State-to-state dynamics of H+HX collisions. I. The H+HX→H2+X (X=Cl,Br,I) abstraction reactions at 1.6 eV collision energy

Abstract

The rotational and vibrational state distributions of the H2 product from the reactions of translationally excited H atoms with HCl, HBr, and HI at 1.6 eV are probed by coherent anti-Stokes Raman scattering spectroscopy after only one collision of the fast H atom. Despite the high collision energy, only the very exoergic (ΔH=−1.4 eV) hydrogen atom abstraction involving HI leads to appreciable H2 product vibrational excitation. For this reaction the H2 vibrational distribution is strongly inverted and peaks in v′=1, with 25% of the total available energy partitioned to vibration. For the mildy exoergic (ΔH=−0.72 eV) reaction with HBr and the nearly thermoneutral (ΔH=−0.05 eV) reaction with HCl, very little energy appears in H2 vibration, 9% and 2%, respectively, and the vibrational state distributions peak at v′=0. However, in all three reactions a significant fraction, 18% to 21%, of the total energy available appears as H2 rotation. All three reactions show a strong propensity to conserve the translational energy, that is the translational energy of the H2+X products is very nearly the same as that of the H+HX reactants. For the reactions with HCl, HBr, and HI the average translational energy of the products is 1.3, 1.7, and 1.7 eV, respectively, and the width of the translational energy distribution is only about 0.5 eV full width at half maximum. The energy disposal in all three reactions is quite specific, despite the fact that this high collision energy is well above the barrier to reaction in all three systems and a large number of product quantum states are energetically accessible. Only a few of these energetically allowed final states are appreciably populated. Although detailed theoretical calculations will be required to account completely for the state specifity, quite simple models of the reaction dynamics can explain much of the dynamical bias that we observe.

Theoretical studies of product polarization and state distributions of the H+HCl reaction

“…The abstraction cross-sections of the H + HCl and D + HCl reactions obtained from the experimental measurements are well justified by QCT calculations on the G3 PES [43]. As another important elementary step in the mechanism of the Cl 2 þ H 2 chain reaction, the dynamical behavior of the H + HCl reaction is also a very interesting subject, but much less information is available concerning this reaction [44][45][46][47]. It is our intention in this work to get more dynamics information, and more about the PES for this system QCT calculations have been performed for the reaction of H + HCl on both BW2 and G3 PESs.…”

“…1 and 2, respectively. Valentini and co-workers [44,45] have measured the absolute reaction crosssections of the H + HCl reaction at the same collision energy and determined values of ð2 AE 1Þ and ð13 AE 3Þ A A for the abstraction channel and Fig. 1(a)) or m 0 ¼ 1 (shown in Fig.…”

“…In order to compare with the experimental measurements of the product state distributions for the H + HCl reaction [44,45], the relative translational energy is chosen to be 1.6 eV in this paper. Calculations made by Aoiz et al [43] have shown that no appreciable dependence of the cross section on the HCl reagent rotational levels was observed for the above reaction, so the trajectory is initiated with the hydrogen chloride in the v ¼ 0 and j ¼ 0 level.…”

Close-Coupling Time-Dependent Quantum Dynamics Study of the H + HCl Reaction

“…1,2 A large number of kinetics studies were carried out for the H + HCl elementary reaction in the temperature range 195 KeTe1200 K, 3 including experiments in which the influence of selective vibrational excitation of HCl on the reaction rate was investigated. [4][5][6][7][8] Recent studies [9][10][11][12][13][14][15][16] of the reaction H + HCl at a collision energy of 1.6 eV have measured the integral cross section for the abstraction channel to be (2(1) Å 2 , 9,10 where as the exchange plus energy-transfer channels gave a combined cross section of (13(3) Å 2 . 9,10 It has been shown by Wight et al 11 that the dominant process is energy transfer.…”

“…[4][5][6][7][8] Recent studies [9][10][11][12][13][14][15][16] of the reaction H + HCl at a collision energy of 1.6 eV have measured the integral cross section for the abstraction channel to be (2(1) Å 2 , 9,10 where as the exchange plus energy-transfer channels gave a combined cross section of (13(3) Å 2 . 9,10 It has been shown by Wight et al 11 that the dominant process is energy transfer. 2 In contrast to numerous experiments, only a few theoretical studies have been reported for this H + HCl reaction.…”