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Reaction of H2O with H produced by the 266 nm photolysis of HI
Volume 180, number 4
CHEMICAL PHYSICS LETTERS
24 May 1991
Reaction of Hz0 with H produced by the 266 nm photolysis of HI K.Honda a, M. Takayanagi b, T. Nishiya b, H. Ohoyama b,’ and I. Hanazaki a The Graduale Universiiyfor AdvancedStudies, Myodaiji. Okazaki444, Japan
a,b
b Institutefor Molecular Science, Myodaiji. Okazaki444, Japan Received 28 January 199 I; in final form 9 March 1991
The elementary reachon, H+H,O+OH+H*, has been studied in the flowing mixture of HI and HZ0 irradiated at 266 nm. The nascent rotational distributions of OH are of Boltzmann-type with a remarkably non-statistical partition over the n-doublet sublevels, while the spin-orbit components are populated statistically. Weak dependence on the collision energy is suggested. These results are shown to be explained with a classical model based on the direct stripping mechanism with the reaction barrier at a later stage of the reaction coordinate.
1. Introduction There have been several reports on the elementary reactions of hot hydrogen which produce OH as a product in flow-cell and molecular-beam experiments [l-4]. It is advantageous to analyse the reaction mechanism in substantial detail by monitoring OH on the basis of its well-known spectroscopic data. Among them, the endothermic reaction, H+H20+OH+H2,
M~62.8
kJ/mol,
(1)
is interesting since it is the reverse reaction of OH + H,+H,O+H, which is an important chainbranching step in the hydrogen combustion [ 51. Kleinermanns and Wolfrum reported the rotational distribution of OH produced in this reaction [ 61. They produced the hot hydrogen atom by photodissociating HBr at 193 nm and probed the rotationalstate distribution of OH with the laser-induced fluorescence (LIF) technique. A trajectory calculation has shown that reaction (1) could exhibit a dependence on the reagent vibrational excitation [ 7,8]. Sinha has accordingly tried to observe the effect of local-mode excitation of water to its 104) - state in this reaction [ 9 1. Bronikowski et al. have reported that a related sys’ Present address: Department of Chemistry, Faculty of Science, Osaka University, Toyonaka 560, Japan.
tern, H-to*, exhibits a dependence on the translational energy of hot hydrogen [ IO]. This suggests that the H t 0, reaction has an early barrier, which is sensitive to the reagent translational energy but not to the vibrational excitation. On the other hand, the H + HZ0 reaction would have a barrier at a later stage of the reaction zone [7,8], being sensitive to vibrational excitation and not to the translational energy. In view of this, it is interesting to study the H +HzO system with different translational energy of hot hydrogen from that produced in the 193 nm photolysis. In this report, we give the result of a flow-cell study of reaction (1) with the hot hydrogen atom produced by photodissociating HI at 266 nm. The 266 nm photodissociation has an additional advantage. It yields iodine atoms in two spin-orbit states (2P,,, and 2P,,Z), corresponding to the translational energies of the hot hydrogen atom of 150 and 60 kJ/mol, respectively [ 1 I 1, in contrast to those of 250 and 200 kJ/mol, respectively, for the photodissociation of HBr at 193 nm [ 121. Since the translational energy of 60 kJ/mol is less than the reaction barrier of 90 kJ/mol for this reaction [a], the contribution of this channel to the OH production is expected to be minor, most of the OH being produced through the I( 2P3,2) channel. This situation would simplify the analysis considerably. In the 0( ‘D) + Ha I80 system reported by Comes and Gericke et al. [ 13,141, the formation of a long-
CMlO9-2614/91/$ 03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
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lived H202 intermediate is shown to be unlikely since the vibrational excitation of OH is different between 160H and ‘*OH; the ‘6oH bond formed by the reaction is vibrationally hotter than the pre-existing “OH bond. The rotational excitation was shown to be nearly the same for i60H and “OH. It is, therefore, interesting to confirm if reaction ( 1) proceeds with a direct mechanism or with an indirect mechanism through an intermediate such as HsO. For this purpose, the reaction of H with D20 has also been examined in the present study.
2. Experimental The measurement was carried out at room temperature (21-23”(Z) under the flow-cell condition. The gaseous samples were continuously flowed in a stainless-steel reaction chamber and irradiated by pulsed lasers operated at 10 Hz. The pump rate was such that the whole irradiated gas was pumped out completely before the next laser shot. HI and Hz0 ( D20) were mixed just before the reaction chamber in 1: 1 ratio. The total gas pressure was varied from 20 to 100 mTorr as monitored by a capacitance manometer (MKS Baratron 220B, O-l Torr). The fourth harmonics of a Nd : YAG laser (Quante157 lC) at 266 nm was used to photodissociate HI. The LIF probe pulse was provided by the second harmonics of a dye laser (Lambda Physik FL 3002; resolution 0.2 cm- ’ ) pumped by a XeCl excimer laser (Lumonics HE-420-SM-B). The dye laser was operated with kiton red and sulforhodamine 640 in the 306-323 nm region to excite the OH A-X (O-O) and ( l-l ) transitions. In order to avoid saturation, the pump- and probe-laser pulse energies were kept at about 4 mJ per pulse and 200 j.tJ per pulse, respeG tively. The pump- and probe-laser beams were incident from the same direction so that they crossed each other at a low angle. Fluorescence was detected in the perpendicular direction by a photomultiplier (Hamamatsu R453) through a filter (Hoya UV30). The photomultiplier signal was amplified and integrated by a boxcar averaging system (Stanford Research SR240, SR250 and SR235) with the gate width of 1 ps. In order to avoid scattered light, the gate was delayed by 80 ns from the probe pulse. The delay time was also 80 ns between the pump- and 322
24 May 1991
probe-laser pulses. The relative LIF intensities for the rotational lines were not affected by varying the total pressure between 20 and 100 mTorr, ensuring that the nascent rotational distribution was obtained. The LlF intensity was obtained from the area below each band. For branches with satellites (such as Q ,1and Ri , ), where main and satellite bands were observed separately for higher K but completely overlapped with each other for lower K, the sum of the main and satellite band areas was determined and divided into the components on the basis of the theoretical Einstein coefficients [ 151. The pump- and probe-laser pulse energies were monitored by photodiodes and used to correct the LIF intensity for energy variation. The rotational population was obtained from the corrected LIF intensity with the tabulated transition probability [ 15 1, HI (Matheson 98n) was used as received. D20 (Merck 99.75%) and distilled water were further degassed by the freeze-pump-thaw method.
3. Results and discussion Fig. la shows a part of the (0,O) band of the LIF spectrum of OH taken under a typical experimental condition (the total gas pressure and the pump-probe delay time are 60 mTorr and 80 ns, respectively). The OH v”= 1 state is expected to be populated on the basis of energetic consideration [ 81, However, we could not observe any LIF signal due to the ( 1,l) transition for the S/N ratio of fig. 1. The low probability of the v” = 1 excitation is presumably due to the fact that the equilibrium distance of the OH bond inH,O(0.95781A[l6])isclosetothatoftheOH radical (0.96966 8, [ 171). Fig. 1b shows the LIF spectrum for the H + D20 reaction. The observed bands can be assigned entirely to OD. No signal due to OH was observed although we measured with the same sensitivity of detection as fig. la. This result indicates that the reaction proceeds by a direct mechanism, in which the hydrogen atom attacks one of the hydrogens in HZ0 and removes it as Hz to leave OH. If this reaction proceeded by an indirect mechanism via a long-lived complex, the signal due to OH should appear with intensity of at least l/2 of that of OD. If we consider the difference of the zero-point energy
Volume 180,number 4
CHEMICALPHYSICSLETTERS
24 May 1991
(b)
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WAV ELENGTHlnm Fig. 1. (a) Part of the OH A2Z+-X’H (v” =O) LIF spectrum for H+HrO+OH+Hr. The hydrogen atom is produced by the 266 nm photodissociation of HI. Gas mixture of 30 mTorr Hz0 and 30 mTorr HI flows through the cell. The delay time between the photolysis and probe pukes is 80 ns. (b) Part of the OD (0” ~0) LIF spectrumfor reaction H+D,O-+OD+HD under the same experimental condition as (a).
in the intermediate, the ratio of the formation of OH should be even higher. The Franck-Condon factors differ only 16Ohbetween the OH and OD transitions. Thus, we can conclude that reaction ( 1) proceeds most probably by a direct, stripping-type mechanism. We shall, show below that this mechanism accounts for several observations reported here. Fig. 2 shows the rotational-state distributions of OH (Y”=O) for the F,A’, F,A’ and F,A” manifolds obtained from the analysis of the R, 1-, Rzz- and Q, ,branch transitions, respectively. Here, F, and F2 denote the spin-orbit components of 0H(211) corresponding to J= 3/2 and l/2, respectively. A’ and A” represent the n-doublet component, where the unpaired electron lies in and perpendicular to the plane of molecular rotation, respectively [ 181. K is the total angular momentum, apart from spin, defined in Hund’s case (b) [ 191.
0
1 5
10
K Fig. 2. Nascent rotational-state distribution of the fine-structure componentsofOH (v”=O). (O):F,A’; (A):F,A’; oJ):F,A”.
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CHEMICAL PHYSICS LETTERS
In the case of the F,A’ state, the rotational distribution is peaked at K=2 and falls to zero at K= 10. This distribution is similar to that reported by Kleinermanns and Wolfrum for the 193 nm photodissociation [ 61. Fig. 3 shows a Boltzmann plot of the data shown in fig. 2. This result shows that the distributions are of Boltzmann-type except for very low rotational levels. The rotational temperatures are 800 + 50 and 550f30 K for the F,A’ and F,A” levels, respectively. These rotational temperatures correspond to the energies of about 8Ohand 5%, respectively, of the available energy (87 kJ/mol). The rotational energy of N 13% of the available energy is lower than the expected value of x25OYofor the completely statistical distribution over whole degrees of freedom of the products. The rotational temperature of the A’ level given by Kleinermanns and Wolfrum [6] for the 193 nm photolysis is about 900 K, slightly higher than our 800 K. The available energy of 87 kJ/mol for the J=3/2 hydrogen produced at 266 nm increases to 187 kJ/mol for J=3/2 produced by the 193 nm photolysis of HBr. Even if we take the J= l/2 channel, it is 137 kJ/mol. Therefore, the relative increase of the rotational energy on changing the excitation from 266 to 193 nm is much smaller than that of the available energy. We conclude, therefore, that the rotational excitation of OH in this reaction is insensitive to the collision energy. The small effect of variation of the collision energy ROTATIONAL 0
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24 May 1991
can be explained as due to the later barrier. Assuming the direct mechanism, we conclude that the attack of H with higher collision energy at one of the hydrogens of Hz0 tends to increase a non-reactive collision but does not enhance the coupling with the H-O stretching motion which brings the system beyond the reaction barrier. This scheme would account for the present observation of the minor effect of collision energy and predicted enhancement of the reaction due to the reagent vibrational excitation
[7,81. Fig. 4 shows the ratio of populations in two spinorbit components F,A’ and Fztl’ as a function of K, where the population has been corrected for the degeneracy 2J+ 1 of the spin-orbit state (2Jf 1=2K + 2 for F, and 2J+ 1= 2K for F2 ). Fig. 4 shows that the F, and F2 states are almost equally populated. The statistical partition seems to be reasonable since the energy and angular momentum conservation conditions give only very loose restrictions; namely, the energy difference between corresponding F, and Fz levels is negligibly small compared with the total available energy. Moreover, the reactants have J= l/2 due to the electron spin of H and the products have J=3/2 or l/2 due to the spin-orbit coupling in OH. The constraints AJ= 1 or 0 can easily be fulfilled since H20 and OH (and possibly HZ) have distribution of rotational angular momentum over a wide range of J. Fig. 5 shows the ratio of populations in two A-dou-
cm-’ 2000
r
Fig. 3. Boltzmann plot of the rotational distribution of OH (v”=(l). Each manifold is normalized at the null rotational energy. (0): F,A’; (A): F,A’; (0): F,A”.
324
Fig. 4. Spin-orbit population ratio, F,K/F,(K+ I ), as a function of K. The ratio has been corrected for degeneracy 2J+ I ( = 2K+ 2 for F, and 2K for Fz).
CHEMICAL PHYSICS LETTERS
Volume 180, number 4
30 0 h
2-
0 0
ir
0 , _..._o.._......._______________
0’
’
’
’
’
K
’
0’
5
Fig. 5. n-doublet population ratio, A’/A”, as a function of K.
blet levels (FiA’ and F,A” ). The ratio is close to unity at K= 1 and increases with K. The value Al/A” ~2.62 0.2 at Kc6 may be compared with the value of 3.2? 1.O at K= 11 given by Kleinermanns and Wolfrum [ 61. This is a result of the different rotational temperatures of the A’ and A” manifolds as shown in fig. 3. The difference in the rotational temperature may be understood on the basis of a classical model; namely, if the hot hydrogen atom attacks Hz0 in the molecular plane and takes one of the hydrogens in HzO, the resultant OH would be preferably in the A’ state since a torque operates in this plane to cause the rotation of OH and the unpaired electron is also left in the plane. On the other hand, the attack of H perpendicular to the Hz0 plane would preferably result in the A” state, where less rotational excitation is expected since the axis of torque is mostly in the direction of the other O-H bond which is left to form the product OH. The ratio of A’ and A” integrated over quantum number K shows also a preference of A’, indicating that not only the rotational temperature but also the total cross section is much larger for the channel producing A’ than for A”. If the reaction had an early barrier, the colliding H would form an H-H bond before the O-H bond distance is elongated, and there would be not so much difference between the total cross sections for the in-plane and perpendicular approaches. However, in the case of a later barrier, the H-H bond formation is not induced by the attack of H but by the coupled O-H stretching motion. Since the perpendicular collision of H with Hz0 has less chance to couple with O-H stretching, it tends to
24May 1991
cause a non-reactive collision in which the Hz0 rotation is simply excited. On the other hand, the inplane approach has more chance to couple with the O-H stretching or bending vibration, resulting in a larger reactive cross section. In conclusion, the observations given here for reaction ( 1) can well be accounted for by the direct mechanism in which the reaction barrier locates at a later stage of the reaction coordinate. The non-statistical partition between the d-doublets can also be explained qualitatively with a classical model of collisional encounter within a framework of this mechanism. For further studies, it would be interesting to see how the mechanism is different for the corresponding reactant-pair reaction, IH~H,O+~U+OH+H,+I.
(2)
It is known in many cases that the complex-initiated reactions have good correlation with the corresponding bimolecular (flow-cell) reaction [l-4]. However, in reaction (2), it is expected that the hydrogen in HI should be hydrogen-bonded to oxygen, which, on irradiation, would attack oxygen rather than hydrogen. This would result in a completely different reaction scheme from the bimolecular reaction studied here. We are planning to perform the supersonic molecular-beam experiment for (2) to study the effect of complexation.
References [ l] C. Wittig, S. Sharpe and R.A. Beaudet, Accounts Chem. Res. 21 (1988) 341. [2] H. Ohoyama, M. Takayanagi, T. Nishiya and 1. Hanazaki, Chem. Phys. Letters 162 ( 1989) 1. [ 31 G. Radhakrishnan, S. Buelow and C. Wittig, J. Chem. Phys. 84 (1986) 727. [4] Y. Chen, G. Hoffmann, D. Oh and C. Wit@, Chem. Phys. Letters 159 (1989) 426. [ 51V.N. Kondratiev, Comprehensive chemical kinetics, Vol. 2, eds. C.H. Banford and C.F.H. Tipper (Elsevier, Amsterdam, 1969) p. 118. [ 61 K. Kleinermanns and J. Wolfrum, Appl. Phys. B 34 ( 1984)
[ 712. Elgersma and G.C. Schatz, Intern. J. Quant. Chem. 15 (1981)611. [8] G.C. Schatz, MC. Colton and J.L. Grant, J. Phys. Chem. 88 (1984) 2971. [ 91 A. Sinha, J. Phys. Chem. 94 ( 1990) 439 I.
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[lo] M.J. Bronikowski, R. Zhang, D.J. Rakestraw and R.N. Zpre, Chem. Phys. Letters 156 ( 1989) 7. [ 1I ] R.D. Clear, S.J. Riley and K.R. Wilson, J. Chem. Phys. 63 ( 1975) 1340. [ 121 G.W. Flynn and R.E. Weston, Ann. Rev. Phys. Chem. 37 (1986) 551. [ 131 F.J. Comes, K.-H. Gericke and I. Marx, I. Chem. Phys. 75 (1981) 2853. [ 141 K.-H. Gericke, F.J. Comes and R.D. Levine, J. Chem. Phys. 74 (1981) 6106.
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[ 151 W.L. Dimpfl and J.L. Kinsey, J. Quant. Spectry. Radiat. Trans. 21 (1979) 233. [ 161 A.R. Hoy and P.R. Bunker, J. Mol. Spectry. 74 (1979) 1. [ 171 K.P. Huber and G. Herzberg, Molecular structure and molecular spectra, Vol. 4. Constants of diatomic molecules (Van Nostrand Reinhold, New York, 1979) p. 508. [ 181 M.H. Alexander et al., J. Chem. Phys. 89 (1988) 1749. [ 191 G. He~berg, Molecular spectra and molecular structure, Vol. 1,Spectra of diatomic molecules (Van Nostrand, Princeton, 1950).
CHEMICAL PHYSICS LETTERS
24 May 1991
Reaction of Hz0 with H produced by the 266 nm photolysis of HI K.Honda a, M. Takayanagi b, T. Nishiya b, H. Ohoyama b,’ and I. Hanazaki a The Graduale Universiiyfor AdvancedStudies, Myodaiji. Okazaki444, Japan
a,b
b Institutefor Molecular Science, Myodaiji. Okazaki444, Japan Received 28 January 199 I; in final form 9 March 1991
The elementary reachon, H+H,O+OH+H*, has been studied in the flowing mixture of HI and HZ0 irradiated at 266 nm. The nascent rotational distributions of OH are of Boltzmann-type with a remarkably non-statistical partition over the n-doublet sublevels, while the spin-orbit components are populated statistically. Weak dependence on the collision energy is suggested. These results are shown to be explained with a classical model based on the direct stripping mechanism with the reaction barrier at a later stage of the reaction coordinate.
1. Introduction There have been several reports on the elementary reactions of hot hydrogen which produce OH as a product in flow-cell and molecular-beam experiments [l-4]. It is advantageous to analyse the reaction mechanism in substantial detail by monitoring OH on the basis of its well-known spectroscopic data. Among them, the endothermic reaction, H+H20+OH+H2,
M~62.8
kJ/mol,
(1)
is interesting since it is the reverse reaction of OH + H,+H,O+H, which is an important chainbranching step in the hydrogen combustion [ 51. Kleinermanns and Wolfrum reported the rotational distribution of OH produced in this reaction [ 61. They produced the hot hydrogen atom by photodissociating HBr at 193 nm and probed the rotationalstate distribution of OH with the laser-induced fluorescence (LIF) technique. A trajectory calculation has shown that reaction (1) could exhibit a dependence on the reagent vibrational excitation [ 7,8]. Sinha has accordingly tried to observe the effect of local-mode excitation of water to its 104) - state in this reaction [ 9 1. Bronikowski et al. have reported that a related sys’ Present address: Department of Chemistry, Faculty of Science, Osaka University, Toyonaka 560, Japan.
tern, H-to*, exhibits a dependence on the translational energy of hot hydrogen [ IO]. This suggests that the H t 0, reaction has an early barrier, which is sensitive to the reagent translational energy but not to the vibrational excitation. On the other hand, the H + HZ0 reaction would have a barrier at a later stage of the reaction zone [7,8], being sensitive to vibrational excitation and not to the translational energy. In view of this, it is interesting to study the H +HzO system with different translational energy of hot hydrogen from that produced in the 193 nm photolysis. In this report, we give the result of a flow-cell study of reaction (1) with the hot hydrogen atom produced by photodissociating HI at 266 nm. The 266 nm photodissociation has an additional advantage. It yields iodine atoms in two spin-orbit states (2P,,, and 2P,,Z), corresponding to the translational energies of the hot hydrogen atom of 150 and 60 kJ/mol, respectively [ 1 I 1, in contrast to those of 250 and 200 kJ/mol, respectively, for the photodissociation of HBr at 193 nm [ 121. Since the translational energy of 60 kJ/mol is less than the reaction barrier of 90 kJ/mol for this reaction [a], the contribution of this channel to the OH production is expected to be minor, most of the OH being produced through the I( 2P3,2) channel. This situation would simplify the analysis considerably. In the 0( ‘D) + Ha I80 system reported by Comes and Gericke et al. [ 13,141, the formation of a long-
CMlO9-2614/91/$ 03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
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Volume 180, number 4
CHEMICAL PHYSICS LETTERS
lived H202 intermediate is shown to be unlikely since the vibrational excitation of OH is different between 160H and ‘*OH; the ‘6oH bond formed by the reaction is vibrationally hotter than the pre-existing “OH bond. The rotational excitation was shown to be nearly the same for i60H and “OH. It is, therefore, interesting to confirm if reaction ( 1) proceeds with a direct mechanism or with an indirect mechanism through an intermediate such as HsO. For this purpose, the reaction of H with D20 has also been examined in the present study.
2. Experimental The measurement was carried out at room temperature (21-23”(Z) under the flow-cell condition. The gaseous samples were continuously flowed in a stainless-steel reaction chamber and irradiated by pulsed lasers operated at 10 Hz. The pump rate was such that the whole irradiated gas was pumped out completely before the next laser shot. HI and Hz0 ( D20) were mixed just before the reaction chamber in 1: 1 ratio. The total gas pressure was varied from 20 to 100 mTorr as monitored by a capacitance manometer (MKS Baratron 220B, O-l Torr). The fourth harmonics of a Nd : YAG laser (Quante157 lC) at 266 nm was used to photodissociate HI. The LIF probe pulse was provided by the second harmonics of a dye laser (Lambda Physik FL 3002; resolution 0.2 cm- ’ ) pumped by a XeCl excimer laser (Lumonics HE-420-SM-B). The dye laser was operated with kiton red and sulforhodamine 640 in the 306-323 nm region to excite the OH A-X (O-O) and ( l-l ) transitions. In order to avoid saturation, the pump- and probe-laser pulse energies were kept at about 4 mJ per pulse and 200 j.tJ per pulse, respeG tively. The pump- and probe-laser beams were incident from the same direction so that they crossed each other at a low angle. Fluorescence was detected in the perpendicular direction by a photomultiplier (Hamamatsu R453) through a filter (Hoya UV30). The photomultiplier signal was amplified and integrated by a boxcar averaging system (Stanford Research SR240, SR250 and SR235) with the gate width of 1 ps. In order to avoid scattered light, the gate was delayed by 80 ns from the probe pulse. The delay time was also 80 ns between the pump- and 322
24 May 1991
probe-laser pulses. The relative LIF intensities for the rotational lines were not affected by varying the total pressure between 20 and 100 mTorr, ensuring that the nascent rotational distribution was obtained. The LlF intensity was obtained from the area below each band. For branches with satellites (such as Q ,1and Ri , ), where main and satellite bands were observed separately for higher K but completely overlapped with each other for lower K, the sum of the main and satellite band areas was determined and divided into the components on the basis of the theoretical Einstein coefficients [ 151. The pump- and probe-laser pulse energies were monitored by photodiodes and used to correct the LIF intensity for energy variation. The rotational population was obtained from the corrected LIF intensity with the tabulated transition probability [ 15 1, HI (Matheson 98n) was used as received. D20 (Merck 99.75%) and distilled water were further degassed by the freeze-pump-thaw method.
3. Results and discussion Fig. la shows a part of the (0,O) band of the LIF spectrum of OH taken under a typical experimental condition (the total gas pressure and the pump-probe delay time are 60 mTorr and 80 ns, respectively). The OH v”= 1 state is expected to be populated on the basis of energetic consideration [ 81, However, we could not observe any LIF signal due to the ( 1,l) transition for the S/N ratio of fig. 1. The low probability of the v” = 1 excitation is presumably due to the fact that the equilibrium distance of the OH bond inH,O(0.95781A[l6])isclosetothatoftheOH radical (0.96966 8, [ 171). Fig. 1b shows the LIF spectrum for the H + D20 reaction. The observed bands can be assigned entirely to OD. No signal due to OH was observed although we measured with the same sensitivity of detection as fig. la. This result indicates that the reaction proceeds by a direct mechanism, in which the hydrogen atom attacks one of the hydrogens in HZ0 and removes it as Hz to leave OH. If this reaction proceeded by an indirect mechanism via a long-lived complex, the signal due to OH should appear with intensity of at least l/2 of that of OD. If we consider the difference of the zero-point energy
Volume 180,number 4
CHEMICALPHYSICSLETTERS
24 May 1991
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1'
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WAV ELENGTHlnm Fig. 1. (a) Part of the OH A2Z+-X’H (v” =O) LIF spectrum for H+HrO+OH+Hr. The hydrogen atom is produced by the 266 nm photodissociation of HI. Gas mixture of 30 mTorr Hz0 and 30 mTorr HI flows through the cell. The delay time between the photolysis and probe pukes is 80 ns. (b) Part of the OD (0” ~0) LIF spectrumfor reaction H+D,O-+OD+HD under the same experimental condition as (a).
in the intermediate, the ratio of the formation of OH should be even higher. The Franck-Condon factors differ only 16Ohbetween the OH and OD transitions. Thus, we can conclude that reaction ( 1) proceeds most probably by a direct, stripping-type mechanism. We shall, show below that this mechanism accounts for several observations reported here. Fig. 2 shows the rotational-state distributions of OH (Y”=O) for the F,A’, F,A’ and F,A” manifolds obtained from the analysis of the R, 1-, Rzz- and Q, ,branch transitions, respectively. Here, F, and F2 denote the spin-orbit components of 0H(211) corresponding to J= 3/2 and l/2, respectively. A’ and A” represent the n-doublet component, where the unpaired electron lies in and perpendicular to the plane of molecular rotation, respectively [ 181. K is the total angular momentum, apart from spin, defined in Hund’s case (b) [ 191.
0
1 5
10
K Fig. 2. Nascent rotational-state distribution of the fine-structure componentsofOH (v”=O). (O):F,A’; (A):F,A’; oJ):F,A”.
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In the case of the F,A’ state, the rotational distribution is peaked at K=2 and falls to zero at K= 10. This distribution is similar to that reported by Kleinermanns and Wolfrum for the 193 nm photodissociation [ 61. Fig. 3 shows a Boltzmann plot of the data shown in fig. 2. This result shows that the distributions are of Boltzmann-type except for very low rotational levels. The rotational temperatures are 800 + 50 and 550f30 K for the F,A’ and F,A” levels, respectively. These rotational temperatures correspond to the energies of about 8Ohand 5%, respectively, of the available energy (87 kJ/mol). The rotational energy of N 13% of the available energy is lower than the expected value of x25OYofor the completely statistical distribution over whole degrees of freedom of the products. The rotational temperature of the A’ level given by Kleinermanns and Wolfrum [6] for the 193 nm photolysis is about 900 K, slightly higher than our 800 K. The available energy of 87 kJ/mol for the J=3/2 hydrogen produced at 266 nm increases to 187 kJ/mol for J=3/2 produced by the 193 nm photolysis of HBr. Even if we take the J= l/2 channel, it is 137 kJ/mol. Therefore, the relative increase of the rotational energy on changing the excitation from 266 to 193 nm is much smaller than that of the available energy. We conclude, therefore, that the rotational excitation of OH in this reaction is insensitive to the collision energy. The small effect of variation of the collision energy ROTATIONAL 0
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ENERGY/ 1000
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24 May 1991
can be explained as due to the later barrier. Assuming the direct mechanism, we conclude that the attack of H with higher collision energy at one of the hydrogens of Hz0 tends to increase a non-reactive collision but does not enhance the coupling with the H-O stretching motion which brings the system beyond the reaction barrier. This scheme would account for the present observation of the minor effect of collision energy and predicted enhancement of the reaction due to the reagent vibrational excitation
[7,81. Fig. 4 shows the ratio of populations in two spinorbit components F,A’ and Fztl’ as a function of K, where the population has been corrected for the degeneracy 2J+ 1 of the spin-orbit state (2Jf 1=2K + 2 for F, and 2J+ 1= 2K for F2 ). Fig. 4 shows that the F, and F2 states are almost equally populated. The statistical partition seems to be reasonable since the energy and angular momentum conservation conditions give only very loose restrictions; namely, the energy difference between corresponding F, and Fz levels is negligibly small compared with the total available energy. Moreover, the reactants have J= l/2 due to the electron spin of H and the products have J=3/2 or l/2 due to the spin-orbit coupling in OH. The constraints AJ= 1 or 0 can easily be fulfilled since H20 and OH (and possibly HZ) have distribution of rotational angular momentum over a wide range of J. Fig. 5 shows the ratio of populations in two A-dou-
cm-’ 2000
r
Fig. 3. Boltzmann plot of the rotational distribution of OH (v”=(l). Each manifold is normalized at the null rotational energy. (0): F,A’; (A): F,A’; (0): F,A”.
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Fig. 4. Spin-orbit population ratio, F,K/F,(K+ I ), as a function of K. The ratio has been corrected for degeneracy 2J+ I ( = 2K+ 2 for F, and 2K for Fz).
CHEMICAL PHYSICS LETTERS
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0 , _..._o.._......._______________
0’
’
’
’
’
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’
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5
Fig. 5. n-doublet population ratio, A’/A”, as a function of K.
blet levels (FiA’ and F,A” ). The ratio is close to unity at K= 1 and increases with K. The value Al/A” ~2.62 0.2 at Kc6 may be compared with the value of 3.2? 1.O at K= 11 given by Kleinermanns and Wolfrum [ 61. This is a result of the different rotational temperatures of the A’ and A” manifolds as shown in fig. 3. The difference in the rotational temperature may be understood on the basis of a classical model; namely, if the hot hydrogen atom attacks Hz0 in the molecular plane and takes one of the hydrogens in HzO, the resultant OH would be preferably in the A’ state since a torque operates in this plane to cause the rotation of OH and the unpaired electron is also left in the plane. On the other hand, the attack of H perpendicular to the Hz0 plane would preferably result in the A” state, where less rotational excitation is expected since the axis of torque is mostly in the direction of the other O-H bond which is left to form the product OH. The ratio of A’ and A” integrated over quantum number K shows also a preference of A’, indicating that not only the rotational temperature but also the total cross section is much larger for the channel producing A’ than for A”. If the reaction had an early barrier, the colliding H would form an H-H bond before the O-H bond distance is elongated, and there would be not so much difference between the total cross sections for the in-plane and perpendicular approaches. However, in the case of a later barrier, the H-H bond formation is not induced by the attack of H but by the coupled O-H stretching motion. Since the perpendicular collision of H with Hz0 has less chance to couple with O-H stretching, it tends to
24May 1991
cause a non-reactive collision in which the Hz0 rotation is simply excited. On the other hand, the inplane approach has more chance to couple with the O-H stretching or bending vibration, resulting in a larger reactive cross section. In conclusion, the observations given here for reaction ( 1) can well be accounted for by the direct mechanism in which the reaction barrier locates at a later stage of the reaction coordinate. The non-statistical partition between the d-doublets can also be explained qualitatively with a classical model of collisional encounter within a framework of this mechanism. For further studies, it would be interesting to see how the mechanism is different for the corresponding reactant-pair reaction, IH~H,O+~U+OH+H,+I.
(2)
It is known in many cases that the complex-initiated reactions have good correlation with the corresponding bimolecular (flow-cell) reaction [l-4]. However, in reaction (2), it is expected that the hydrogen in HI should be hydrogen-bonded to oxygen, which, on irradiation, would attack oxygen rather than hydrogen. This would result in a completely different reaction scheme from the bimolecular reaction studied here. We are planning to perform the supersonic molecular-beam experiment for (2) to study the effect of complexation.
References [ l] C. Wittig, S. Sharpe and R.A. Beaudet, Accounts Chem. Res. 21 (1988) 341. [2] H. Ohoyama, M. Takayanagi, T. Nishiya and 1. Hanazaki, Chem. Phys. Letters 162 ( 1989) 1. [ 31 G. Radhakrishnan, S. Buelow and C. Wittig, J. Chem. Phys. 84 (1986) 727. [4] Y. Chen, G. Hoffmann, D. Oh and C. Wit@, Chem. Phys. Letters 159 (1989) 426. [ 51V.N. Kondratiev, Comprehensive chemical kinetics, Vol. 2, eds. C.H. Banford and C.F.H. Tipper (Elsevier, Amsterdam, 1969) p. 118. [ 61 K. Kleinermanns and J. Wolfrum, Appl. Phys. B 34 ( 1984)
[ 712. Elgersma and G.C. Schatz, Intern. J. Quant. Chem. 15 (1981)611. [8] G.C. Schatz, MC. Colton and J.L. Grant, J. Phys. Chem. 88 (1984) 2971. [ 91 A. Sinha, J. Phys. Chem. 94 ( 1990) 439 I.
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[lo] M.J. Bronikowski, R. Zhang, D.J. Rakestraw and R.N. Zpre, Chem. Phys. Letters 156 ( 1989) 7. [ 1I ] R.D. Clear, S.J. Riley and K.R. Wilson, J. Chem. Phys. 63 ( 1975) 1340. [ 121 G.W. Flynn and R.E. Weston, Ann. Rev. Phys. Chem. 37 (1986) 551. [ 131 F.J. Comes, K.-H. Gericke and I. Marx, I. Chem. Phys. 75 (1981) 2853. [ 141 K.-H. Gericke, F.J. Comes and R.D. Levine, J. Chem. Phys. 74 (1981) 6106.
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[ 151 W.L. Dimpfl and J.L. Kinsey, J. Quant. Spectry. Radiat. Trans. 21 (1979) 233. [ 161 A.R. Hoy and P.R. Bunker, J. Mol. Spectry. 74 (1979) 1. [ 171 K.P. Huber and G. Herzberg, Molecular structure and molecular spectra, Vol. 4. Constants of diatomic molecules (Van Nostrand Reinhold, New York, 1979) p. 508. [ 181 M.H. Alexander et al., J. Chem. Phys. 89 (1988) 1749. [ 191 G. He~berg, Molecular spectra and molecular structure, Vol. 1,Spectra of diatomic molecules (Van Nostrand, Princeton, 1950).