The dynamics of the reactions H+H2O→OH+H2 and H+D2O→OD+HD at 1.4 eV

Abstract

OH(OD) quantum state populations, rovibrational quantum state-resolved center-of-mass angular scattering distributions, and H2(HD) coproduct internal energy release distributions have been determined for the hot H atom reactions with H2O and D2O at mean collision energies close to 1.4 eV. The experiments employ pulsed laser photolysis coupled with polarized Doppler-resolved laser induced fluorescence detection of the radical products. The OH(2Π1/2,v′=0,N′=1,A′) and OD(2Π1/2,v′=0,N′=1,A′) angular distributions generated by the two isotopic reactions are quite distinct: that for the reaction with H2O shows intensity over a wide range of center-of-mass scattering angles, and peaks in the sideways direction, while the state-resolved angular distribution for the reaction with D2O displays more scattering in the backward hemisphere. For higher OH(OD) angular momentum states the differences in the angular distributions for the two reactions are less marked, with both systems showing a slight preference for backward scattering. The kinetic energy release distributions are insensitive to OH(OD) quantum state and to isotopic substitution, and reveal that the H2(HD) coproducts are born internally cold at 1.4 eV. OH(OD) quantum state averaged energy disposals in the two reactions are also presented. The new experiments provide detailed mechanistic information about the two reactions and clarify the dominant sources of product OH(OD) rotational excitation. Current theoretical understanding of the reaction is critically assessed.

The Dynamics of the H+H2O Reaction

“…Most of the experiments [50,52,56,74] have found that the spin-orbit population ratios P 3/2 /P 1/2 (weighted by N'/(N' 1) factors) is almost unity. A recent experiment performed by the group of Brouard, [76] at a collision energy of 1.4 eV, showed that for N' < 2 this ratio tends to the value of two, implying that the reaction populates preferentially the P 3/2 spin-orbit state for low N'. On the other hand, the A'/A'' ratio in the P 3/2 manifold have been measured by Wolfrum et al [52] and rises from 1 at N' 1 to 3 ± 6 at N' > 6.…”

“…[76] The reported population data have been averaged over OH(OD) spin-orbit and l-doublet levels allowing proper comparison with theoretical calculations that do not take into account the splitting. Figure 7 compares the OH rotational distribution with the theoretical QCT prediction using the OC PES [75] and the most recent YZCL2 PES.…”

“…Table 2 Experimental results from ref. [76];~: QCT results using the OC PES. [75] b) As a), but comparing the experimental data with those calculated using the QCT method on the YZCL2 PES.…”

“…The fact that the kinetic energy of the H atom does not effectively promote rotational excitation of OH raises the intriguing question about the origin of this excitation. Several mechanisms have been proposed [52] but the most convincing has been forwarded by Brouard et al [76] They realized that OH rotational distributions generated via photodissociation of H 2 O at 193 and 157 nm are very similar to those observed in the reaction at 1.4 and 2.5 eV, respectively. Furthermore, using a modified Frank ± Condon model devised for the photodissociation of H 2 O(A ƒ ), they managed to reproduce the rotational distributions of the OH product.…”

“…However, some minor discrepancies between the predictions of this model and the experimental rotational distributions leave some scope for a impulsive contribution from the breaking of the HHÀOH bond. [76] The experiments of Brouard and co-workers [76] provide also OH and OD quantum-state-resolved kinetic energy release distributions. Due to energy conservation they reflect the distributions over internal energies in the H 2 or HD co-products.…”