distribution of the resulting
OH fragments is probed by using laser-induced fluorescence at sub-Doppler resolution. We find that for the
2 2
NO ) molecule vibrationally excited in the vicinity of its first OH stretching overtone
(
1
-
1
minor OH + NO
two fragments and 45% into internal excitation. In addition, electronically excited NO
dominant HO + NO channel is also observed. The visible fluorescence from the excited NO
sufficiently intense to allow vibrational action spectra of HO NO to be recorded with good signal-to-noise.
As vibrational states in the vicinity of the 2ν level can also undergo unimolecular dissociation on the ground
3
channel, 55% of the ∼19 600 cm of available energy goes into relative translation of the
associated with the
fragments is
2
2
2
2
2
2
1
electronic surface, comparing relative integrated intensities of spectral features appearing in the vibrational
state selected action spectra with their known total infrared absorption cross sections provides a means for
estimating unimolecular dissociation quantum yields from these levels under collision-free conditions. The
present results indicate that at 298 K the unimolecular dissociation quantum yield for the HO
is ∼30 ( 5%. Evidence is also presented for the generation of HOONO impurity in the standard synthesis
route for HO NO
2
NO
2
1
(2ν ) state
2
2
.
Introduction
Specifically, the measurements of Rohel et al.10 at 248 nm find
the following quantum yields:
Peroxynitric acid (PNA/HO2NO2) is formed in the atmosphere
via a three-body recombination reaction involving the HO2 and
NO2 radicals and, hence, provides a temporary reservoir for
HO NO f OH + NO (34%)
(1a)
(1b)
2
2
3
1
these species. Apart from its reaction with OH, photodisso-
f HO + NO (56%)
2
2
ciation constitutes an important removal mechanism for atmo-
spheric HO2NO2. The relatively weak HO2-NO2 bond strength
implies that PNA molecules can be dissociated not only via
the absorption of UV photons, associated with promotion to a
repulsive excited electronic state, but also through unimolecular
dissociation initiated by infrared excitation on its ground
electronic surface.2 The latter mechanism is expected to be
important, for example, at high solar zenith angles (>90°) where
the extended optical path lengths lead to selective depletion of
the short wavelength component in the solar actinic flux relative
The quantum yields for the above two channels were determined
by respectively measuring the total yield of OH and NO2
fragments resulting from 248-nm photolysis and thus do not
preclude the possibility that some of the fragments shown in
the above equations are formed in excited electronic states. In
,3
fact emission from electronically excited NO has been reported
2
in the 248-nm photodissociation study of Macleod et al.11
Dissociation of PNA can also occur readily on its ground
electronic surface and several aspects of this unimolecular
reaction have been investigated both experimentally and
4
-6
to the longer wavelengths due to scattering and absorption.
6,12-14
9
Thus, to fully quantify the photochemical loss mechanisms of
atmospheric PNA, a thorough understanding of both its vibra-
tional and electronic photochemistry is needed.
The UV absorption spectrum of PNA has been investigated
by several groups and consists of a broad absorption feature
theoretically.
Using available thermo chemical data, one
readily estimates the threshold energies for opening the lowest
energy product channel from simple bond fission of PNA, HO
2
-
1
+ NO2 to be ∼8110 cm . The threshold for opening the HO
-1
+ NO3 channel is much higher at ∼13 780 cm . The difference
between the threshold energy for opening the HO2 + NO2
7
-9
starting around 360 nm and extending well below 190 nm.
Ab initio calculations10 suggest the presence of two excited
singlet electronic states in the near-UV region both of which
are dissociative along the HO2-NO2 and the HO-ONO2
-1
channel, 8110 cm , and the band center for exciting the first
-
1 15
OH stretching overtone (2ν1) of PNA, 6910 cm , suggests
that unimolecular dissociation can occur upon excitation to the
2ν1 level only with the assistance of substantial thermal energy.
At room temperature (298 K) the average internal thermal
1
coordinates. The first excited singlet state, 2 A, is located 5.05
eV above the ground state while the second is at 5.52 eV.
Quantum yield measurements from the photodissociation of
PNA have been investigated at 248 nm and indicate that the
primary channels are those associated with the formation of OH
and HO2 photofragments, even though energetically several
other dissociation pathways are also possible at this wavelength.
-
1 16
energy (vib. + rot.) in PNA is estimated to be 700 cm . Thus,
only states associated with the high-energy tail of the Boltzmann
-
1
distribution have sufficient energy to make up the ∼1200 cm
energy deficit required to open the HO2 + NO2 pathway. Roehl
et al. have investigated the quantum yield for unimolecular
dissociation of PNA upon excitation to several vibrational levels
in the vicinity of the 2ν1 band as a function of temperature and
†
Part of the special issue “Richard Bersohn Memorial Issue”.
1
0.1021/jp040285s CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/04/2004

Photodissociation of Vibrationally Excited Pernitric Acid
J. Phys. Chem. A, Vol. 108, No. 39, 2004 8135
particular, fixing λ2 and λ3 while varying the wavelength of the
infrared excitation laser (λ1) generates vibrational action spectra
that reveal information about the fraction of states within the
selected vibrational level that do not undergo unimolecular
dissociation and thus survive for at least a time period set by
the time delay between λ1 and λ2. As we discuss below,
comparing vibrational band intensities appearing in these double-
resonance action spectra with their known total integrated
infrared absorption cross sections provides a means for estimat-
ing the unimolecular dissociation quantum yields of the bands
in the absence of collisions.
Experimental Section
The experimental apparatus is similar to that used in our
previous studies investigating unimolecular dissociation of
18
HOCl. Briefly, the photolysis chamber consists of a glass cell
equipped with a viewing window for monitoring laser-induced
fluorescence (LIF), two sets of mutually orthogonal sidearms
for introducing laser light, and several inlet ports for adding
reagents. The inside of the cell is coated with halocarbon wax
to minimize sample wall loss. We generate HO2NO2 (PNA) in
a manner similar to that described in the literature using the
Figure 1. Schematic energy level diagram illustrating photodissociation
7
,17
of vibrationally excited HO
room-temperature PNA molecules to the 2ν
2
NO
2
(PNA). Laser λ
1
vibrationally excites
subse-
following reaction scheme:
H O + BF NO f HO NO + HF + BF
3
1
level where λ
2
quently further promotes these state-selected molecules to a dissociative
electronic excited state. The resulting OH photofragments are probed
(2)
2
2
4
2
2
2
via LIF by using λ
3
. PNA molecules excited to states of the 2ν
1
vibrational level having sufficient initial thermal energy can also
undergo unimolecular dissociation on the ground electronic surface.
The threshold energies for opening the various product channels shown
are estimated by using the enthalpy data of refs 6 and 9.
The above synthesis is realized in practice by slowly adding
2 g of BF4NO2 to 5 mL of precooled H2O2 (∼90%) kept in
∼
a jacketed glass cell maintained at -13 °C using a chiller
circulating an ethylene glycol-water mixture. The entire
reaction is conducted inside a glovebag filled with dry N2.
Immediately after the synthesis the sample cell is capped off,
removed from the glovebag, and further cooled to -17 °C. The
sample cell is then pumped on to remove volatile species such
as N2 and NO2. During the experimental runs the chiller
reservoir temperature is maintained between -13 and -10 °C
to regulate the partial pressure of HO2NO2 in the sample cell.
The HO2NO2 sample is slowly flowed into the photolysis cell
and varying the extent to which a two-inch diameter bellows
valve, connecting the photolysis cell to a mechanical vacuum
pump, is opened regulates the pressure. Although the sample
reservoir is kept cold, the photolysis cell is at room temperature
and typically operates in the pressure range between ∼80 and
100 mTorr. All components coming in contact with the reagent
prior to entering the photolysis cell are made of either glass or
Teflon to minimize loss of PNA. Under typical operating
conditions, the PNA samples last about 7-10 days before
requiring replacement. Concentrated H2O2 used in the synthesis
is prepared by bubbling N2 through a 50% H2O2 solution over
a period of several days to remove the more volatile water
component; the final H2O2 concentration is estimated by noting
its volume change.
6
pressure. Consistent with the energy constraints noted above
they find that at 295 K, corresponding to the upper limit of
their reported temperature range, the quantum yield for unimo-
lecular dissociation for the 2ν1 band is ∼27% and independent
6
of pressure over the 2-40 Torr range used in their study.
An invariable complication associated with investigating the
laboratory photochemistry of PNA is the presence of impurities.
Previous studies have identified H2O2, HNO3, and NO2 as being
the major impurities present in PNA samples generated by using
6,7,15,17
the traditional synthesis route involving H2O2 + NO2BF4.
In this study we present evidence that HOONO can also form
during the synthesis of PNA. As many of the above impurities
have overlapping absorption bands and give rise to common
photofragments, great care is needed to ensure that contribution
from impurity photochemistry is kept to negligible levels. Below
we present the first results of using vibrational state selection
to selectively excite and investigate the near-UV photochemistry
of PNA while minimizing interference from these impurities.
Figure 1 illustrates the approach used in these double-resonance
experiments. Infrared light (λ1) is used to vibrationally state
select PNA molecules by excitation in the vicinity of the 2ν1
band, and the vibrationally excited molecules are then subse-
quently dissociated by promotion to an electronic excited state
by using a second photon at ∼390 nm (λ2). Finally the OH
fragments resulting from the photodissociation are probed by
laser-induced fluorescence with use of a third laser (λ3). Thus
the effective total energy imparted to the PNA molecule through
the double-resonance excitation corresponds to an equivalent
single photon excitation energy of ∼307 nm. Apart from
obtaining information regarding the electronic photochemistry
of PNA, the double-resonance technique also provides the means
for probing several aspects of the unimolecular dissociation
dynamics occurring on the ground electronic surface. In
Infrared radiation (λ1) for exciting the first OH stretching
overtone of PNA (2ν1) is generated by an optical parametric
oscillator (OPO: Spectra Physics MOPO-730), which is pumped
by the third harmonic of an injection seeded Nd:YAG laser
(Spectra Physics GCR-270). The idler beam from the OPO laser
provides the required tunable radiation between 6600 and 7150
-1
-1
cm with a bandwidth of ∼0.4 cm and pulse energies ranging
from 4 to 6 mJ. Radiation at ∼390 nm for exciting the electronic
transition in the second step of the double resonance (λ2) is
generated by frequency mixing the visible output of another
Nd:YAG laser pumped dye laser operating with R-640 dye with
the Nd:Yag laser’s fundamental. Typical output pulse energies

8136 J. Phys. Chem. A, Vol. 108, No. 39, 2004
Matthews et al.
from the mixing process are between 8 and 10 mJ. The OH
photofragments resulting from the double-resonance excitation
are probed via the A-X transition at ∼308 nm with use of laser
induced fluorescence (LIF). The 308-nm radiation, which has
-
1
a bandwidth of 0.17 cm , is generated by frequency doubling
the output of a third Nd:YAG laser (Continuum: NY81-20)
pumped dye laser (Continuum: ND60). The probe and pho-
tolysis laser beams are combined on a dichroic mirror and
directed into the photolysis cell after passing through a
collimating lens system. The vibrational excitation laser propa-
gates counter to the other two beams and is focused into the
center of the cell by using a 400-mm lens. For some of the
Doppler profile measurements, the probe laser is introduced
orthogonal to the other two beams. All three laser systems
operate at 20 Hz and have temporal widths (fwhm) of ∼7 ns.
The time delay between λ1 and λ2 is typically fixed at 20 ns,
although for experiments probing the unimolecular dissociative
lifetime of the vibrational states, this delay is varied between 7
and 50 ns. The probe laser pulse λ3 typically occur ∼80 ns after
the OPO (λ1) and its intensity is greatly attenuated to avoid
saturation of the OH transitions as well as prevent photolysis
of the PNA. The OH fluorescence excited by the probe laser is
collected by using an f/1 lens system and imaged onto an end-
on photomultiplier (EMI 9635QB). The combination of a color
glass filter (Schott UG-11), a 308-nm interference filter, and a
3
55-nm edge filter located in front of the photomultiplier
provides discrimination against scattered laser light. For detec-
tion of NO2 fluorescence the Schott UG-11 color glass filter is
replaced with a GG-420 filter and the 308-nm interference and
edge filters are removed. The signal from the PMT is sent to a
gated charge integrator (LeCroy, 2249SG ADC) and subse-
quently digitized and passed to a laboratory computer for storage
and analysis.
Figure 2. (a) HOONO (2ν
1 2 2
) action spectrum arising from the H O
+ BF NO source. The spectrum is generated by scanning the IR laser
4
2
(λ
3
1
) while monitoring OH fragment yield through the Q
1
(2) transition
(
λ
). (b) HOONO(2ν ) action spectrum generated from the HO + NO
1
2
+
M three-body recombination reaction. The OH radicals required for
reaction
through a
Results and Discussion
the three-body reaction are generated by using the H + NO
2
with hydrogen atoms being produced by passing H
microwave discharge in a sidearm reactor.
2
As a first step in using double resonance to investigate the
photochemistry of PNA we examine the influence of infrared
excitation (λ1) alone on the sample. The right side of Figure 1
illustrates the simple bond fission pathways associated with the
dissociation of HO2NO2 (PNA) and their corresponding thresh-
old energies. As noted above, in these room-temperature
measurements the initial thermal energy associated with the high
energy tail of the Boltzman distribution combined with the 2ν1
photon energy is sufficient for opening the HO2 + NO2 pathway.
By contrast the OH + NO3 channel is not accessible at these
vibrational excitation energies. Thus based on energetics we
do not expect OH fragments to be generated solely from the
infrared excitation of PNA in the 2ν1 spectral region. In actual
fact, however, we find that freshly prepared PNA samples do
give rise to OH signal when exposed to infrared radiation tuned
to the vicinity of the 2ν1 vibration. Monitoring the yield of the
OH fragments by using the Q1(2) transition, for example, while
scanning the wavelength of the infrared excitation laser (λ1),
generates an action spectrum like the one shown in Figure 2a.
We believe that this signal is due to OH fragment generated
through the unimolecular dissociation of HOONO impurity
present in the PNA sample. Not only is the thermochemistry
we have also generated HOONO using the three-body recom-
bination mechanism and the corresponding action spectrum is
shown in Figure 2b. Although we have not characterized all
aspects of the HOONO formation, we find that the yield of
HOONO from the BF4NO2 + H2O2 source is typically high
when BF4NO2 is in slight excess of that reported in the
Experimental Section. We note that recent studies have reported
generating HOONO by using the reaction of BF4NO + H2O2.22
Hence if BF4NO was present in our system as an impurity it
could account for the formation of HOONO. The HOONO
signal from our PNA source remains strong only for the first 1
to 2 days of use of a freshly prepared PNA sample, suggesting
that it is rather volatile and disappears following extended
pumping. Interestingly, to the best of our knowledge prior
studies have not reported detecting HOONO impurity in their
PNA samples, although it is clear that its presence can
potentially interfere with the study of PNA photochemistry. All
experiments on PNA that we report below are conducted after
the OH signal from HOONO (2ν1) reached negligible levels.
After eliminating the HOONO interference we are able to
more clearly probe the influence of infrared excitation on PNA
itself. As expected on the basis of known thermochemistry and
also reported by previous investigators, the 0 f 2ν1 excitation
of PNA leads to opening the HO2 + NO2 pathway. The
production of HO2 is readily verified by conducting the infrared
excitation experiments in the presence of added NO. The
for opening the HOONO f OH + NO2 (D0 ≈ 19.8 kcal/
mol)1
9,20
consistent with the energy associated with the 2ν1
excitation, but also the band centers of the four primary spectral
features observed in Figure 2a match up exactly with those
recently reported in ref 21 for HOONO generated by using the
high-pressure three-body recombination reaction: OH + NO2
+
M f HOONO + M. To facilitate a more direct comparison,

Photodissociation of Vibrationally Excited Pernitric Acid
J. Phys. Chem. A, Vol. 108, No. 39, 2004 8137
further promoted to an electronic excited state by using ∼390-
nm light (λ2) (see Figure 1). The choice of wavelength λ2 is
dictated by a desire to maximize the double-resonance signal
from PNA while minimizing background signal initiated by λ2
alone. In particular, the absorption cross-section of both HNO3
and HOOH is weak at 390 nm and NO2 is not expected to
fluoresce when excited at this wavelength as it leads to
dissociation. One advantage of this experimental approach is
that the excited electronic state of PNA naturally gives rise to
OH fragments, and hence it is not necessary to add nitric oxide
to record a vibrational action spectrum. Figure 3b shows a
typical vibrational overtone action spectrum of HO2NO2 (2ν1)
obtained with the double-resonance technique. The spectrum
is generated by scanning λ1 while monitoring the yield of OH
photofragments with use of the Q1(4) transition. We find that
action spectra generated by monitoring yields of other OH
rotational states give similar results. In conjunction with the
action spectra of PNA, we have also recorded double-resonance
spectra of pure HOOH and HNO3 in separate experiments using
the same combination of excitation wavelengths. Through these
cross checks we are able to verify that the spectral feature
-
1
observed at 6910 cm in Figure 3b is due to PNA while the
-
1
feature at 7075 cm is due to hydrogen peroxide. Apparently
the combination of λ1 and λ2 effectively prevents HNO3 species
from contributing at these wavelengths. Thus it is clear that by
-
1
tuning λ1 to ∼6910 cm , the peak of the PNA 2ν1 band, and
subsequently photolyzing the vibrationally excited molecules
using 390 nm light, we can investigate the near-UV photo-
chemistry of PNA with out interference. We note that the
combination of 2ν1 + 390 nm corresponds to an effective single
photon excitation of ∼307 nm.
Figure 3. (a) Vibrational overtone action spectrum of HO
This spectrum is generated by exciting PNA to the 2ν level and then
converting the unimolecular dissociation products, HO , to OH through
its reaction with NO, which is also added to the reaction cell. In the
spectrum the yield of OH radicals is monitored with the Q (2) transition
as the 0 f 2ν excitation frequency is scanned. (b) Vibrational overtone
action spectrum of HO NO (2ν ) from the 2ν + 390 nm photodis-
sociation. This spectrum is generated by scanning λ while monitoring
OH photofragments through the Q (4) transition. No nitric oxide is
present and the time delay between λ
Vibrational overtone action spectrum of HO
the 2ν
fluorescence while scanning λ
is shown in the inset. The assignment is from ref 6.
2
NO
2
(2ν
1
).
1
2
1
1
2
2
1
1
Another aspect of the PNA double-resonance action spectra
that we have investigated is to examine whether they change
as the time delay between λ1 and λ2 is varied. By varying this
time delay we hoped to probe the lifetime distribution of those
states of the 2νOH vibrational band that have sufficient energy
to undergo unimolecular dissociation on the ground electronic
surface. Specifically for a given time delay (∆t) between λ1
and λ2, only ro-vibrational states that do not undergo uni-
molecular dissociation during ∆t can subsequently interact with
λ2 and produce OH photofragements through promotion to the
repulsive electronic excited state. Thus, action spectra of these
predissociative vibrational states taken under different time delay
settings may exhibit different intensity distributions. The double-
resonance action spectrum presented in Figure 3b corresponds
to ∆t ) 25 ns, and we find that the spectrum does not change
noticeably as ∆t is reduced to ∼7 ns, the temporal resolution
limit of our laser system. Thus, these first room-temperature
double-resonance measurements suggest that the majority of
PNA states that undergo unimolecular dissociation upon excita-
tion through the 0 f 2ν1 band apparently do so on a time scale
much faster than ∼7 ns. This observation is consistent with the
lack of pressure dependence reported in the quantum yield
1
1
1
and λ
2
is set at 25 ns. (c)
(2ν ) generated via
2
NO
2
1
1
+ 390 nm photodissociation, but monitoring total NO
2
1
. The weak ν + 2ν combination band
1
3
addition of NO initiates the following reaction with the HO2
photofragments:
2
HO + NO f OH + NO2
-12
3
(
k ) 8.6 × 10 cm /(molecule s)) (3)
Hence this reaction rapidly converts HO2 to OH and since the
latter species can be detected by LIF, the above reaction provides
a convenient, although indirect, means for monitoring the
formation of HO2 photofragments. Figure 3a displays a vibra-
tional overtone action spectrum of HO2NO2 over the region of
the 0 f 2ν1 transitions obtained by this method. To facilitate
the conversion of HO2 photofragments to OH, the delay between
the probe and vibrational excitation lasers is increased to ∼11
µs in these measurements; the resulting OH molecules are
monitored via LIF by using the Q1(2) transition. Comparing
Figure 2a and Figure 3a we clearly see that although the
HOONO and HO2NO2 spectra are distinct, there are substantial
regions of overlap in the vicinity of their respective 2ν1 bands.
Consequently care must be taken to avoid excitation of the
unwanted species.
6
measurements of Roehl et al. over the 2-40 Torr range. We
also note that the width of the 2ν1 band generated monitoring
HO2 formation by the addition of NO (Figure 3a) is broader
than that obtained by using the double-resonance excitation
(Figure 3b). We believe this is due to the fact that the action
spectrum generated by monitoring unimolecular reaction is
biased (due to energy constraint) in favor of states in the 2ν1
band with high internal energy while the action spectra generated
by state-selected electronic photodissociation are expected to
be equally sensitive to all the states of 2ν1. Using sub-Doppler
resolution LIF we have also investigated the energy disposal
After examining the predissociation spectra from the 2ν1
vibrational level we focused on the double-resonance excitation
experiments. In these measurements, PNA molecules vibra-
tionally excited to the 2ν1 level by the OPO (λ1) are subsequently

8138 J. Phys. Chem. A, Vol. 108, No. 39, 2004
Matthews et al.
vibrational action spectra with their known integrated infrared
absorption cross sections, we can estimate the quantum yield
for unimolecular dissociation associated with the 2ν1 state. As
6
noted above, Roehl et al. determined the quantum yield for
unimolecular dissociation of several near threshold vibrational
levels of PNA as a function of temperature and pressure by
comparing the relative yield of HO2 fragments from these levels
with those from the much higher energy 3ν1 band, whose
dissociation quantum yield they assumed to be unity. Their
experiments relied on using the HO2 + NO titration reaction to
convert HO2 photofragments to OH for the purposes of
monitoring HO2 fragment yields and, thus, were conducted
under conditions of high pressure (>2 Torr). The present
experiments provide a complementary method for estimating
the unimolecular dissociation quantum yields at low pressure
(
∼70-80 mTorr) in the absence of collisions and without
requiring nitric oxide. This is accomplished by noting that the
integrated signal intensity, S , for the ith vibrational band
2
Figure 4. Nascent rotational state distribution of the OH (V ) 0, Π3/2
manifold resulting from the 2ν + 390 nm excitation of PNA. The
inset shows the Doppler width associated with the Q (4) transition with
)
i
1
appearing in the double-resonance action spectrum is directly
proportional to the product of the following factors:
1
-
1
the 0.17 cm probe laser line width convoluted out.
i
i
i
i
i
for the OH + NO3 channel resulting from the double-resonance
photodissociation of PNA. Taking into account the photon
energies associated with λ1 and λ2, the average thermal energy
S ∝ n σ f σ UVσ
EM
(4)
0
IR
S
In the above expression n is the number density of PNA
molecules in the excitation volume, σIR is the band’s integrated
infrared absorption cross section, f is the fraction of Vibra-
tionally excited molecules that surViVe and do not undergo
unimolecular dissociation, σUV is the cross section for absorption
of UV light by these vibrationally excited molecules, and σEM
is their cross section for producing electronically excited NO2.
Since excitation to the first excited singlet electronic state of
HO NO involves an n f π* transition that is primarily
0
-
1
in PNA (700 cm ), and the bond energy for opening the OH
-
1
+
NO3 channel (13 780 cm ), we estimate the available energy
S
-
1
associated with this dissociation pathway to be ∼19 500 cm .
By scanning the probe laser over the rotational transitions of
the OH fragment’s A r X band and examining both their
-
1
intensity and Doppler line widths, we find that 470 cm of the
-
1
available energy goes into rotational and 8420 cm into
translational excitation of the OH fragment. The nascent OH
energy disposal results are shown in Figure 4. Within our
detection limit, we do not observe any vibrationally excited OH.
Applying momentum and energy conservation, we infer that
2
2
1
0
localized on the NO2 chromophore, the Franck-Condon
factors associated with electronic excitation are likely to be most
sensitive to initial vibrational motion in the parent molecule
involving the NO moiety. Since the 2ν and ν + 2ν vibrations
-1
the partner NO3 fragment receives ∼2310 cm of translational
2
1
1
3
-
1
and ∼8250 cm of internal excitation. The amount of internal
energy available for NO3 is insufficient to produce it in its
electronic excited state. We are unable to obtain detailed
information regarding energy disposal for the HO2 + NO2
pathway in these experiments. However, it is clear from
monitoring total visible fluorescence over the 420-650 nm
range, that a substantial amount of the NO2 fragments is
generated in electronic excited states. That these excited NO2
fragments are indeed being formed from the double-resonance
photodissociation of PNA is confirmed by recording an action
spectrum that monitors the total visible fluorescence as a
function of λ1 with λ2 held fixed at 390 nm. Figure 3c shows
an action spectrum obtained by this method. The signal-to-noise
ratio in this spectra is considerably better than that obtained by
monitoring OH fragments in a single quantum state. In fact
taking into account the photomultiplier response, filter transmis-
sions, and relative fluorescence signals from NO2 versus OH,
we estimate that the HO2 + NO2 channel is at least ∼10 times
more favored over the OH + NO3 channel at these excitation
energies. This is to be compared with the results at 248 nm,
where the yield of the HO2 + NO2 channel is ∼2 times that of
the OH + NO3 channel.10 The improved signal to noise
associated with monitoring the NO2 fluorescence in these
double-resonance experiments permits us to detect even rela-
tively weak vibrational bands such as the ν1 + 2ν3 combination
band (involving O-H stretch and OOH bend) located at 6250
involve primarily motion on the OH/HOO portion of PNA, these
states are likely to have comparable Franck-Condon factors.
In addition, as the OH stretching state is the “bright” state in
overtone spectroscopy, it is likely that the intensity of the ν +
1
2ν combination band arises from mixing of this state with the
3
2
3
“bright” 2ν state. This is also consistent with the fact that
1
stretch-bend coupling is very common in many other mol-
2
4
ecules. Thus assuming that state mixing is strong in vibra-
tionally excited PNA, excitation of these vibrational spectral
features having comparable energies is expected, on average,
2
to differ little with regards to either their ability to absorb λ or
their propensity for producing excited NO fragments upon
2
double-resonance photodissociation; hence they will have similar
values for σUV and σEM. Strong state mixing and concomitant
statistical behavior also seem reasonable given that the vibra-
tional state density of PNA at energies corresponding to 2ν is
1
3
-1
∼10 states/cm . Hence, under these conditions taking the ratio
of integrated intensities of two spectral features in the action
i
j
spectra, S and S , gives:
i
j
i
j
i
j
(S /S ) ≈ (σ /σ )( f / f )
(5)
IR
IR
S
S
Thus if the total integrated infrared absorption cross sections
are known, then one can determine the unimolecular dissociation
quantum yield of one vibrational level relative to another. If in
j
addition one of the vibrational states is nondissociative, so f S
-
1
i
cm . One immediate benefit of being able to detect this lower
energy combination band is that by comparing the integrated
intensities of the ν1 + 2ν3 and 2ν1 bands appearing in the
) 1.0, then the quantum yield for the dissociative state f S, is
obtained directly. In PNA the ν1 + 2ν3 band is lower in energy
-
1
compared to the 2ν1 band by about 650 cm , and its

Photodissociation of Vibrationally Excited Pernitric Acid
J. Phys. Chem. A, Vol. 108, No. 39, 2004 8139
unimolecular dissociation quantum yield is observed to be
negligible due to energy constraints. In fact the data of Roehl
et al. show that the unimolecular dissociation quantum yield
to undergo unimolecular dissociate at room temperature do so
on a time scale faster than ∼7 ns. In addition the quantum yield
for unimolecular dissociation of the HO2NO2 2ν1 state is
determined to be ∼30 ( 5% in the absence of collisions at 298
K. The present results are in good agreement with the earlier
results of Roehl et al. obtained at high pressure with the HO2
+ NO f OH + NO2 reaction to monitor formation of HO2
6
for the ν1 + 2ν3 band is ∼6% at room temperature (implying
j
that f S ) 0.94). Consequently we use the ν1 + 2ν3 band as our
j
reference state and take f S ≈ 1 for this combination band. Using
the experimentally determined integrated intensity ratio for the
ν1 + 2ν3 and 2ν1 bands (found to be 0.041 from the action
spectra), and their corresponding integrated infrared absorption
6
from excitation of HO2NO2 in the region of the 2ν1 level. In
future experiments we hope to use the double-resonance
technique to investigate the dependence of HO2NO2 unimo-
lecular dissociation quantum yield on temperature.
-
19
-20
cross-sections (2ν1: 9.5 × 10
and ν1 + 2ν3: 2.7 × 10
2
-1
6
j
cm molecule cm ), we estimate f S for the 2ν1 band to be
∼
70%; hence at room temperature its dissociation quantum yield
is ∼30%. We have examined the extent to which the relative
UV absorption probability of the two vibrational levels change
with λ2 by carrying out separate experiments using λ2 ) 367
nm. Consistent with our assumption that the two vibrational
levels exhibit similar behavior in the second step of the double-
resonance excitation, the vibrational band intensity ratio did not
change when λ2 was changed from 390 to 367 nm. In addition
we have confirmed, through power dependence studies involving
the λ1 laser, that the vibrational band intensity ratio is not
affected by saturation of the infrared transitions of the stronger
Acknowledgment. We thank NSF and the academic senate
of UCSD for partial support of this work. We thank Melanie
McWilliams for assistance on various aspects of the experiment.
References and Notes
(
1) Brasseur, G. P.; Orlando, J. J.; Tyndall, G. S. Atmospheric
Chemistry and Global Change; Oxford University Press: New York, 1999.
2) Donaldson, D. J.; Frost, G. J.; Rosenlof, K. H.; Tuck, A. F.; Vaida,
V. Geophys. Res. Lett. 1997, 24, 2651.
3) Donaldson, D. J.; Tuck, A. F.; Vaida, V. Phys. Chem. Earth C
(
(
2
000, 25, 223.
2
ν1 band. Our finding for the 2ν1 dissociation quantum yield is
(4) Salawitch, R. J.; et al. Geophys. Res. Lett. 1994, 21, 2551.
(5) Wennberg, P. O.; et al. Geophys. Res. Lett. 1999, 26, 1373.
6
consistent with the earlier results of Roehl et al., who report a
tu e nmi mp eo rl ae tcuu r lea .r dissociation quantum yield of ∼27% at room
(6) Roehl, C. M.; Nizkorodov, S. A.; Zhang, H. G.; Blake, A.;
Wennberg, P. O. J. Phys. Chem. A 2002, 106, 3766.
(7) Molina, L. T.; Molina, M. J. J. Photochem. 1981, 15, 97.
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Summary
Chem. Phys. 2002, 4, 1432.
9) DeMore, W. B.; et al. JPL Publication 94-26, NASA/JPL, Pasadena,
CA, 1994.
(
In summary we have used double-resonance excitation to
investigate the photochemistry of HO2NO2 free from interfer-
ence of various impurities that are typically present in PNA
samples. The combination of 2ν1 + 390 nm used in these state-
selected photodissociation experiments corresponds to an ef-
fective single-photon excitation of ∼307 nm. These first
measurements provide information on the energy disposal for
the minor OH + NO3 channel resulting from electronic
photodissociation of PNA. The primary channel, associated with
the NO2 + HO2 pathway, is detected through monitoring NO2
excited-state fluorescence. The NO2 fluorescence intensity is
sufficiently intense to allow us to record action spectra of both
the 2ν1 and ν1 + 2ν3 bands. Comparing integrated intensities
of these bands appearing in the state-selected action spectra with
their known infrared absorption cross section allows us to
determine the quantum yield for unimolecular dissociation for
the predissociative 2ν1 level. This parameter is important in
accessing the infrared initiated photochemistry of HO2NO2 in
the atmosphere. The present measurements indicate that the
states associated with the 2ν1 level that have sufficient energy
(10) Roehl, C. M.; Mazely, T. L.; Friedl, R. R.; Li, Y.; Francisco, J. S.;
Sander, S. P. J. Phys. Chem. A 2001, 105, 1592.
(
11) Macleod, H.; Smith, G. P.; Golden, D. M. J. Geophys. Res. 1988,
9
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(
13) Staikova, M.; Donaldson, D. J.; Francisco, J. S. J. Phys. Chem. A
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(15) Zhang, H.; Roehl, C. M.; Sander, S. P.; Wennberg, P. O. J. Geophys.
Res. 2000, 105, 14593.
2
(
(
16) Calculated using data from ref 14.
(17) Kenley, R. A.; Trevor, P. L.; Lan, B. Y. J. Am. Chem. Soc. 1981,
1
03, 2203.
(18) Dutton, G.; Barnes, R. J.; Sinha, A. J. Chem. Phys. 1999, 111,
4
(20) Dixon, D. A.; Feller, D. C.; Zhan, G.; Francisco, J. S. J. Phys.
Chem. A 2002 106, 3191.
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8
5
55.
(22) Drouin, B. J.; Fry, J. L.; Miller, C. E. J. Chem. Phys. 2004, 120,
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