Dynamics of molecular reactions in solids: Photodissociation of F2 in crystalline Ar

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Abstract
Classical molecular dynamics simulations of F2 photodissociation in a host Ar crystal are presented. At temperature T=12 K, the photodissociation yield shows a sharp threshold for an excess energy of ∼0.6 eV, and it reaches nearly unity for excess energies above 2 eV. For a given excess energy, the quantum yield at 4 K is higher than a 12 K, and is predicted to remain finite even at 0 K. The transition state for photofragment exit from the reagent cage is found to be located in well‐defined windows in the unit cell of the surrounding solid. The quantum yields (or photodissociation probabilities) are extremely high, especially at low T, in comparison with the values found in previous studies, e.g., for Cl2 in Xe and in Ar. Indeed, for high excess energy the near‐unit quantum yields indicate the virtual absence of an inhibiting cage effect on the reaction. The anomalous behavior of F2 in Ar is attributed to the short effective range of the repulsive F/Ar interaction, which enables the F atom to exit the cage and migrate in the crystal. It is also due in part to the F/Ar attractive potential, which is found strong enough to focus and stabilize the migration of the F product in ‘‘channels’’ within the lattice, following photolysis. Classical trajectories show long‐range migration of the product atoms, of the scale of 30 Å, following the initial impulse provided by the photodissociation. This is the first system for which such long‐range impulse‐induced migration was found. The results of the simulations are analyzed focusing on the role of the initial state of F2 in the crystal, on the final sites occupied by the product atoms, and on the migration dynamics. Implications of the results for mechanisms of reactions in solids are discussed.