Dynamics of molecular reactions in solids: Photodissociation of HI in crystalline Xe

Abstract

The photodissociation of HI impurities in a crystalline Xe host is studied by molecular dynamics simulations. From the calculated trajectories, analyses are given for: The behavior of the HI molecule before dissociation, the motions of the fragments following photon absorption, and the sites and vibration dynamics of the H fragments long after dissociation. The main findings include: (i) The photodissociation yield as a function of temperature is not monotonic (0 at 0 K, 0.2 at 17 K, 0.1 at 35 K). (ii) The nascent H atoms, at early time (∼0.5 ps), exhibit well defined, high frequency (∼900 cm−1) vibrations in the cage. The H trajectories acquire increasingly a random walk character with the progress of time. (iii) H exit from the cage is virtually never direct. (iv) After relaxation to equilibrium, the product H atoms occupy dilated interstitial sites (nearest-neighbor xenons displace by 0.3 and 0.6 Å at the octahedral and tetrahedral interstitial sites, respectively). (v) The H atom dynamically distorts the octahedral site and exhibits three well-separated local vibrational frequencies, corresponding to motions along well-defined axes of the site. (vi) The reagent HI molecular rotations are strongly hindered at low temperatures, and are more aptly described as large amplitude bendings associated with the complex Xe⋅⋅⋅HI. The experimental implications of the above findings and the possible consequences of quantum effects are discussed.