Distributions of rovibrational states of secondary product NO X 2Π from photodissociation of nitric acid at 193 nm

Article abstract of DOI:10.1063/1.474963

The distributions of vibrational, rotational, spin-orbit and lambda-doublet states of nascent NO are obtained using the A ²Σ-X ²Π transition via the technique of laser-induced fluorescence. NO is produced mainly from secondary dissociation of fragment NO₂ from nitric acid after photolysis at 193 nm. According to the measured distribution the partition of energy in NO X ²Π is calculated to be ～840cm^-1 in vibration and ～1410cm^-1 in rotation. The spin-orbit state ²Π_1/2 of NO is populated about twice of that in ²Π_3/2 for the vibrational levels υ=0, 1, and 2. The measured populations of vibrational states υ=0, 1, and 2 are 0.75, 0.19, and 0.07, respectively; these populations agree with results calculated with the statistical model, prior theory. The distributions of rotational states of NO X ²Π for these three vibrational levels have single maxima of Gaussian shape. The experimental data indicate that although NO₂, produced photochemically from nitric acid, was proposed to be in an electronically excited state, the state distributions of NO from unstable NO₂ agree with those from NO₂ excited with monochromatic light in the UV-vis range after averaging for a broad distribution of internal energy. In the UV-vis range of excitation the NO fragment dissociates from the ground electronic surface of NO₂. Hence, this unknown electronic state of NO₂ is expected to be coupled to the ground electronic surface then leading to dissociation.

Full text of DOI:10.1063/1.474963

Distributions of rovibrational states of secondary product NO X 2 Π from
photodissociation of nitric acid at 193 nm
Gen-Hou Leu and I-Chia Chen
Citation: The Journal of Chemical Physics 107, 7223 (1997); doi: 10.1063/1.474963
View online: http://dx.doi.org/10.1063/1.474963
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/107/18?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Communication: Direct measurements of nascent O(3P0,1,2) fine-structure distributions and branching ratios of
correlated spin-orbit resolved product channels CO(ã 3Π; v) + O(3P0,1,2) and CO( X
VUV photodissociation of CO2
̃ Σ + 1 ; v) + O(3P0,1,2) in
J. Chem. Phys. 140, 231101 (2014); 10.1063/1.4883515
Detection of OH radical in laser induced photodissociation of tetrahydrofuran at 193 nm
J. Chem. Phys. 122, 124309 (2005); 10.1063/1.1867354
Photodissociation dynamics of enolic-acetylacetone at 266, 248, and 193 nm: Mechanism and nascent state
product distribution of OH
J. Chem. Phys. 118, 2590 (2003); 10.1063/1.1535424
Production of HCO from propenal photolyzed at 193 nm: Relaxation of excited states and distribution of internal
states of fragment HCO
J. Chem. Phys. 114, 8964 (2001); 10.1063/1.1357437
Nascent rotational and vibrational state distributions of NH (X 3 Σ − ) and ND (X 3 Σ − ) produced in the reactions
of N( 2 2 D) with H 2 and D 2
J. Chem. Phys. 106, 4985 (1997); 10.1063/1.473546
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
132.174.255.116 On: Tue, 02 Dec 2014 18:41:33

2
Distributions of rovibrational states of secondary product NO X ⌸
from photodissociation of nitric acid at 193 nm
Gen-Hou Leu and I-Chia Chena)
Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China
͑
Received 27 May 1997; accepted 1 August 1997͒
The distributions of vibrational, rotational, spin–orbit and lambda–doublet states of nascent NO are
2
2
obtained using the A ⌺–X ⌸ transition via the technique of laser-induced fluorescence. NO is
produced mainly from secondary dissociation of fragment NO from nitric acid after photolysis at
2
2
1
93 nm. According to the measured distribution the partition of energy in NO X ⌸ is calculated to
be ϳ840 cm in vibration and ϳ1410 cm in rotation. The spin–orbit state ⌸1 of NO is
populated about twice of that in ⌸3 for the vibrational levels ␷ϭ0, 1, and 2. The measured
Ϫ1
Ϫ1
2
/2
2
/2
populations of vibrational states ␷ϭ0, 1, and 2 are 0.75, 0.19, and 0.07, respectively; these
populations agree with results calculated with the statistical model, prior theory. The distributions of
2
rotational states of NO X ⌸ for these three vibrational levels have single maxima of Gaussian
shape. The experimental data indicate that although NO , produced photochemically from nitric
2
acid, was proposed to be in an electronically excited state, the state distributions of NO from
unstable NO agree with those from NO excited with monochromatic light in the UV-vis range
2
2
after averaging for a broad distribution of internal energy. In the UV-vis range of excitation the NO
fragment dissociates from the ground electronic surface of NO . Hence, this unknown electronic
2
state of NO is expected to be coupled to the ground electronic surface then leading to dissociation.
2
©
1997 American Institute of Physics. ͓S0021-9606͑97͒00642-9͔
2
ϩ
I. INTRODUCTION
a second photon at 193 nm to the D ⌺ state. These pho-
toproducts NO and HONO from nitric acid photolyzed in
x
Solar photolysis of nitric acid is an important source of
the UV range play important roles in the ozone cycle in the
atmosphere; hence it is important to understand the photodis-
sociation dynamics of nitric acid.
OH radicals in the stratosphere. Nitric acid also plays as a
1
reservoir for NO in the atmosphere. Turnipseed et al. have
x
measured the quantum yield for the formation of OH using
8
Previously in this laboratory we found OH from nitric
laser-induced fluorescence ͑LIF͒ to be 0.33Ϯ0.06. Schiffman
2
acid photolyzed at 193 nm to possess only 3% of available
energy distributed to internal degrees of freedom of the OH
fragment. The translational energies of OH and NO2 are es-
timated from Doppler widths of OH and momentum conser-
et al. used direct infrared absorption to detect OH and found
3
a quantum yield equal to 0.47Ϯ0.06. Felder et al. measured
photofragment translational spectra of nitric acid and found
4
the OH yield to be 0.6. Myers et al. used similar technique
Ϫ1
with a different method to determine the OH yield and ob-
vation to be 2330 and 860 cm , respectively, and 87.8% of
tained 0.33Ϯ0.04 for formation of NO and OH, in agree-
the excess energy attributed to the internal energy of NO . In
order to have so much energy and from the estimated distri-
2
2
1
ment with the value of Turnipseed et al. In addition, Myers
4
et al. reported yields 0.53 for singlet channel HONO
bution of internal energy, NO is expected to be formed in an
2
˜
˜
1
1
9
(
(
X AЈ)ϩO( D) and 0.13 for triplet channel HONO
electronically excited state. Bai and Segal suggested that the
NO2 2B state ͑ B shown in their paper which is incorrect ͒
is correlated to nitric acid 2 AЈ, accessed by a 193 nm pho-
ton. Furthermore, Myers et al. assigned stable NO pro-
3
3
a AЉ)ϩO( P). Conclusively, one third of the product
2
4
2
1
channels of nitric acid at 193 nm photolysis forms OH and
1
3
4
NO . Felder et al. and Myers et al. found NO formed
4
2
2
2
from two pathways; one resulted in a stable electronic state
2
duced on photolysis of nitric acid to be in the B state and
2
whereas the other, about 69% of fragment NO , is unstable,
2
unstable NO to be in an unknown excited state lying above.
2
undergoing secondary dissociation to NOϩO. Dissociation
Ϫ1 5
Hence both experimental results and theoretical calculations
to form NO and OH requires energy D Ϸ47.6 kcal mol
.
2
0
indicate that NO produced from nitric acid is in an elec-
2
Hence, the fragment NO can have energy that exceeds the
2
Ϫ1
6
tronically excited state. Because the strong vibronic interac-
dissociation energy, 25 132 cm to produce NOϩO. Yeh
et al. observed emission of NO in the transition
D ⌺ –A ⌺ in time-resolved Fourier-transform spectra;
2
2
7
tion between the B and the ground electronic state A ,
2 1
2
2
ϩ
2
ϩ
NO initially formed in the B state would undergo relax-
2 2
ation in a subpicosecond time scale to vibrationally excited
they concluded that NO formed in the first vibrationally ex-
cited state is a secondary fragment from NO produced from
nitric acid photolyzed at 193 nm. The product NO absorbed
states in the ground electronic surface with mainly electronic
2
2
character of A .
1
The dissociation dynamics of NO are investigated by
2
many research groups.6
,10–33
Visible absorption spectra of
a͒Author to whom all correspondence should be addressed; E-mail address:
icchen@faculty.nthu.edu.tw
NO are complicated because of strong interactions among
2
J. Chem. Phys. 107 (18), 8 November 1997
0021-9606/97/107(18)/7223/7/$10.00
© 1997 American Institute of Physics
7223
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
132.174.255.116 On: Tue, 02 Dec 2014 18:41:33

7224
G.-H. Leu and I-C. Chen: Photodissociation of nitric acid
the lowest electronic states. When an NO molecule, pre-
2
pared in a supersonic jet, is excited to exceed the dissociation
threshold of formation of NOϩO with a monoenergetic light,
the rotational state distributions of NO fluctuate, but, on av-
16–18
erage, follow a statistical behavior.
In this work, nitric
acid was photolyzed at 193 nm, and the state distributions of
2
ϩ
2
nascent NO were measured using LIF on the A ⌺ –X ⌸
17
transition. Statistical models such as the prior theory and
separated statistical ensembles ͑SSEs͒34 are used to fit the
2
experimental data. Internal state distributions of NO X ⌸
produced photochemically from nitric acid and from pho-
tolysis of NO directly in jet-cooled and thermalized condi-
2
tions are compared.
II. EXPERIMENT
FIG. 1. Part of fluorescence excitation spectrum of nascent NO
Fuming nitric acid with a purity greater than 99.5%
Merck͒, after degassing, flowed continuously through a gas
2
ϩ
–X 2⌸, ͑0,2͒ transition produced on photodissociation of nitric acid
A
⌺
͑
at 193 nm.
cell at a pressure 20–30 mTorr and was photolyzed with an
ArF excimer laser ͑Lambda Physik, Compex 100͒ at a wave-
length 193 nm. The produced secondary photofragment NO
2
vibrational levels of NO X ⌸ were obtained on recording a
long spectrum including parts of these two bands. Scans
made on separate days were averaged to give the intensity
ratios to determine the vibrational population of NO. Only
the transitions from high rotational states of ␷ϭ0 were used
in determining the vibrational population of ␷ϭ0.
2
ϩ
2
was probed with LIF on the transition A ⌺ –X ⌸. The
probe beam is from a dye laser ͑Lambda Physik, LPD3002͒,
pumped with an XeCl excimer ͑Lambda Physik, LPX220͒,
after frequency doubling in a BBO crystal. The probe pulses,
at a repetition rate 10 Hz, were collinear and delayed
ϳ50–150 ns after the photolysis pulses. The energy of the
probe pulses was kept below 3 ␮J/pulse ͑spot size 3
III. RESULTS AND MODEL CALCULATIONS
2
ϩ
2
mm diam͒ to avoid saturation in exciting the A ⌺ –X ⌸
transition. We ensured a single-photon process by checking
the intensity ratio of the weak satellite peaks to that of the
A portion of the fluorescence spectrum and assignments
2
ϩ
2
for nascent NO in the A ⌺ –X ⌸ ͑0,2͒ transition are
2
2
main branches of NO to be small. Only 50–100 ␮J/cm per
shown in Fig. 1. The NO X ⌸ state has two spin–orbit
pulse of an ArF excimer laser was used to photolyze nitric
acid to avoid multiphoton processes occurring in dissociation
states 2⌸1/2 and 2⌸3/2 ; each of them has two lambda–
doublet components. These components can be selectively
detected on choosing different branches of transitions. In NO
2
of nitric acid. The power range of 10–200 ␮J/cm per pulse
2
ϩ
2
of 193 nm light was checked and found to be in the linear
region of the LIF intensity of NO vs photolysis power. The
total fluorescence of the excited NO was collected with a
photomultiplier ͑PMT, Hamamatsu R212UH͒ after an inter-
ference filter ͓Corion (239Ϯ12) nm͔, mounted perpendicu-
larly to both laser beams. The signal from the PMT was sent
to a gated integrator, to an analog-to-digital ADC converter
via a GPIB interface then to a personal computer for data
processing.
A ⌺ ϪX ⌸, there are in total twelve branches. Limited
by the experimental resolution, only eight branches are re-
solved; the Q11 branch overlaps with the P21 branch and Q22
with R12 . These overlapped branches are from the same
lower state but to upper states that are spin components, only
slightly split. The spectral intensities of transitions were di-
vided by their H o¨ nl–London factors35 to obtain the relative
rotational populations. All observed branches were analyzed
to obtain the relative population of the NO states for the
various lambda–doublet and spin–orbit states. Figure 2
shows a distribution of relative population of rotational states
Thermalized NO remaining in the gas cell interfered
with detection of nascent NO in the vibrational ground state.
Hence, during the scan we recorded a spectrum by alternat-
ing the probe pulses 100 ns before and after the photolysis
pulses; from each data point was subtracted the background
signal from NO taken before the photolysis pulses then av-
eraged over 60 laser shots. After application of the subtrac-
tion, the uncertainty of spectral intensity for JϽ15.5 is too
large to be included in the data. Spectra of NO
of ␷ϭ1 obtained from eight branches of the ͑0,1͒ band. The
2
population of the AЉ component of ⌸1 is determined from
/2
the spectral intensity of Q ϩP and R21 and the AЈ com-
11
21
ponent from P11 and R ϩQ , and analogously the AЉ
11
21
2
component of ⌸3 from Q ϩR and P12 and AЈ from
/2
22
12
P ϩQ and R22 . The populations obtained from separate
22
12
branches are approximately equal within the experimental
error for the same spin–orbit state except for the ␷ϭ2 level.
The obtained populations for the same rotational state are
2
ϩ
2
A ⌺ –X ⌸ ͑0,0͒, ͑0,1͒, and ͑0,2͒ bands were recorded
and the intensity was normalized for fluctuations of both
photolysis and probe pulses detected with photodiodes.
About 30–50 laser shots were averaged for each data point.
The ratios of spectral intensity between transitions from two
2
averaged, and displayed in Fig. 3 for NO X ⌸ ␷ϭ0, 1, and
2 states. The populations for Jр15.5, ␷ϭ0 are not shown in
the plot because of a large experimental uncertainty arising
J. Chem. Phys., Vol. 107, No. 18, 8 November 1997
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
132.174.255.116 On: Tue, 02 Dec 2014 18:41:33

G.-H. Leu and I-C. Chen: Photodissociation of nitric acid
7225
FIG. 2. Distributions of population of rotational states of nascent NO X 2
⌸
␷
ϭ1 obtained from various branches corrected for the rotational line
strength.
from background NO remaining in the gas cell. For the mea-
2
sured vibrational states, the population of ⌸ is approxi-
1/2
2
2
mately twice of ⌸ , shown in Fig. 4, although ⌸ is
3/2
1/2
Ϫ1
only 120 cm lower in energy. These ratios ͑ϳ1.9 on aver-
age͒ seem to be invariant with rotational quantum number.
The maximal populations for the three vibrational levels are
about Jϭ20.5–25.5. The ␷ϭ2 state has a smaller maximum
J value ϳ42.5 than the other vibrational states limited by the
available energy, shown in Fig. 3. For ␷ϭ2 state the ob-
served population of the AЈ exceeds slightly than that of the
AЉ state but not for the other vibrational states. All rotational
distributions of these three vibrational levels show a non-
Boltzmann behavior ͑Fig. 5͒ and can be fitted with Gaussian
functions ͑Fig. 3͒.
The population ratios among vibrational states were ob-
tained on comparing the spectral intensities of transitions for
various vibronic bands corrected for the Franck–Condon
3
6
factors,
oscillator strengths, frequency factors, and
weighted for the rotational population for that vibrational
level. Because all three levels were obtained on exciting to
the same upper level, there is no need to correct the spectral
response of the detector and optical filters. In Table I we list
experimental measurements for vibrational populations of
2
NO X ⌸. Vibrational levels up to ␷ϭ2 were observed.
Most NO produced was formed in the vibrational ground
state, ϳ75%. From the distributions of product states, the
energy distributed in average to vibration of NO is ϳ840 and
Ϫ1
ϳ1410 cm in rotation.
In previous work of photodissociation of HNO at wave-
3
8
length 193 nm, the distribution of internal energy NO was
2
FIG. 3. Distributions of rotational states of nascent product NO X 2⌸1/2 and
estimated from the Doppler widths of product OH transitions
and from assuming conservation of momentum. In the same
work, OH was formed in only the ground vibrational state
2
X
͑
⌸3/2 ͑a͒ ␷ϭ2 ͑b͒ ␷ϭ1, and ͑c͒ ␷ϭ0 levels for lambda–doublet states AЈ
᭿͒ and AЉ ͑᭺͒ of ⌸1/2 and AЈ ͑ϫ͒ and AЉ ͑᭡͒ of 2⌸3/2 . The best-fit
2
Gaussian functions for these distributions are displayed in solid line.
J. Chem. Phys., Vol. 107, No. 18, 8 November 1997
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
132.174.255.116 On: Tue, 02 Dec 2014 18:41:33

7226
G.-H. Leu and I-C. Chen: Photodissociation of nitric acid
TABLE I. Comparison of vibrational populations of NO X 2⌸ from experi-
mental data and results calculated with prior and SSE models.
Priora
SSEa
Vibrational level
NO X 2
⌸
Experimental
͑1͒
͑2͒
͑1͒
͑2͒
0
1
2
3
0.75
0.19
0.07
Ϸ0
0.71
0.21
0.07
0.02
0.77
0.19
0.04
0.01
0.56
0.26
0.12
0.05
0.61
0.28
0.10
0.02
a͑1͒ and ͑2͒ using the internal state distribution of NO from curve ͑—͒ and
2
͑
ؠؠؠؠ͒ in Fig. 6, respectively.
other assumption, that the velocity of the OH fragment is
isotropic, was made. However, within the experimental un-
certainties, the distributions from these two methods agree
with each other, shown in Fig. 6. Nevertheless, the second
method is a direct measurement of the translational energy;
FIG. 4. Population ratio of NO X 2⌸1/2 /X 2⌸3/2 as a function of NO rota-
tional quantum number for three vibrational levels.
presumably, the distribution of internal energy of NO is
2
expected to be more accurate. Hence, we use this distribution
to perform model calculations and compare the results from
model calculations with the experimental data. According to
and with little rotational energy, indicating that much avail-
able energy is expected to be distributed as internal energy of
the other fragment NO . The second distribution for the NO2
2
the second distribution of NO internal energy, the internal
2
internal energy, PNO2(E ), was calculated from the distri-
int
Ϫ1
energy is, on average, to be ϳ27 100 cm , and the average
available energy Eavailϳ3700 cm
bution of translational energy, P(E ) obtained by Myers
Ϫ1
t
.
4
8
et al. and the average internal energy of OH, E (OH)
ϭ950 cm given the relationship that E ͑NO ͒ϷE ϪE
ϪE ͑OH͒ϪE . The photon energy E ϭ51 810 cm and
rot
Ϫ1
int
2
ph
Ϫ1
th
(PNO2(E )ϫ͑E Ϫ25 130 cmϪ1͒
int int
Ϫ1
Eavail
ϭ
͚
ͫ
ͬ
rot
t
ph
E
Ͼ25 130 cm
int
the threshold energy of formation NO ϩOH from nitric acid
2
Ϫ1
is E ϭ16 660 cm . The distribution from the OH Doppler
th
Ϭ
͓PNO2͑E ͔͒ .
͑1͒
͚
int
width is expected to have a large experimental uncertainty
because the Doppler width was obtained from convolution
the spectral width of a transition with the laser width. The
line shape of the laser beam was not purely Gaussian. An-
ͩ
ͪ
Ϫ1
E
Ͼ25 130 cm
int
FIG. 6. Distributions of internal energy of fragment NO from nitric acid
2
photolyzed at 193 nm, obtained from subtracting the internal and transla-
7
tional energies of the fragment OH obtained from LIF spectrum of OH ͑—͒
and translational energy of products from molecular beam experiment ͑Ref.
4͒ ͑ؠؠؠؠ͒, respectively; see text. The position of the arrow shows the disso-
ciation threshold for formation of NOϩO.
FIG. 5. Boltzmann plots of NO X 2⌸ ␷ϭ0, 1, and 2 states rovibrational
energy of NO.
J. Chem. Phys., Vol. 107, No. 18, 8 November 1997
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
132.174.255.116 On: Tue, 02 Dec 2014 18:41:33

G.-H. Leu and I-C. Chen: Photodissociation of nitric acid
7227
FIG. 7. Plot of experimental distribution of nascent NO X 2⌸1/2 ␷ϭ0, 1,
and 2 from photolysis of nitric acid at 193 nm and calculated curves dis-
played in solid and dashed lines with the prior model using the distributions
FIG. 8. Plot of rotational distribution of secondary product NO X 2⌸1/2
ϭ2 from experimental data and according to the prior model with internal
␷
energy of NO given below: Solid line—distribution obtained from transla-
2
of internal energy of NO shown in Fig. 6 as ͑—͒ and ͑ؠؠؠؠ͒, respectively.
2
tional energy spectroscopy ͓shown in Fig. 6 as ͑ؠؠؠؠ͔͒, dashed line—
distribution obtained from LIF spectrum of OH ͓shown in Fig. 6 as ͑—͔͒,
Ϫ1
and dotted–dashed line—fixed available energy Eavailϭ6000 m . The cal-
culated curves were scaled on the ordinate axis to obtain a best fit to the
experimental distribution.
An oscillatory behavior was observed in the distribution
of rotational states of NO dissociated from jet-cooled NO
2
1
6–21,27,28,30–33
photolyzed with monochromatic light.
Reisler
1
6,17
et al.
showed that, on average, these distributions of NO
rotational states agree with the calculated results using a sta-
Another statistical model, separated statistical ensemble
theory ͑SSE͒, was used to calculate the vibrational popula-
tions because from the results of Reisler et al.17 the SSE
model agrees with experimental data better than the PST.
The SSE model assumes the vibrational adiabaticity during
the process of dissociation. We used a loose, bending fre-
quency, 100 cm suggested by Ionov et al. for the transi-
tion state of the dissociation channel to form NOϩO and the
frequency for stretching mode is similar to that of the NO
fragment. Both calculated values and experimental data are
listed in Table I. The bending mode cannot be treated as an
internal rotor in which the SSE model will predict equal
population for the vibrational level given a fixed photolysis
energy. This will overestimate the vibrational excitation dif-
ferent from the experimental observation.
tistical model, phase-space theory ͑PST͒; this theory is de-
1
7
scribed elsewhere. According to this model both total en-
ergy and angular momentum are conserved during
dissociation. In our experimental conditions, thermalized ni-
tric acid has a large average rotational angular momentum.
Although OH was not produced in highly rotationally ex-
cited states, the average rotational angular momentum of
Ϫ1
22
NO is expected to be large. In addition, given the broad
2
distribution of internal states of NO , the prior model is ex-
2
pected to give results similar to those of the PST. Hence, we
used the prior theory that includes no constraint on angular
momentum. In Fig. 7 we display a comparison of experimen-
tal and calculated prior distributions for three vibrational
states given internal energy distributions of NO shown in
2
Fig. 6. For the ␷ϭ0 level, the prior distribution using the
internal energy of NO2 calculated from the translational
spectroscopy agrees with the experimental data but not for
the other vibrational states. Energetically, the ␷ϭ2 state is
dissociated from NO with high internal energy. Hence, we
IV. DISCUSSION
2
compare the ␷ϭ2 distribution with the calculated distribu-
tion shown in Fig. 8 in which the calculated distribution was
scaled on the ordinate axis to obtain a best fit to the experi-
mental distribution. As in the case of singlet ketene
dissociation, at high available energy the prior model over-
estimates the NO population for the states of low J and un-
derestimates the population for states with high J. Similar
phenomenon is observed for the ␷ϭ1 distribution when
compared with the prior distribution. The prior distributions
predict the shape of distributions of rotational states of NO
incorrectly.
7
Yeh et al. discussed the production mechanism of NO
in the ground electronic state from nitric acid photolyzed at
193 nm and concluded that the main mechanism is from NO2
undergoing secondary dissociation. They reported that the
NO was produced within the pulse duration of the excimer
laser Ϸ20 ns. Ionov et al.22 And Miyawaki et al. have
measured directly the apparance of NO at the dissociation
threshold of NO to be in the picosecond range. Combining
all the results, the secondary dissociation rate of nitric acid is
expected to be fast. From the recent results of the transla-
3
7
25
2
J. Chem. Phys., Vol. 107, No. 18, 8 November 1997
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
132.174.255.116 On: Tue, 02 Dec 2014 18:41:33

7228
G.-H. Leu and I-C. Chen: Photodissociation of nitric acid
TABLE II. Summary of photodissociation work on NO2.
E
⍀
⌳
Preparation
avail
(
cmϪ1)
v
J
(2⌸1/2 /2⌸3/2
)
(AЈ/AЉ) conditions Reference
1
1
2
949
72.9% vϭ0,
2
Oscillatory
Oscillatory
Oscillatory
3.4
Ϸ1
Ϸ1
Ϸ1
Jet-cooled
Jet-cooled
Jet-cooled
17
17
17
7.1% vϭ1
998–2061 79.5% vϭ0,
0.5% vϭ1
200–3038 56%–58% vϭ0,
4%–42% vϭ1
3.4–3
2.7–2.4
2
4
3
7
360
222
57% vϭ0, 44% vϭ1 Oscillatory
44% vϭ0, 28% vϭ1 Oscillatory
Jet-cooled
Jet-cooled
31
32
1.9
Ϸ1
Ϸ1
16% vϭ2, 12% vϭ3
7
1
336
29% vϭ0, 28% vϭ1, Single peaked
1.3–1.9
298 K
11
14
1
2
8% vϭ2, 23% vϭ3,
.5% vϭ4
5 530
Inverted,
peaked at vϭ5
vϭ0–1, 3–8 single peaked,
vϭ2 double peaked
Ϸ1
1.6
Effusive jet
HNO3
This work 75% vϭ0, 19% vϭ1 vϭ0, 1, 2
1.9
Ϸ1
͑0–10 000͒ 7% vϭ2
Gaussian shaped
4
3
tional spectroscopy a minor channel of HONO (
˜
a AЉ) pro-
model incorrectly predicts the shape of the rotational distri-
butions. The rotational distributions of NO from thermalized
duction was observed. Larrieu et al.38 performed the quan-
tum chemical calculations and suggested that this excited
NO2 photolyzed at various wavelengths display singly
state correlates to fragment OHϩNO without an energy bar-
10,11,13
maxima.
The oscillatory phenomena in the rotational
3
rier. Hence, HONO (
˜
a AЉ) is expected to be unstable. Ken-
distributions were averaged from contribution of various dis-
3
9
ner et al. observed a long-lived intermediate with a lifetime
sociating NO states. According to the same explanation the
2
greater than 20 ␮s from nitric acid after photolysis at 193 nm
oscillatory behavior is expected to be smoothed for NO from
nitric acid. At a low available energy, PST was found to
predict the averaged rotational distribution correctly,
3
and assigned this intermediate to be HONO (
˜
a AЉ). Hence,
in the time scale of detection used in this work the amount of
HONO (a AЉ) dissociation is expected to be small. Two
˜
3
32
whereas not for large excess energy. Given the shape of the
experimental rotational distributions it leads the assumption
that at energy greater than the dissociation threshold the dis-
tribution of rotational states of NO is resulted from Franck–
photon processes like HONO absorbing another 193 nm pho-
ton will produce NO. In this work we kept the power of
photolysis laser low to be in the linear region to minimize the
NO production from these multiphoton processes. Very
small amount of NO may be produced via two photon pro-
cesses, but the major mechanism of NO production is from
the unstable NO2.
Comparison of this work with results obtained on NO2
dissociation under thermalized and jet-cooled conditions is
listed in Table II. Comparing the population ratios of
2
2
⌸1/2
/ ⌸ obtained at various photolytic energies, as the
3/2
available energy increases this ratio decreases and reaches
two. At Eavailϭ15 530 m the 2⌸1/2 / ⌸
Ϫ1
2
ratio reaches
3/2
1
4
unity. The production of spin–orbit states in our work was
unequal. This experimental ratio ⌸1 / ⌸ , ϳ1.9, agrees
2
2
/2
3/2
with the results of NO from photolysis of NO at low avail-
2
able energy and of theoretical prediction40 but shows no de-
pendence on the rotational quantum number. According to
our work, the two lambda doublet states have about equal
population similarly to other results except that at E
avail
ϭ15 530 m the population ratio AЈ/AЉ is 1.6.14
Ϫ1
Under the cold jet conditions of NO the populations of
2
product NO in the ground vibrational level lie between
Ϫ1 17
80%–56% at Eavailϭ1949–3360 cm
.
Our experimental
value falls within this range and agrees with those results
that the energy distribution in vibrational degrees of freedom
behaves statistically. However, our value agrees slightly bet-
ter with the prior prediction than that of the SSE but the prior
FIG. 9. Stick spectra of emission of the P branch of NO D 2⌺ ϪA 2⌺ϩ
obtained previously ͑Ref. 6͒ ͑solid stick͒ and simulated data using experi-
mental population of rotational states of NO ␷ϭ1 obtained in this work
͑shaded stick͒. The simulation was done as described previously. ͑Ref. 7͒.
ϩ
J. Chem. Phys., Vol. 107, No. 18, 8 November 1997
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
132.174.255.116 On: Tue, 02 Dec 2014 18:41:33

G.-H. Leu and I-C. Chen: Photodissociation of nitric acid
7229
1
Condon overlap of wave functions of the transition state of
A. A. Turnipseed, G. L. Vaghijani, J. E. Thompson, and A. R. Ravishan-
kara, J. Chem. Phys. 96, 5887 ͑1992͒.
A. Schiffman, D. D. Nelson, Jr., and D. J. Nesbitt, J. Chem. Phys. 198,
NO and the rotational state of NO. After summing over all
2
2
internal states of NO , the rotational distributions can be
2
6935 ͑1993͒.
4
1
Gaussian-like. This condition explains our observations
that the rotational distributions of NO can be fitted with
Gaussian functions. At an available energy much in excess
of the threshold of dissociation, the position of the transition
state can move toward that of the parent molecule and dy-
namics in the exit channel may determine the product distri-
butions. Similar results were obtained by Knepp et al.32 on
3
4
P. Felder, X. Yang, and J. R. Huber, Chem. Phys. Lett. 215, 221 ͑1993͒.
T. L. Myers, N. Forde, D. C. Kitchen, B. Hu, and L. J. Butler, J. Chem.
Phys. 107, 5361 ͑1997͒.
5
JANAF Thermochemical Tables, J. Phys. Chem. Ref. Data, Suppl. 14,
͑
1985͒.
6
7
U. Robra, H. Zacharias, and K. H. Welge, Z. Phys. D 16, 175 ͑1990͒.
P.-S. Yeh, G.-H. Leu, Y.-P. Lee, and I-C. Chen, J. Chem. Phys. 103, 4879
͑1995͒.
G.-H. Leu, C.-W. Hwang, and I-C. Chen, Chem. Phys. Lett. 257, 481
͑1996͒.
8
photolysis of NO at 309 nm. The SSE calculated with both
2
9
0
internal energy distribution of NO seems to slightly overes-
Y. Y. Bai and G. A. Segal, J. Chem. Phys. 92, 7479 ͑1990͒.
H. Zacharias, M. Geilhaupt. K. Meier, and K. H. Welge, J. Chem. Phys.
2
1
1
timate the vibrational excitation. These results imply that the
prior model without considering the adiabaticity of vibra-
74, 218 ͑1981͒.
1
H. Zacharias, K. Meier, and K. H. Welge, Energy Storage and Redistri-
bution in Molecules, edited by J. Hinze ͑Plenum, New York, 1983͒, p.
107.
M. Mons and I. Dimicoli, Chem. Phys. Lett. 131, 298 ͑1986͒.
M. Mons and I. Dimicoli, Chem. Phys. 130, 307 ͑1989͒.
J. McFarlane, J. C. Polanyi, and J. G. Shapter, J. Photochem. Photobiol.
A: Chem. 58, 139 ͑1991͒.
tional energy of NO works properly in this case.
4
Myers et al. reported that unstable NO which dissoci-
2
1
1
2
3
ates to NOϩO is in none of the lowest four electronic states
nor in a quartet state. When NO is excited with a photon in
2
14
the range 350–400 nm, the dissociating states are considered
2
2
15
to have characters of mixed B and A states, because of
J. Miyawaki, K. Yamanouchi, and S. Tsuchiya, Chem. Phys. Lett. 180,
2
1
287 ͑1991͒.
strong vibronic coupling between these two lowest electronic
states. From agreement between distributions of internal
16
D. C. Robie, M. Hunter, J. L. Bates, and H. Reisler, Chem. Phys. Lett.
93, 413 ͑1992͒.
1
states of NO from nitric acid with NO after photolysis, our
17
2
M. Hunter, S. A. Reid, D. C. Robie, and H. Reisler, J. Chem. Phys. 99,
1093 ͑1993͒.
S. A. Reid, J. T. Brandon, M. Hunter, and H. Reisler, J. Chem. Phys. 99,
data imply that the dissociating channel underwent fragmen-
18
tation via the ground electronic surface. The unstable NO is
2
4860 ͑1993͒.
expected to be in an electronic state that couples strongly to
the ground electronic surface. Hence, in the case of dissocia-
tion of nitric acid, even NO is a secondary product; the dis-
tributions of internal states can be understood from dynamics
obtained from the experiments using cold molecular jets.
We used the population obtained in this work to simulate
19
N. Changlong, L. Hua, and J. Pfab, J. Chem. Phys. 97, 7458 ͑1993͒.
J. Miyawaki, K. Yamanouchi, and S. Tsuchiya, J. Chem. Phys. 99, 254
͑1993͒.
V. P. Hradil, T. Suzuki, S. A. Hewitt, P. L. Houston, and B. J. Whitaker,
J. Chem. Phys. 99, 4455 ͑1993͒.
S. I. Ionov, G. A. Brucker, C. Jaques, Y. Chen, and C. Wittig, J. Chem.
Phys. 99, 3420 ͑1993͒.
S. J. Klippenstein and T. Radivoyevitch, J. Chem. Phys. 99, 3644 ͑1993͒.
C. Wittig and S. I. Ionov, J. Chem. Phys. 100, 4714 ͑1994͒.
J. Miyawaki, K. Yamanouchi, and S. Tsuchiya, J. Chem. Phys. 100, 4716
20
21
22
23
2
ϩ
2
ϩ
the emission spectrum of NO D ⌺ ϪA ⌺ obtained in
2
2
4
5
7
this group previously via time-resolved Fourier-transform
spectroscopy. The method of simulation was described
͑
1994͒.
7
26
previously. Both spectra agree well ͑Fig. 9͒ and confirm the
S. I. Ionov, H. F. Davis, K. Mikhaylichenko, L. Valachovic, R. A. Beau-
det, and C. Wittig, J. Chem. Phys. 101, 4809 ͑1994͒.
S. A. Reid and H. Reisler, J. Chem. Phys. 101, 5683 ͑1994͒.
S. A. Reid, D. C. Robie, and H. Reisler, J. Chem. Phys. 100, 4256 ͑1994͒.
S. A. Reid and H. Reisler, J. Chem. Phys. 101, 5683 ͑1994͒.
J. Miyawaki, K. Yamanouchi, and S. Tsuchiya, J. Chem. Phys. 101, 4505
results observed previously that the NO emission is the state
2
2
7
8
2
X ⌸ ␷ϭ1 state absorbing a 193 nm photon. In the experi-
mental spectrum, the low spectral intensities for NЈϭ32 and
29
30
2
ϩ
42
3
5 occur because these D ⌺ states are perturbed. The
͑
1994͒.
small oscillatory phenomenon observed previously in the
relative spectral intensities of even and odd parity states of
NO, are not confirmed in the present work. With LIF the
uncertainty is greater than that obtained using a time-
resolved Fourier-transform spectrometer in determining the
relative spectral intensity of rotational states. However, other
factors excluded from the simulation may contribute inten-
sity variation in even–odd parity states.7
31
J. A. Harrison, X. Yang, M. R o¨ sslein, P. Felder, and J. R. Huber, J. Phys.
Chem. 98, 12260 ͑1994͒.
P. T. Knepp, A. C. Terentis, and S. H. Kable, J. Chem. Phys. 103, 194
͑1995͒.
S. A. Reid, D. C. Robie, and H. Reisler, J. Phys. Chem. 100, 474 ͑1996͒.
C. Wittig, I. Nadler, H. Reisler, M. Noble, J. Catanzarite, and G.
Radhakrishnan, J. Chem. Phys. 83, 5581 ͑1985͒.
32
3
3
3
4
35
L. T. Earls, Phys. Rev. 48, 423 ͑1935͒.
36
H. Scheingraber and C. R. Vidal, J. Opt. Soc. B 2, 343 ͑1985͒.
E. A. Wade, H. Clauberg, S. K. Kim, A. Mellinger, and C. B. Moore, J.
Chem. Phys. A 101, 732 ͑1997͒.
3
3
3
7
8
9
C. Larrieu, A. DarGelos, and M. Chaillet, Chem. Phys. Lett. 91, 465
͑
1982͒.
R. D. Kenner, F. Rohrer, Th. Papenbrock, and F. Stuhl, J. Phys. Chem. 90,
294 ͑1986͒.
ACKNOWLEDGMENTS
1
4
4
0
1
H. Katagiri and S. Kato, J. Chem. Phys. 99, 8805 ͑1993͒.
R. Schinke, Photodissociation Dynamics ͑Cambridge University Press,
Cambridge, 1993͒.
The authors thank L. J. Butler for providing a preprint,
and National Science Council of the Republic of China
42
͑Contract No. NSC 86-2113-M-007-38͒ for support.
E. Miescher, J. Chem. Phys. 73, 3088 ͑1980͒.
J. Chem. Phys., Vol. 107, No. 18, 8 November 1997
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
132.174.255.116 On: Tue, 02 Dec 2014 18:41:33