Photofragment translational spectroscopy of nitric acid at 248 nm with VUV photoionization detection of products

Article abstract of DOI:10.1016/j.cplett.2004.08.070

This study examines the 248-nm photodissociation of nitric acid (HNO ₃) and characterizes the translational energy distribution of the nascent photofragments. Photofragment translational spectroscopy with VUV photoionization detection evidenced one product channel, cleavage of HNO ₃ to form OH + NO₂, and established an upper limit on the contribution from the O + HONO formation channel of 3%. These data contribute an independent measurement to a literature debate regarding the branching between these two channels.

Full text of DOI:10.1016/j.cplett.2004.08.070

Chemical Physics Letters 397 (2004) 21–25
www.elsevier.com/locate/cplett
Photofragment translational spectroscopy of nitric acid at
48 nm with VUV photoionization detection of products
2
a,
M.J. Krisch , M.C. Reid , L.R. McCunn , L.J. Butler , J. Shu
a
a
a
b
*
a
Department of Chemistry, James Franck Institute, The University of Chicago, 5640 S. Ellis Avenue, Chicago, IL 60637, USA
b
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
Received 19 June 2004; in final form 19 June 2004
Available online 12 September 2004
Abstract
This study examines the 248-nm photodissociation of nitric acid (HNO
3
) and characterizes the translational energy distribution
of the nascent photofragments. Photofragment translational spectroscopy with VUV photoionization detection evidenced one prod-
uct channel, cleavage of HNO to form OH + NO , and established an upper limit on the contribution from the O + HONO for-
3
2
mation channel of 3%. These data contribute an independent measurement to a literature debate regarding the branching between
these two channels.
ꢀ
2004 Elsevier B.V. All rights reserved.
1
. Introduction
tion, the latter producing a shoulder on the strong
absorption band peaking near 270 nm (r = 1.64 ·
10
ꢀ
20
2
cm ) [4]. Both states result from exciting one elec-
Nitrogen oxides, molecules with complex electronic
structure, test our ability to predict product branching
and energy partitioning in photodissociation. Nitric acid
photodissociation is a useful model system for examin-
ing electronic nonadiabaticity in reactions where indi-
vidual orbital symmetry is not conserved along the
reaction coordinate [1,2]. Furthermore, several groups
tron from non-bonding O atom orbitals to the p*(NO2)
orbital [5,6].
Energetically, upon 248 nm excitation, HNO could
3
undergo three-bond cleavage reactions: OH + NO2,
O + HONO, or H + NO . [7] Previous work found the
3
H + NO channel to be negligible (U248(H) = 0.002) [8].
3
have used HNO as an OH precursor, generating inter-
3
While dissociation to OH + NO is clearly the dominant
2
est in the OH quantum yield [3]. This study addresses a
discrepancy in the literature regarding the product
branching upon HNO photodissociation at 248 nm.
product channel, the branching to the O + HONO chan-
nel is in debate. The only direct O quantum yield measure-
ment found U248(O) = 0.031 ± 0.010 [8]. Turnipseed et al.
used pulsed laser-induced fluorescence, calibrated to
H O , to obtain absolute OH quantum yields for HNO
3
of U248(OH) = 0.95 ± 0.09 and U193(OH) = 0.33 ± 0.06
[8]. Work utilizing electron bombardment ionization at
193 nm found U193(OH) = 0.33 ± 0.04 via photofragment
translational spectroscopy, in agreement with the Turnip-
seed study [1]. In contrast, Schiffman et al. [3] measured
3
The UV absorption spectrum of HNO exhibits two
3
broad peaks arising from electronic excitations centered
on the NO moiety. A strong absorption (r = 1.63 ·
2
2
2
ꢀ
17
2
cm ) [4] centered at 185 nm is due to excitation
1
0
1
0
to the bright 2 A state through a p ! p*(NO ) transi-
2
1
tion [5,6]. Excitations to the lower energy 2 A and
00
1
00
1
A states contribute to the longer wavelength absorp-
OH quantum yields from HNO with infrared flash ki-
3
*
netic spectroscopy, finding U248(OH)= 0.75 ± 0.10 and
U193(OH) = 0.47 ± 0.06; this low 248 nm OH quantum
Corresponding author. Fax: +1 773 702 5863.
E-mail address: ljb4@midway.uchicago.edu (L.J. Butler).
0
009-2614/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2004.08.070

22
M.J. Krisch et al. / Chemical Physics Letters 397 (2004) 21–25
yield suggests another significant product channel. Until
now, a photofragment translational spectroscopy meas-
urement at 248 nm was precluded by the lower absorption
The available energy (Eavl) following a photodissoci-
ation event is distributed amongst translational (ET),
rovibrational (E ), and electronic (E ) energy,
v,r
el
ꢀ20
2
cross section at this wavelength (r248 = 2.0 · 10
cm )
4,9]. This study takes advantage of the low background
E
avl ¼ Eparent þ hm ꢀ D
0
ðHO–NO Þ
2
[
¼
E þ E þ Eel;
ð1Þ
associated with VUV photoionization, coupled with
photofragment translational spectroscopy, to make an
independent determination of the OH quantum yield at
T
v;r
where D (HO–NO ) is the bond dissociation energy, hm
is the photon energy (114.9 kcal/mol), and Eparent is ta-
ken as ÆE æ = 0.79 kcal/mol at the nozzle temperature,
0
2
2
48 nm.
vib
as calculated from literature vibrational frequencies [13].
After secondary photodissociation of the NO2,
2
. Experimental
Eavl ¼ Eparent þ 2hm ꢀ D ðHO–NO Þ ꢀ D ðO–NOÞ
0
2
0
These photofragment translational spectroscopy
¼
Formula 1 and a D (HO–NO ) of 47.5 ± 0.5 kcal/mol
E
v;r þ Eel þ E
T
ðOH þ NO
2
Þ þ E
T
ðO þ NOÞ: ð2Þ
experiments used a crossed laser-molecular beam scat-
tering apparatus at Endstation 9.0.2.1 of the Advanced
Light Source at the Lawrence Berkeley National Labo-
ratory, described previously [10,11]. A pulsed molecular
0
2
[
7], give Eavl = 68.2 kcal/mol following the primary dis-
=
sociation. After secondary photodissociation, Eavl
1
7
11.2 kcal/mol from formula 2 and a D (O–NO) of
0
beam of 4% HNO was generated by seeding 100% fum-
3
1.84560 ± 0.00015 kcal/mol [14].
We assigned signal at m/e = 17(OH ) (Fig. 1) to OH
produced from HO–NO bond fission in HNO , and de-
ing HNO (Spectrum Laboratory Products) at 5.5 ꢁC in
3
+
helium to a total backing pressure of 475 Torr. The mix-
ture was expanded through a 0.5 mm diameter pulsed
nozzle at 83 ꢁC and photolyzed by a 248.5 nm KrF exci-
2
3
rived the recoil kinetic energy distribution, P(E ), from
T
ꢀ
1
forward convolution fitting of the data. The P(E ) is
T
mer laser. The photolysis beam was 225 mJ pulse , fo-
2
cused onto a 10.4-mm beam spot. The low HNO3
centered at 12 kcal/mol (Fig. 2). There was little or no
þ
evidence of products at m=e ¼ 46ðNO Þ, with an inte-
absorption cross-section required a high laser fluence
to see primary photoproducts, unavoidably causing sec-
ondary photodissociation of NO . The molecular beam
2
grated signal of 44.5 ± 33 counts after 300 000 laser
shots and 9 ± 19 counts after 150 000 laser shots at pho-
toionization energies of 10.7 and 10.2 eV, respectively.
2
typically peaked between 1488 and 1546 m/s, with a
FWHM between 17% and 18.8%, as measured with a
chopper wheel.
This suggests that NO may undergo secondary photo-
2
dissociation and/or daughter ion fragmentation within
+
the ionizer. Indeed, the m/e = 16(O ) data shows evi-
The velocity of the neutral photofragments is ob-
tained from the time to travel 15.1 cm to the detector.
There, they are ionized by tunable [12] vacuum-ultra-
violet (VUV) synchrotron light and filtered by mass
with a 2.1-MHz quadrupole (with a mass-dependent
resolution from 0.3 to 0.45 amu) and a Daly detector.
The time-of-flight constant, used to correct for the ion
dence of both these processes.
We could satisfactorily fit the m/e = 16(O ) data
+
without assuming that HNO + hm ! O + HONO
3
occurred (Fig. 3a). The slower peak is momentum-
matched to the lower recoil kinetic energy portion of
1
/2
transit time, was 5.7 ls/amu . Higher harmonics of
the synchrotron light were attenuated by an argon
gas filter and, at wavelengths below 10.8 eV, a
MgF2 filter. At masses where this was insufficient,
shot-to-shot subtraction of a laser-off, beam-on back-
ground was used.
m/e = 17, (OH+)
1
5o, 14.5 eV
3
. Results
The data indicate that dissociation of HNO into OH
3
and NO is the dominant primary dissociation channel
2
40
60
80
100
120
140
following 248.5 nm excitation of HNO ; we estimate that
3
Time of Flight, t (µs)
the O + HONO channel has no more than a 3% quantum
yield. We looked for dissociation products which
might give parent or daughter ion signal at m=e ¼
+
Fig. 1. TOF spectrum at m/e = 17(OH ), signal averaged for 600 000
laser shots at a 15ꢁ source angle and a 14.5-eV photoionization energy.
We obtained the forward convolution fit to these data, shown with the
þ
þ
þ
þ
þ
2
1
6ðO Þ; 17ðOH Þ; 30ðNO Þ; 46ðNO Þ; 47ðHNO Þ;
2
T
solid line, from the P(E ) in Fig. 2. Open circles show experimental
þ
and m=e ¼ 63ðHNO ; parent; to detect clustersÞ.
data.
3

M.J. Krisch et al. / Chemical Physics Letters 397 (2004) 21–25
23
m/e = 16, (O +)
5o, 15 eV
0.1
(a)
1
0
0
0
0
.08
.06
.04
.02
0
NO + hν
O + NO
2
HNO + hν
OH + NO2
3
0
10
20
30
40
E (kcal/mole)
m/e = 16, (O +)
T
(b)
1
5o, 15 eV
Fig. 2. The P(E
T
2
) for OH–NO bond fission, derived from the forward
+
NO + hν
O + NO
convolution fit of the m/e = 17(OH ) spectra (Fig. 1), is given by the
solid line. The small-dashed line shows the truncated P(E ) used in the
forward convolution fit of m/e = 16(O ) signal as dissociative ioniza-
tion of high Eint NO photofragments (Fig. 3).
2
T
+
HNO
+ hν
O + HONO
3
2
HNO + hν OH + NO2
3
the data at m/e = 17, consistent with dissociative ioniza-
tion of NO , and is fit with the P(E ) in Fig. 2, truncated
at 12 kcal/mol. This cut-off, implying that only the
2
T
higher internal energy NO photofragments dissocia-
2
+
tively ionize to O , corresponds to ꢁ2.3 eV of NO
0
40
80
120
160
200
240
280
2
internal energy (see Section 4). Combined with the
Time of flight, (µs)
1
energy to reach the O ion, consistent with the appear-
5-eV photoionization energy, this provides 17.3 eV of
+
+
Fig. 3. TOF spectrum at m/e = 16(O ) at a 15ꢁ source angle and a
1
5-eV photoionization energy, signal averaged for 789 000 laser shots.
Open circles show experimental data. (a) The forward convolution fit
shown in the small-dashed line, calculated from the associated P(E ) in
Fig. 2, corresponds to dissociative ionization of high Eint NO
photofragments. The forward convolution fit shown in the long-
dashed line corresponds to secondary photodissociation of NO to
form NO + O; it was obtained from the P(E ) in Fig. 4. (b) Similar to
(a) but adding an O + HONO channel, shown by the gray line, from a
P(E ) peaked at 3.0 kcal/mol, with a FWHM of 1.7 kcal/mol. We
deleted 3% of the low kinetic energy side of the NO
+
ance energy of O from NO , reported as 16.8–17.6 eV [15].
2
The P(E ) generated to fit the faster peak in the
T
T
m/e = 16 data, assigned to secondary photodissociation
of NO product, peaks at 17 kcal/mol but extends to
2
2
2
74 kcal/mol (Fig. 4). This signal is momentum-matched
to the m/e = 30 data (Fig. 5), as NO and O are sibling
T
co-fragments of NO photodissociation. Thus, the same
2
T
2
! NO + OP(E
T
)
secondary P(E ) fits the data in both spectra. Although
T
+
in Fig. 4 to attribute the maximum possible O signal to O + HONO.
+
The appearance energy of NO from HONO is unknown, so this fit
+
assumes that all the HONO produced can give NO daughter
the fit shown for the m/e = 30 data includes no contribu-
+
tion from NO dissociatively ionizing to NO in the
2
mass spectrometer, signal from 90 to 115 ls may be
attributed to this source. Including this contribution
fragments.
does not substantially affect the P(E ) for the secondary
T
dissociation channel. Although the primary dissociation
channels were studied with an unpolarized laser, so the
derived P(E ) does not depend on any anisotropy in
0
0
0
.032
.028
.024
0.02
.016
.012
.008
.004
0
T
the dissociation, in the fitting of the secondary channels
we assumed the angular distribution was isotropoic.
We searched for evidence of the O + HONO product
0
0
0
0
+
channel. No m/e = 47(HONO ) signal appeared above
the background with photoionization at 11.5, 13.3,
1
4.3, and 16.3 eV. In the two spectra taken at 15.3 eV,
integration of the m/e = 47 data over the time range of
the slow NO peak (see below) resulted in a tiny inte-
grated signal of 69 ± 38 counts after 250 000 laser shots
and 94 ± 32 counts after 125 000 laser shots. The
HONO photoionization energy was recently measured
as 10.97 ± 0.03 eV [16], in agreement with computa-
tional results [17]. HONO gave strong daughter ions at
0
10
20
30
40
50
60
70
80
ET (kcal/mole)
Fig. 4. The P(E
T
) for secondary photodissociation of NO
2
, derived
+
from the forward convolution fit of the m/e = 16(OH ) spectra (Fig. 3),
is given by the long-dashed line.

24
M.J. Krisch et al. / Chemical Physics Letters 397 (2004) 21–25
yield of OH from 248 nm photolysis of HNO is 0.97–
3
m/e = 30, (NO+)
(
a)
1
.0, assuming unit quantum yield for photodissociation.
1
5o, 11 eV
NO + hν O + NO
4. Discussion
2
Photofragment translational spectra indicate that
photodissociation of HNO at 248 nm occurs primarily
3
through the OH + NO2 channel. As mentioned, this
study was spurred by conflicting previous measurements
of the quantum yield of OH at 248 nm; our results are in
closer agreement with Turnipseed et al. [8] than Schiff-
m/e = 30, (NO+)
(
b)
NO + hν
2
man et al. [3]. In a previous HNO photodissociation
3
o
O + NO
15 , 11 eV
study at 193 nm, this laboratory also arrived at a result
consistent with that of Turnipseed et al. [1]. The differ-
ence in U248(OH) values for HNO in the two spectro-
3
HNO + hν
O + HONO
3
scopic studies arises from their disagreement over
U(OH) for H O at 248 nm. Schiffman et al. [3] meas-
2
2
ured the quantum yield from H O as U248(OH) =
2
2
1
.58 ± 0.23 and U193(OH) = 1.22 ± 0.13 while Turnip-
seed et al. used U248(OH) = 2.00 ± 0.05 [18] and
U193(OH) = 1.51 ± 0.18 [19]. Despite extensive investiga-
tion of H O , no current U(OH) measurements resolve
2
2
this discrepancy, as reviewed elsewhere [3,8]. If one ad-
justs the branching to OH + NO at 248 nm reported
50
100
150
200
250
Time of Flight, t(µs)
2
by Shiffman et al. by using U248(OH) = 2.0 for 248 nm
+
Fig. 5. TOF spectrum at m/e = 30(NO ) at a 15ꢁ source angle and a
H O photodissociation, their corrected quantum yield
2
2
1
1-eV photoionization energy, averaged for 700 000 laser shots.
+
Potential contribution from dissociative ionization of NO to NO is
2
is 0.95, in agreement (within stated error bars) with
our results here and those of Turnipseed et al. A similar
correction cannot explain the disagreement at 193 nm.
This study also characterized the translational and
not shown (see Section 3). (a) The forward convolution fit shown with
the long-dashed line corresponds to the secondary photodissociation
T
P(E ) from Fig. 4. Open circles indicate experimental data points. (b)
Fit obtained when a 3% branching to O + HONO is allowed, as
discussed for Fig. 3b. The gray line shows dissociative ionization of
+
HONO to NO .
internal energy distribution of the OH + NO products.
2
The P(E ) describing this channel peaks well away from
T
zero at 12 kcal/mol. Myers et al. [1] constructed a re-
stricted adiabatic correlation diagram linking HNO3
electronic states to likely NO products. This treatment
+
+
+
þ
2
OH , O , NO , and NO in a study of 193 nm HNO
3
2
photolysis with electron bombardment detection; [1] this
48 nm study showed little evidence of HONO at parent
or any daughter ions following VUV photoionization.
[1,2] assumes a propensity towards electronic rearrange-
ments that occur primarily within the NO moiety and
2
2
1
suggests that the 1 A state, which is likely to be ac-
00
2
cessed at 248 nm, correlates to OH + NO (1 A ). How-
The m/e = 30 data showed a small apparent peak near
1
2
2
30 ls that was not fit by the P(E ) generated by the
ever, it is also energetically possible to produce
T
2
1
00
NO (1 B ) via excitation to the 2 A state. From calcu-
2 1
m/e = 16 data. The peak is too fast to obviously attri-
bute to dissociating dimers; the absence of signal at
the parent mass also suggests that no clusters were pre-
lated vertical excitations, neither state [1] should gener-
ate a P(E ) peaked at 12 kcal/mol. Additionally,
T
+
sent. The NO signal was too slow to assign to daughter
þ
threshold energies for these states [20] indicate that they
are energetically inaccessible at the high translational
energy edge of the P(E ); thus, some ground state
ion fragmentation from m=e ¼ 46ðNO Þ, so we consid-
2
ered that the slow peak might be dissociative ionization
of HONO. To ascertain the maximum possible branch-
ing to O + HONO, we used the P(E ) derived from the
T
NO2 is produced. We lack the sensitivity to assign
NO electronic states based on structure in the P(E ),
T
2
T
+
small peak at m/e = 30, assuming it was from the NO
daughter of HONO, to determine the expected arrival
times of the momentum-matched O co-fragments in a
as was possible at 193 nm.
We can compare the result here to a P(E ) for the flu-
int
orescing portion of the NO photoproduct from HNO
2
3
+
modified m/e = 16(O ) fit (Fig. 3b). From this fit, which
photolysis at 248 nm, measured by Miller and Johnson
[9]. Although broader in shape than our distribution,
is not as good as that with no O + HONO contribution,
an upper limit of 3% on the contribution from O +
HONO was calculated. We conclude that the quantum
that P(E ) is also unimodal, peaking at ꢁ54 kcal/mol
int
NO internal energy. For this to be consistent with the
2

M.J. Krisch et al. / Chemical Physics Letters 397 (2004) 21–25
25
current study, the OH fragment must have little internal
excitation. Photodissociation of HNO at wavelengths of
[2] N.R. Forde, T.L. Myers, L.J. Butler, Faraday Discuss. 108 (1997)
21.
3] A. Schiffman, D.D. Nelson Jr., D.J. Nesbitt, J. Chem. Phys. 98
1993) 6935, and references therein.
4] F. Biaume, J. Photochem. 2 (1973) 139.
2
3
[
[
1
93 [21], 280 [22], 266 [23], and 241 nm [24] produced OH
(
excitation, of only 3.3, 3.7, 3.7 and 3.0 kcal/mol, respec-
tively. If 3 kcal/mol goes into OH, the distribution re-
ported here would peak at Eint (NO ) = 53 kcal/mol,
[5] Y.Y. Bai, G.A. Segal, J. Chem. Phys. 92 (1990) 7479.
[6] A.M. Gra n˜ a, T.J. Lee, M. Head-Gordon, J. Phys. Chem. 99
2
(
7] J. Chase, W. Malcolm (Eds.), NIST-JANAF Thermochem-
1995) 3493.
similar to the Miller and Johnson study.
If the small peak at m/e = 30 corresponds to
O + HONO, the only energetically accessible product
[
ical Tables, American Chemical Society, Washington, DC,
998.
1
1
channel is the spin-forbidden HONO(X A ) + O( P)
0
3
[8] A.A. Turnipseed, G.L. Vaghjiani, J.E. Thompson, A.R. Ravish-
ankara, J. Chem. Phys 96 (1992) 5887.
3
25,26]. Turnipseed et al. observed O( P) atoms follow-
[
ing 248 nm HNO photolysis. Additionally, one channel
[9] C.E. Miller, H.S. Johnston, J. Phys. Chem. 97 (1993) 9924.
3
[
10] P.A. Heimann, M. Koike, C.W. Hsu, D. Blank, X.M. Yang,
A.G. Suits, Y.T. Lee, M. Evans, C.Y. Ng, C. Flaim, H.A.
Padmore, Rev. Sci. Instrum. 68 (1997) 1945.
in the 193 nm photolysis of HNO was assigned to the
3
3
1
0
spin-forbidden O( P) + HONO(X A ) by Li et al. [27]
At higher energies, the spin-conserving product combi-
[11] X. Yang, J. Lin, Y.T. Lee, D.A. Blank, A.G. Suits, A.M. Wodtke,
Rev. Sci. Instrum. 68 (1997) 3317.
1
1
nation would be O( D) + HONO(X A ).
0
[
12] The wavelength of the synchrotron light was tuned through
changes in the gap of a U10 undulator, as calibrated in
unpublished work by D. Peterka and M. Ahmed (2002).
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Phys. Chem. Ref. Data 32 (2003) 879.
The crucial atmospheric role of HNO3 motivates
many studies [28]. This work shows that excitation to
1
the 1 A state in the small shoulder near 270 nm results
00
[
in a near unit OH + NO quantum yield. The solar acti-
2
nic flux in the stratosphere between 230 and 275 nm is
low, resulting in only the red side of this absorption
shoulder impacting stratospheric photochemistry on
[14] R. Jost, J. Nygøard, A. Pasinski, A. Delon, J. Chem. Phys. 105
1996) 1287.
(
[
[
[
15] J.W. Au, C.E. Brion, Chem. Phys. 218 (1997) 109, and references
therein.
1
0
par with the 200 nm absorption to the 2 A state, as
quantified by modeled HNO photolysis rates [4,29].
16] C.A. Taatjes, D.L. Osborn, T.A. Cool, K. Nakajima, Chem.
Phys. Lett. 394 (2004) 19.
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3
(
1999) 147.
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96, The error bars of the value in this reference are different from
Acknowledgements
9
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The National Science Foundation supported this
work under Grant No. CHE-0109588 (L.J.B.). M.J.K,
M.C.R, and L.R.M. acknowledge a National Science
Foundation Graduate Research Fellowship, Summer
Environmental Research Award, and GAANN fellow-
ship, respectively, for salary support. The Advanced
Light Source is supported by the Director, Office of Sci-
ence, Office of Basic Energy Sciences, Materials Sciences
Division, of the US Department of Energy under Con-
tract No. DE-AC03-76SF00098 at Lawrence Berkeley
National Laboratory. The Chemical Dynamics Beamline
is supported by the Director, Office of Science, Office of
Basic Energy Sciences, Chemical Sciences Division of the
US Department of Energy under the same contract.
[
[
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