Time-resolved vibrationally mediated photodissociation of HNO3: Watching vibrational energy flow

Article abstract of DOI:10.1063/1.474443

Ultrafast excitation of an O-H stretching vibrational followed by photodissociation of the energized molecules allows direct observation of the time for intramolecular energy redistribution in isolated nitric acid. We excite the first overtone of the O-H stretch vibration in HNO₃ with a 100 fs laser pulse. A second, time-delayed pulse preferentially photodissociates molecules having vibrational excitation in modes orthogonal to the O-H stretch. The photodissociation yield increases as a function of time because energy flows out of the initially excited O-H bond into other more efficiently dissociated vibrations. The single exponential time constant for this intramolecular vibrational relaxation is 12 ps, consistent with moderate coupling of the O-H stretch to states close in energy.

Full text of DOI:10.1063/1.474443

Time-resolved vibrationally mediated photodissociation of HNO 3 : Watching
vibrational energy flow
Dieter Bingemann, Michael P. Gorman, Andrew M. King, and F. Fleming Crim
Citation: The Journal of Chemical Physics 107, 661 (1997); doi: 10.1063/1.474443
View online: http://dx.doi.org/10.1063/1.474443
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/107/2?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Photodissociation of CH3CHO at 248 nm by time-resolved Fourier-transform infrared emission spectroscopy:
Verification of roaming and triple fragmentation
J. Chem. Phys. 140, 064313 (2014); 10.1063/1.4862266
Selective bond breaking mediated by state specific vibrational excitation in model HOD molecule through
optimized femtosecond IR pulse: A simulated annealing based approach
J. Chem. Phys. 139, 034310 (2013); 10.1063/1.4813127
Photodissociation of gaseous CH3COSH at 248 nm by time-resolved Fourier-transform infrared emission
spectroscopy: Observation of three dissociation channels
J. Chem. Phys. 138, 014302 (2013); 10.1063/1.4768872
Selective photodissociation of O–H and O–D bonds from ground vibrational state of HOD using simple UV pulses
J. Chem. Phys. 127, 024305 (2007); 10.1063/1.2751185
Effective chromophore potential, dissipative trajectories, and vibrational energy relaxation: Br 2 in Ar matrix
J. Chem. Phys. 123, 244506 (2005); 10.1063/1.2138028
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:
141.210.2.78 On: Wed, 26 Nov 2014 11:33:37

LETTERS TO THE EDITOR
The Letters to the Editor section is divided into four categories entitled Communications, Notes, Comments, and Errata.
Communications are limited to three and one half journal pages, and Notes, Comments, and Errata are limited to one and
three-fourths journal pages as described in the Announcement in the 1 July 1997 issue.
COMMUNICATIONS
Time-resolved vibrationally mediated photodissociation of HNO3:
Watching vibrational energy flow
Dieter Bingemann, Michael P. Gorman, Andrew M. King, and F. Fleming Crim
Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706
͑
Received 28 February 1997; accepted 1 May 1997͒
Ultrafast excitation of an O–H stretching vibrational followed by photodissociation of the energized
molecules allows direct observation of the time for intramolecular energy redistribution in isolated
nitric acid. We excite the first overtone of the O–H stretch vibration in HNO with a 100 fs laser
3
pulse. A second, time-delayed pulse preferentially photodissociates molecules having vibrational
excitation in modes orthogonal to the O–H stretch. The photodissociation yield increases as a
function of time because energy flows out of the initially excited O–H bond into other more
efficiently dissociated vibrations. The single exponential time constant for this intramolecular
vibrational relaxation is 12 ps, consistent with moderate coupling of the O–H stretch to states close
in energy. © 1997 American Institute of Physics. ͓S0021-9606͑97͒02426-4͔
INTRODUCTION
rectly the flow of energy from the initially excited vibration
to other vibrational modes of the molecule.
The redistribution of vibrational energy within a mol-
ecule is at the heart of reaction dynamics. Not surprisingly,
intramolecular vibrational energy redistribution ͑IVR͒ has at-
tracted many experimental studies, mostly in the frequency
VIBRATIONALLY MEDIATED PHOTODISSOCIATION
Vibrationally mediated photodissociation is a double
resonance technique: one laser pulse prepares a vibrationally
excited molecule in its electronic ground state, and a second
laser pulse, tuned to an electronic transition, photodissociates
the molecule. In an ideal vibrationally mediated photodisso-
ciation experiment ͑see Fig. 1͒, the photodissociation laser
pulse has too little energy to dissociate molecules with no
initial vibrational excitation. However, since the excited state
surface is repulsive and its potential energy decreases along
the dissociation coordinate, the energy difference between
the electronic ground state and the excited state decreases
along this coordinate. Therefore, a less energetic photolysis
photon can transfer vibrationally excited molecules that are
elongated along the dissociation coordinate to the excited
state surface. Even in less than ideal vibrationally mediated
1
domain. The absorption frequencies and intensities in high
resolution spectra of isolated molecules reveal the energies
of molecular eigenstates and coupling constants between
zero-order bright and dark states, respectively. The analysis
of such spectra allows one to infer the dynamics of an ini-
tially localized excitation, if it were prepared, by calculating
the time evolution of the zero-order bright state wave
2
function.
There is less work on IVR in the time domain. Typically
in time-domain experiments, a short laser pulse prepares a
vibrationally excited state in the first electronically excited
state S through an electronic transition to a Franck–Condon
1
bright state, and a second short pulse probes the IVR dynam-
3
4–7
ics by fluorescence depletion, pulsed field ionization,
or
photodissociation experiments, vibrational excitation pro-
other spectroscopic techniques.8 The molecules investigated
in such studies are fairly large. Smaller molecules with fewer
normal modes and lower densities of states potentially allow
a more specific modeling of their IVR dynamics. Further,
comparing the results for molecules of different size could
reveal trends that help establish a detailed molecular descrip-
tion of IVR.
,9
duces a substantial increase in dissociation yield.1
1–14
For the
same photolysis energy, the excitation of the vibration along
the dissociation coordinate improves the Franck–Condon
overlap of the ground state wave function with the wave-
function of the dissociative state.
Time domain vibrationally mediated photodissociation
experiments use short, broadband laser pulses for the vibra-
tional overtone excitation and photodissociation. Information
about the molecular dynamics comes from varying the time
delay between the two laser pulses while their frequencies
remain constant. The vibrational overtone excitation initially
prepares the zero-order bright state. Because the bright state
is not an eigenstate of the molecular Hamiltonian, it evolves
We have studied the IVR dynamics of a small molecule,
HNO , on its ground state surface in the time domain using
3
10
the technique of vibrationally mediated photodissociation
to monitor the vibrational energy redistribution. Vibra-
tionally mediated photodissociation allows one to specify the
nature of the initial vibrational excitation and to monitor di-
J. Chem. Phys. 107 (2), 8 July 1997
0021-9606/97/107(2)/661/4/$10.00
© 1997 American Institute of Physics
661
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:
141.210.2.78 On: Wed, 26 Nov 2014 11:33:37

662
Letters to the Editor
overtone absorption, and a second short laser pulse at 266
nm subsequently photolyzes the vibrationally excited mol-
ecules.
We generate the two pulses by nonlinear frequency con-
version in an optical parametric amplifier ͑OPA͒ and a fre-
quency tripling stage, respectively, starting with 80 fs
Ti:Sapphire laser pulses. The regeneratively amplified
Ti:Sapphire laser system ͑Clark MXR, CPA 1000͒ produces
8
1
00 nm pulses at a repetition rate of 1 kHz and an energy of
mJ. We frequency double 100 ␮J of the 800 nm pulses in
a 0.3 mm BBO crystal ͑type I, ␪ϭ29°͒ and mix the generated
00 nm pulses with leftover 800 nm light in a 0.2 mm BBO
4
crystal ͑type I, ␪ϭ42°͒ to make 1 ␮J pulse at 266 nm. The
remaining Ti:Sapphire output pumps a continuum seeded,
double pass OPA ͑Ref. 15͒ ͑5 mm BBO, type II, ␪ϭ27°͒,
which gives 90 ␮J pulses at 1.44 ␮m. These roughly 100 fs
Ϫ1
pulses have a bandwidth of approximately 280 cm ͑1.9
times the Fourier limit͒. A 500 mm lens focuses the 4 mm
diameter infrared beam to a diameter of about 250 ␮m in the
center of a fluorescence cell, where it overlaps with the ul-
traviolet pulses, which are focused down from a 1.5 mm
diameter with a 300 mm lens. The polarizations of the two
laser pulses are perpendicular to each other.
A photon counting photomultiplier ͑EMI 9813QB͒ de-
tects the fluorescence of the electronically excited NO prod-
2
uct in the range of 480–580 nm through a Schott GG475
cutoff filter. The signal passes to a gated photon counter
FIG. 1. Schematic drawing of ground and excited state potential energy
surfaces of nitric acid (HNO ) illustrating time-resolved vibrationally me-
3
͑
SR400͒ with a 1.2 ␮s wide gate that opens 40 ns after the
laser pulses arrive. An externally synchronized chopper
PAR, Super Chopper 300͒ blocks every other infrared pump
diated photodissociation. ␭ovt is the vibration overtone excitation laser
wavelength ͑1.44 ␮m͒, ␭diss is the photolysis wavelength ͑266 nm͒, and
⌬
t is the time delay between excitation pulse and photolysis pulse. In this
͑
schematic, the HO–NO stretch coordinate represents all vibrational modes
2
pulse for separate background detection. We average 5000
shots per time delay step and record up to about 5000 signal
counts on top of a background of about 15 000 counts. The
with a strong increase in photodissociation yield after vibrational excitation.
in time as vibrational energy leaks into dark background
states. The vibrational wave packet spreads, and probability
moves into vibrationally excited states other than the zero-
order bright state. Some of these states have a higher pho-
tolysis yield from the second laser pulse than the initially
prepared state. Varying the time delay between vibrational
overtone excitation laser and photolysis laser pulses directly
measures the time for energy to flow out of the zero-order
bright state.
HNO vapor flows through the room temperature fluores-
cence cell at a pressure of 250 mTorr and an approximate
3
rate of 200 scc/h. We prepare the anhydrous HNO by mix-
3
ing concentrated HNO and H SO in a 1:3 ratio and collect-
3
2
4
ing the HNO vapor in a trap at dry ice temperature.
3
RESULTS
Our primary observation is the time evolution of the
photodissociation yield as a function of time delay between
the vibrational excitation pulse and the photolysis pulse. Fig-
ure 2 shows the increase in the fluorescence from the NO2
product of the vibrationally mediated photodissociation sig-
EXPERIMENTAL DETAILS
Using nitric acid (HNO ) as a model system has several
3
advantages in a time-resolved vibrational mediated photodis-
sociation experiment. Its photodissociation yields a small
nal of HNO for the first 80 ps ͑circles͒. From 80 ps through
3
1 ns there is no statistically significant change in the signal
level. A single exponential rise with a time constant of ␶ϭ12
ps ͑solid line͒ fits these data well.
amount of electronically excited NO , which we can detect
2
using its spontaneous fluorescence. Furthermore, the vibra-
tionally mediated photodissociation of HNO is well studied
We obtained additional information about the initially
prepared zero-order state by changing the wavelength of the
vibrational overtone excitation laser while recording the vi-
brationally mediated photodissociation signal with the time
delay fixed at 50 ps. The result is plotted in Fig. 3 as filled
circles. For comparison, we also include a photoacoustic
3
in the frequency domain.14 Figure 1 schematically shows a
potential energy surface for the time-resolved vibrationally
mediated photodissociation of HNO . In this simple picture,
3
the N–O stretch coordinate represents modes with a strong
increase in photodissociation yield after vibrational excita-
tion. A 100 fs pulse at 1.44 ␮m prepares the zero-order
bright state, the O–H stretch (vϭ2) local mode, via its first
spectrum of HNO recorded with the same fs pulses ͑open
3
circles͒ and a conventional infrared absorption spectrum of
J. Chem. Phys., Vol. 107, No. 2, 8 July 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:
141.210.2.78 On: Wed, 26 Nov 2014 11:33:37

Letters to the Editor
663
energy of the first O–H stretch overtone. A simple analysis
of the data treats all states equally and uses Fermi’s Golden
Rule17 ͑energies in cm ),
Ϫ1
1
/␶ϭ4␲ 2c 2␳͑E͒
͉V͉
to relate the observed lifetime ␶ to the coupling strength
in the molecule. Using the measured time constant of
ϭ12 ps and the calculated state density ␳(E)Ϸ10 states/
͉V͉
␶
Ϫ1
cm , we find an average coupling matrix element of ͉V͉
Ϫ1
ϭ0.08 cm . For a state density of 10 vibrational states/
Ϫ1
cm , this matrix element mixes only a few dark states with
the zero-order bright state. In that situation, the vibrational
energy should beat back and forth between these few
coupled zero-order states, producing oscillations ͑‘‘quantum
FIG. 2. Increase in fluorescence signal from the NO product of the vibra-
2
tionally mediated photodissociation of nitric acid (HNO ) as a function of
3
2
time delay ⌬t between the vibrational excitation laser pulse and the photo-
dissociation laser pulse ͑filled circles͒. The solid line is a single exponential
rise with a time constant of 12 ps.
beats’’͒ in the time-resoled vibrationally mediated photodis-
sociation signal. Instead, we observe a simple exponential
rise. The excitation laser bandwidth in our experiment ͑280
Ϫ1
cm ), which spans the entire thermal rotational bandwidth
HNO vapor ͑dotted line͒. The widths of the action and pho-
3
Ϫ1
of the overtone absorption band ͑50 cm ), could explain the
toacoustic spectra reflect the bandwidth of our infrared laser
pulse. Both spectra have the same wavelength dependence,
consistent with our simple picture of the first overtone of the
O–H stretch vibration as the zero-order bright state in this
time-resolved vibrationally mediated photodissociation ex-
periment. The rise of the signal establishes a characteristic
time of 12 ps for relaxation of the initially excited O–H
absence of quantum beats. This pulse initially prepares an
ensemble of molecules with a thermal distribution of rota-
tional quantum numbers and two quanta of O–H stretch vi-
bration. Because the rotational constants depend on the vi-
brational state, the energy differences between the rovibronic
eigenstates that mix to form the zero-order bright state are
slightly different for each rotational state. Molecules with
different rotational quantum numbers will therefore beat at
different frequencies. In our ensemble with a broad distribu-
tion of rotational excitation, these beats add destructively,
and we observe an unstructured time evolution reflecting the
average rate.
Absorption of a linearly polarized laser pulse leads to a
preferred orientation of excited molecules. For the thermal
distribution of rotational states in our experiment, this initial
anisotropy decays rapidly compared to the observed signal
rise time, but an orientational recurrence should occur after
about 40 ps. The absence of such a recurrence in Fig. 2
suggests that orientational effects are unimportant in our ex-
periments.
stretch in a HNO molecule at an energy level of 7000
3
Ϫ1
cm
.
ANALYSIS
A count of vibrational states in HNO ͑Ref. 16͒ assum-
3
ing the vibrations are harmonic oscillators gives a vibrational
Ϫ1
Ϫ1
state density of ␳(E)Ϸ10 states/cm near the 7000 cm
Time-resolved IVR experiments performed thus far used
larger probe molecules such as stilbene, fluorene,3 or
8
6
p-difluorobenzene with considerably higher state densities
at much lower vibrational excitation energies. These mea-
surements found IVR time constants on the order of 10–20
Ϫ1
ps for excitation energies around 1500 cm at a density of
Ϫ1
states of several 100 states/cm . The much lower density of
Ϫ1
states ͑10 states/cm ͒ necessary to yield an IVR time con-
stant of 12 ps in HNO could be the consequence of larger
3
coupling constants in nitric acid. It could equally well reflect
the participation of only a portion of the available states in
the initial IVR. Such selective coupling of a subset of all the
available states is consistent with results from frequency do-
main IVR experiments such as those on acetylene
homologs.1
FIG. 3. Dependence of the vibrationally mediated photodissociation ͑VMP͒
signal on the vibrational excitation wavelength at a delay of 50 ps between
excitation and photolysis laser pulse ͑filled circles͒. A photoacoustic spec-
trum of nitric acid recorded with the same infrared laser pulses ͑open
circles͒ and a conventional infrared absorption spectrum ͑dashed line͒ are
plotted for comparison.
8–21
J. Chem. Phys., Vol. 107, No. 2, 8 July 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:
141.210.2.78 On: Wed, 26 Nov 2014 11:33:37

664
Letters to the Editor
ACKNOWLEDGMENTS
R. L. Vander Wal, J. L. Scott, F. F. Crim, K. Weide, and R. Schinke, ibid.
4, 3548 ͑1991͒; Y. Cohen, I. Bar, S. Rosenwaks, ibid. 102, 3612 ͑1995͒;
9
This research is funded by the Air Force Office of Sci-
entific Research. D. B. gratefully acknowledges a scholar-
ship by the German Science Foundation. We appreciate in-
formative discussions with Professor D. J. Nesbitt.
D. David, A. Strugano, I. Bar, and S. Rosenwaks, ibid. 98, 409 ͑1993͒.
T. M. Tichich, M. D. Likar, H.-R. D u¨ bal, L. J. Butler, and F. F. Crim, J.
Chem. Phys. 87, 5820 ͑1987͒.
S. S. Brown, R. B. Metz, H. L. Berghout, and F. F. Crim, J. Chem. Phys.
1
1
1
1
2
3
4
5
105, 6293 ͑1996͒.
A. Sinha, R. L. Vander Wal, and F. F. Crim, J. Chem. Phys. 91, 2929
͑1989͒.
M. Nisoli, S. De Silvestri, V. Magni, O. Svelto, R. Danelius, A. Piskars-
kas, G. Valiulis, and A. Varanavicius, Opt. Lett. 19, 1973 ͑1994͒ describe
a similar system.
F. M e´ len and M. Herman, J. Phys. Chem. Ref. Data 21, 831 ͑1992͒.
C. Cohen-Tannoudji, B. Diu, and F. Lalo e¨ , Quantum Mechanics ͑Wiley,
1
D. J. Nesbitt and R. W. Field, J. Phys. Chem. 100, 12 735 ͑1996͒, and
references therein.
P. R. Stannard and W. M. Gelbart, J. Phys. Chem. 85, 3592 ͑1981͒.
J. F. Kauffman, M. J. C oˆ te, P. G. Smith, and J. D. McDonald, J. Chem.
2
3
1
6
Phys. 90, 2874 ͑1989͒.
J. M. Smith, C. Lakshminarayan, and J. L. Knee, J. Chem. Phys. 93, 4475
4
17
͑
1990͒.
New York, 1977͒, Vol. 2, p. 1301.
5
6
7
8
9
0
1
18
19
J. M. Smith, X. Zhang, and J. L. Knee, J. Chem. Phys. 99, 2550 ͑1993͒.
X. Zhang, J. M. Smith, and J. L. Knee, J. Chem. Phys. 100, 2429 ͑1994͒.
J. M. Smith, X. Zhang, and J. L. Knee, J. Phys. Chem. 99, 1768 ͑1995͒.
P. M. Felker and A. H. Zewail, Adv. Chem. Phys. 70, 265 ͑1988͒.
P. G. Smith and J. D. McDonald, J. Chem. Phys. 96, 7344 ͑1992͒.
F. F. Crim, Annu. Rev. Phys. Chem. 44, 397 ͑1993͒.
A. McIllroy and D. J. Nesbitt, J. Chem. Phys. 92, 2229 ͑1990͒.
E. R. T. Kerstel, K. K. Lehmann, T. F. Mentel, B. H. Pate, and G. Scoles,
J. Phys. Chem. 92, 8282 ͑1991͒.
J. E. Gambogi, R. P. L’Esperance, K. K. Lehmann, B. H. Pate, and G.
Scoles, J. Phys. Chem. 98, 1112 ͑1993͒.
20
21
1
1
J. E. Gambogi, K. K. Lehmann, B. H. Pate, G. Scoles, and X. Yang, J.
Phys. Chem. 98, 1748 ͑1993͒.
D. H a¨ usler, P. Andresen, and R. Schinke, J. Chem. Phys. 87, 3949 ͑1987͒;
J. Chem. Phys., Vol. 107, No. 2, 8 July 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:
141.210.2.78 On: Wed, 26 Nov 2014 11:33:37