Dynamics of the H+D2→HD+D reaction: dependence of the product quantum state distributions on collision energy from 0.98 to 1.3 eV

Measurement of the nascent HD product rotational and vibrational state distributions for the H+D2→HD+D reaction is reported. Ultraviolet photolysis of HI using a pulsed laser at 291 or 280 nm is used to create H atoms in an HI/D2 gas mixture, giving H+D2 collisions with relative energies of 0.98 or 1.1 eV. Pulsed-laser coherent anti-Stokes Raman scattering (CARS) spectroscopy is used to record rotationally and vibrationally resolved Raman spectra of the HD product of the photolytically initiated H+D2 reaction, under effectively single-collision conditions. The HD product quantum state distributions are extracted from these CARS spectra. The present data are combined with results we obtained previously at 1.3 eV collision energy to reveal the collision energy dependence of the product quantum state distributions. We find that at all three collision energies the product distributions can be quite accurately represented by a linear rotational and vibrational surprisal function. The rotational surprisal parameter is large, positive, and nearly constant (θR=3.0–3.5) over this energy range, indicating a strong and consistent dynamical bias against product rotational excitation. The vibrational surprisal parameter, in contrast, varies much more, and is not even a monotonic function of collision energy (λV=2.2, 3.3, and 2.6 at 0.98, 1.1, and 1.3 eV, respectively). This behavior indicates enhancement of the cross section for the vibrationally nonadiabatic reaction channel at certain energies. At all collision energies 69%–73% of the available energy appears as translational energy of the products, with 22%–24% in rotation, and only 4%–7% in vibration. Product quantum state distributions derived from quasiclassical trajectory calculations at 0.98 and 1.3 eV collision energies agree very well with our measurements, indicating that classical mechanics provides an adequate description of the dynamics at these energies, and that the ab initio H3 potential surface used in these calculations must be at least reasonably accurate.