Molecular dynamics simulations of reactions in solids: Photodissociation of Cl2 in crystalline Xe

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Abstract
The photodissociation of isolated Cl2 impurities in a Xe crystal was investigated by molecular dynamics simulations. The calculations were carried out for a photodissociation energy of 3.775 eV, and for several temperatures in the range from 10 to 150 K (the melting point is 162 K). The focus was on the physical mechanisms whereby the product atoms exit from the cage, on the properties of the final sites occupied by the Cl atom, and on the temperature dependence of the processs. The main findings were: (1) exit of a Cl atom from the original reagent cage, when it occurs, is always direct upon photodissociation, and does not involve multiple collisions with the surrounding cage walls. This is in qualitative contrast with the dynamics of cage exit in the case of HI photodissociation in Xe at very low temperatures, found in a previous study. (2) The occurrence of product exit from the cage depends entirely on whether the reagent molecule has been oriented at the direction of a transition state for the exit at the instant of photodissociation. (3) The temperature threshold of Cl exit from the cage is 95 K, and essentially coincides with onset of free rotation for the reagent molecules in the host crystal. (4) The temperature dependence of the probability for cage exit is strongly nonmonotonic: The probability increases as T increases from 95 to 110 K, falls off to 0 around 125 K, then increases again as T approaches melting. (5) At the photodissociation energy used, the only site that the Cl atoms occupy in the new cage is the octahedral interstitial site. Various aspects of reaction dynamics in crystalline solids are discussed in the light of the above results and by their comparison with findings of a previous study on photodissociation of HI in Xe.