J . Phys. Chem. 1990, 94, 4391-4393
Bimolecular Reaction of a Local Mode Vibrational State: H OH(V,J) H2
+
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4391
+ H20(4vOH)
Amitabha Sinha Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 (Received: February 12, 1990)
Vibrational overtone excitation of water to the 104)- local mode state is used to initiate the endothermic bimolecular reaction of hydrogen atoms with water molecules. A probe of the nascent OH product using laser-induced fluorescence indicates that this fragment receives less than 10% of the -9000 cm-l of available energy consistent with a spectator model. Most of the OH fragments (-98%) are formed in the vibrational ground state with an average of 360 cm-l of rotational energy and -350 cm-' of translational excitation. The present results on the H + H 2 0 system demonstrate the feasibility of using vibrational overtone excitation to promote bimolecular reactions involving reagents containing light hydrogen atom stretching motion and, thus, investigate the influence of local mode excitation on chemical reactivity.
Introduction Understanding the role vibrational energy plays in promoting bimolecular reactions is both of practical and fundamental importance. The practical utility of such studies is largely a result of the ubiquitous nature of vibrationally excited species which play a central role in many chemical processes such as combustion, atmospheric, and plasma chemistry. From a theoretical perspective, understanding how various forms of reagent energy affect reaction dynamics and correlating these findings with the topology of the potential energy surface have long been of interest to chemists.' As a result, there have been a number of experimental studies investigating the effect of reagent vibration on reaction dynamics.* However, the difficulty of cleanly preparing reactants with high levels of vibrational excitation on the ground electronic surface has restricted most of these studies to chemical systems containing relatively low levels of vibrational excitation and involving primarily diatomic reagent^.^ For molecules containing light atom stretching motions, such as those involving C-H, 0-H, and N-H bonds, vibrational overtone excitation has proven to be a useful technique for depositing energy into the vibrational degrees of f r e e d ~ m . In ~ this Letter we present the first results on the vibrational overtone initiated bimolecular reaction of water molecules with hydrogen atoms AHoo = 62 kJ/mol H HZO(4vOH) OH(u,J) H2
+
-
+
(1)
Figure 1 illustrates the energetics for this process. The H + H 2 0 reaction is 62 kJ/mol endoergic5 and has a calculated barrier of 90 kJ/mole6 In the present experiment we overcome this activation energy by exciting the third overtone of the 0-H stretching vibration in water, more specifically the 104)- vibrational state occurring in the region of 13 900 cm-'.'+* The nascent product state distribution of the resulting O H fragments is probed by laser-induced fluorescence. The large rotational constant of the water molecule along with its relatively low density of states at this energy allows us to selectively excite single rotational transitions of the 104)- band and, thus, investigate the reaction in a highly state specific manner. The present experiments involving vibrational overtone excitation complement the work of Klein( I ) Levine, R. D.; Bernstein, R. B. Molecular Reaction Dynamics and Chemical Reactioiry; Oxford University Press: Oxford, UK, 1987. (2) Kneba, M.; Wolfrum, J. Annu. Rev. Phys. Chem. 1980,3/,47. Moore, C. B.;Smith, 1. W. M. Faraday Discuss. Chem. Soc. 1979, 67, 146. (3) Wolfrum, J. In Reactions of Small Transient Species;Academic Press: London, 1983. (4) Crim, F. F. Annu. Reo. Phys. Chem. 1984, 35, 657. (5) Baulch, D. L.; Cox, R. A.; Hampson, R. F.; Kerr, J. A,; Troe, J.; Watson, R. T. J. Phys. Chem. Ref. Data 1984, 13, 1259. (6) Schatz, G. C.; Elgersma, H.Chem. Phys. Lerr. 1980, 73, 21. Walch, S. P.; Dunning, T. H. J . Chem. Phys. 1980, 72, 1303. (7) Grossman, B. E.; Browell, E. V. J . Mol. Specfrosc. 1989, 136, 264. (8) Child, M. S.; Lawton, R. T. Chem. Phys. Left. 1982,87, 217. Child, M.S. Arc. Chem. Res. 1985, 18.45. Child, M. S.; Halonen, L. Ado. Chem. Phys. 1984,57, I . Watson, 1. A.; Henry, B. R.; Ross, 1. G . Spectrochim. Acfa, Parr A 1981. 37A, 8 5 7 .
0022-3654/90/2094-4391$02.50/0
ermanns and W o l f r ~ mwho , ~ have used hot hydrogen atoms to study the translational energy dependence of this reaction. The H H 2 0 reaction is particularly interesting for investigating the influence of reagent vibrational excitation because quasiclassical trajectory studies'O indicate that this system can potentially exhibit mode-specific behavior. In addition, due to the change in vibrational character of the O H stretching states of water from normal mode to local mode with increasing energy8 and the slow rate of intramolecular energy redistribution in water, the H + H 2 0 reaction system provides an unique opportunity to study the influence of local mode excitation on chemical reactivity. Because local mode excitations are characterized by the vibrational energy being predominantly confined to the light hydrogen atom stretching motion, direct reactions involving hydrogen atom abstraction should be particularly sensitive to this type. of excitation, since it is for this class of chemical reactions that the local mode excitation coordinate most closely resembles the reaction coordinate. In these first measurements, we illustrate the technique and present preliminary results on the nascent O H product state distribution resulting from the reaction.
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Experimental Approach The experimental apparatus uses the standard discharge-flow technique to generate radicals, vibrational overtone excitation to energize the reactant molecule, and laser-induced fluorescence to detect the product. The reaction chamber consists of a glass cell equipped with a viewing window for monitoring laser-induced fluorescence, two Brewster angled side arms for introducing laser light, and several inlet ports for introducing reagents. The reagents are continuously flowed through the cell, and the total pressure is typically maintained between 80 and 90 mTorr by using a partially throttled mechanical pump. The interior walls of the cell and the glass tubing bringing in the radical reagent from the discharge zone are coated with halocarbon wax in order to minimize loss of radicals due to heterogeneous chemistry. We generate hydrogen atoms by flowing a mixture of H2and He through a microwave discharge (Raytheon 30 W, 2450 MHz) located on the side of the cell. Water vapor is introduced through a separate port, and its partial pressure is regulated between 15 and 20 mTorr. The partial pressures of H2 and He in the cell are 20 and 50 mTorr, respectively. A Nd:YAG laser pumped dye laser provides 50-mJ pulses in the region of 720 nm for vibrational overtone excitation. The diameter of the excitation beam is reduced to 2 mm at the center of the cell using a 50-cm focal length lens. In order to identify the various rovibrational transitions of water, a small portion (-4%) of the vibrational overtone excitation beam is split off and sent into a photoacoustic cell containing water vapor. The photoacoustic signal is continuously monitored and provides a con(9) Kleinermanns, K.; Wolfrum, J. Appl. Phys. B 1984, 34. 5. (IO) Schatz, G. C.; Colton, M. C.; Grant, J. L. J . Phys. Chem. 1984.88, 2971.
0 1990 American Chemical Society
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Letters
4392 The Journal of Physical Chemistry, Vol. 94, No. 11, 1990 H
+ H20(4Vo~)
OH + H2
120
/03>'
L -
x
F W w
6
P-
102>-
4
t
1
I
ILiS
h
P
al c w
u H
I
BO
7220
WAVELENGTP
40
2L0
Photoacoustic Spectrum
1,
H,O
Figure 1. Schematic potential energy curve illustrating the energetics of the H + H20reaction and the relevant vibrational states for the reactant and products.
venient diagnostic of the vibrational excitation process. For most of the measurements, an intracavity etalon is inserted into the vibrational excitation laser to reduce its bandwidth (-0.05 cm-I) and thus provide more efficient excitation of the narrow water transitions. We probe the OH product state distribution by partially saturated laser-induced fluorescence on the A X transition using frequency-doubled light around 308 nm from a XeCl excimer laser pumped dye laser. The probe laser light counterpropagates relative to that from the overtone laser with the delay between the two lasers set between 50 and 1000 ns depending on the particular measurement. For determination of the nascent product state distribution the delay is typically set at 100 ns. A photomultiplier (EM1 9635QB)views the OH fluorescence excited by the probe laser through an f / l optical system, and a gated integrator captures the resulting signal. A laboratory computer accumulates the fluorescence signal along with those from the photoacoustic cell and photodiodes that monitor the energy of each laser pulse. Two colored glass filters (Corning 7-54) and baffles in the side arms leading into the chamber reduce scattered light. We observe that the O H signal from the reaction disappears when either the microwave discharge is turned off or when pure He is discharged (Le., no H2 flow), consistent with the reaction being due to hydrogen atoms and not He metastables."
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Results and Discussion The energy level diagram in Figure 1 illustrates the thermochemistry of the H + H20reaction. Because the reaction is endoergic with a classical barrier to reaction of 90 kJ/mol, energy must be added to the reagents to initiate reaction. In the present experiment we provide the necessary energy by exciting to the 104)- local mode vibrational state of water in the region of 13 900 cm-1.8 Because we excite single rovibrational transitions in water, we are able to prepare vibrational states of the reagent that maintain their integrity on a time scale comparable to the gas kinetic collision frequency. It is this ability to prepare a single eigenstate of the polyatomic reagent that allows us to investigate the chemical reactivity of the initially prepared local mode state (1 1) The experimental configuration is such that species generated in the microwave dischargedo not have a straight path into the reaction cell. A bend in the tubing carrying radicals from the discharge zone to the reaction cell ensures that molecules undergo many collisions with the walls of the tubing and with each other before reaching the reaction zone. Thus, electronically excited species formed in the discharge are quenched before they reach the reaction cell. Vibrationally excited H2, which are also formed by the discharge, do not have enough energy to dissociate H20(4voH)and thus do not present a problem in this study.
(8)
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Figure 2. Vibrational overtone excitation spectra of water in the region of the 104)-band: (a) action spectrum obtained by probing the Pl(2) transition of the OH product from the H H20(104)-)reaction. Total gas pressure is 80 mTorr and the delay between the vibrational excitation laser and the probe laser light is 100 ns. (b) Room temperature pho-
+
toacoustic spectrum.
without interference from intramolecular energy redistribution. For the H + H20system we observe no reaction products for vibrationall unexcited water but find that reaction occurs within a few gas kinetic collisions for H20(104)-).12 The designation 104)- indicates that in the local mode basis, which is most appropriate for describing highly excited stretching states of water, the major basis function contributing to the description of the excited vibrational state is the antisymmetric linear combination involving the 104) and 140) functions (Le., qnb= 2-1/2[104) 140)]). In the local mode formalism the functions 104) and 140) designate vibrational states of water containing zero quanta of excitation in one 0-H bond and four quanta in the other.I3 The bending mode is not excited and contains zero quanta. Two types of measurements are important for the work described here. In the first, monitoring a particular OH rovibrational state while varying the vibrational overtone excitation laser wavelength generates an action spectrum of those water molecules that react to produce O H fragments in the interrogated quantum state. In the second, varying the probe laser wavelength with the vibrational overtone excitation laser tuned to a particular rovibrational transition of water yields an OH laser induced fluorescence excitation spectrum from which we extract the nascent distribution of products among their quantum states. Figure 2 compares the vibrational overtone action spectrum of the 104)- band with a photoacoustic spectrum over the same region. The action spectrum, shown in Figure 2a, comes from fixing the probe laser on the P,(2) rotational transition of the OH(0,O) band and monitoring the total laser induced fluorescence signal as a function of the vibrational overtone excitation laser frequency. In principle the intensities of the various rotational transitions occurring in the action spectrum depend not only on the absorption cross section for vibrational overtone excitation but also on the cross section for reaction. Thus, a comparison of relative spectral intensities occurring in a rotationally unrelaxed action spectrum with those from a photoacoustic measurement can reveal information on the rotational-state dependence of the (12) A very rough estimate for the rate of this reaction is made by determining the yield of OH product at short reaction time and using the relationship A[OH]/Af N k[H20(4voH)][H]. At a delay of 100 ns between the vibrational overtone excitation laser and the probe laser, the OH signal e x d s our detection limit (- 1 X 106 molecules cm3state-') by about a factor of 20. Using this as an estimate of the OH concentration, a time delay At = 100 ns, an estimated microwave discharge dissociation efficiency of -25% for H2.and the excitation efficiency for vibrational overtone excitation' to the 4uOH of we find a rate constant of k 1O-l' cm' molecule-l s-I. For an estimate of the dissociation efficiency of H, in a microwave discharge see: Shaw, T. M. J. Chem. Phys. 1959,30, 1366. (1 3) In the more traditional normal mode basis the 104)- vibrational state correlates with the (301) level, with the three indices representing the quantum numbers for the symmetric stretch, bend, and antisymmetric stretch, respectively
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The Journal of Physical Chemistry, Vol. 94, No. 11, 1990 4393
Letters
H
+
+
Hz0(4~0,)-0H A2Z+-X211
H2
OH R Branch
less than a factor of 2. On physical grounds these trends are not too surprising, since it is the H2 bond that has to be broken in the H2 + O H reaction and, thus, should be most strongly coupled to the reaction coordinate. Studies on the effect of translational energy in enhancing reactivity of the H2 O H reaction have also been conducted. Zellner and Steinertl' find that if the amount of energy corresponding to excitation of H2 from u = 0 to u = 1 is instead put into relative translation of the reagents, a rate enhancement of 5 X lo3 is observed. This finding suggests that the H2 + O H reaction has an early barrier. The above experimental findings are supported by theoretical calculations20which confirm that the barrier to the H2 OH reaction is indeed located in the entrance channel and that vibrational excitation of H2 is more efficient than vibrational excitation of O H in reducing the vibrationally adiabatic threshold energy for reaction. The O H bond appears as a spectator in the H2 + O H reaction, and as a result its vibration is not strongly coupled to the reaction coordinate. On the basis of the above findings for the H2 + OH reaction and microscopic reversibility we expect that the forward reaction involving H H 2 0 exhibits a late barrier and that excitation of a local mode vibrational state of water, where the energy is predominantly confined to the O H stretches, should be particularly efficient in promoting this reaction.I0 The spectator behavior exhibited by O H in the H2 + O H reaction should correlate with the unreacted O H bond in water behaving as a spectator in the H H 2 0 reaction. Our present findings that the title reaction occurs within a few gas kinetic collisions12and that the OH product receives less than 10%of the available energy are consistent with the above spectator model for reaction. We are currently planning experiments to probe the H2 product in order to determine the energy partitioning completely.
+
+
Figure 3. Excitation spectrum of the OH(2113,2,u=O) product obtained by scanning the frequency of the probe laser while the vibrational overtone laser wavelength is fixed on the 202 lo, transition of the 104)vibrational band in water.
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reaction probability. The spectrum shown in Figure 2a is one where the time delay between the vibrational excitation laser and the probe laser is adjusted such that the energized water molecules have undergone on the average only 0.08 total collisions. As a result, it represents an action spectrum that is not completely re1a~ed.I~ The similar relative intensities occurring in this spectrum and the photoacoustic spectrum shown in Figure 2b indicate that the cross section for the H + H 2 0 reaction is not strongly dependent on the initial rotational state of H20, in accord with theoretical predictions.I0 The reaction of water molecules excited to the 104)- vibrational state with hydrogen atoms provides -9000 cm-I of energy to be partitioned between the OH and H2 products.15 Although the measurement of the product rotational and fine structure state distribution is presently incomplete, preliminary analysis of O H excitation spectrum like the one shown in Figure 3 indicates that most of the O H fragments (-98%) are formed in the vibrational ground state with an average of -360 cm-' of rotational energy. Our estimate of OH translational excitation comes from line-width measurement of the spectral features appearing in the O H excitation spectrum. The observed laser line-width deconvoluted Doppler width of -0.15 cm-' for the R,(7) transition indicates that roughly 350 cm-' of energy appears in O H translation.I6 Experiments using a narrow-bandwidth (-0.06 cm-I) probe laser are currently underway in order to refine these measurements. From conservation of energy, the remaining -8200 cm-' of energy must appear as internal and translational excitation of the H2 product. Although there are no prior experimental studies on the influence of reagent vibrational excitation on the H + H 2 0 reaction, we can gain valuable insight into this question by applying microscopic reversibility to the results of earlier studies on the reverse reaction involving O H H2. Independent experimental studies by Zellner and Steinert" as well as by Glass and ChaturvediI8 indicate that vibrational excitation of the H2moiety enhances the rate of the H2 O H reaction by over 2 orders of magnitude. In contrast, upon vibrationally exciting the O H reactant, Spencer et a1.I9find that the rate of the H2 + O H reaction increases by
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+
(14) Although the rate for rotational relaxation of water has not been measured, we expect it to be of similar magnitude to that found for HICO (Le., approximately a IO times gas kinetic). See: Bewick, C. P.; Haub, J. G.; Hynes, R. G.; Martins, J. F.; Orr, 9. J. J . Chem. Phys. 1988, 88, 6350. = hv, + E,", - M O P . (15) ( I 6) This represents the translational energy in the laboratory frame. (17) Zellner, R.; Steinert, W. Chem. Phys. Lett. 1981, 81, 568. (18) Glass, G. P.; Chaturvedi, 9. K. J . Chem. Phys. 1981, 75, 27499. (19) Spencer, J . E.; Endo, H.; Glass, G. P. Sixteenth Symposium (International) on Combustion, [Proceedings];Combustion Institute: Pittsburgh, PA. 1977.
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Summary Local mode excitation to produce a nearly pure 0-H stretching state of water is shown to be an effective means for promoting the H H 2 0 reaction. A probe of the nascent O H product state distribution resulting from the reaction of water excited to the 104)-vibrational state indicates that this product receives less than 10% of the available energy consistent with the spectator model. The present results on the H H 2 0 system demonstrate the feasibility of using vibrational overtone excitation to initiate bimolecular radical-molecule reaction involving reagents with light atom stretching motion. Furthermore it suggests that, in favorable cases where intramolecular interactions are weak and single excited eigenstates can be prepared by vibrational overtone excitation, it should be possible to study the effect of initial local mode excitation on bimolecular reactions involving polyatomic reagents. We are currently using the technique to prepare various other vibrationally excited states of water and its isotopes in order to fully characterize the influence of initial nuclear motion on the H + H 2 0 reaction. A particularly interesting aspect of local mode excitation is the opportunity it offers for observing bond-selective chemistry. Our recent measurements2I on the H + HOD system show that the reaction of an HOD molecule containing four quanta of 0 - H stretching excitation almost exclusively produces the H2 + OD products demonstrating that, for systems where intramolecular coupling is weak, local mode excitation can lead to bond-selective chemistry.
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Acknowledgment. We thank Professor F. Fleming Crim for critically reading the manuscript, for several helpful discussions, and for his encouragement during these initial experiments. We also thank Professor J. W. Taylor for the loan of the microwave power supply and Mr. Mark Hsiao for assistance with the experiment and figures. Thanks also to Dr. B. Sinha and Mr. R. Krishna for helpful discussions and the reviewer for several useful comments. This work is supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Department of Energy. (20) Dunning, T. H.; Kraka, E.; Eades, R. A. Faraday Discuss. Chem.
Soc. 1987,84427. Rashed, 0.;Brown, N. J. J . Chem. Phys. 1985,82,5506.
(21) Sinha, A.; Hsiao, M.; Crim, F. F. J . Chem. Phys., in press.
