A microscopic, theor., and exptl. description of an elementary solid state photoreaction, i.e. the dissocn. of a small mol. in a rare lattice is presented. The ingredients and results of mol. dynamics calcns. for the dissocn. of F2, Cl2, HI, and H2O mols. are discussed and the concepts of cage effect, impulsive and delayed exit are illustrated. Examples for the detn. of the structure and dynamics of and products in the matrix from ESR, IR and neutron expts. are combined with exptl. investigations of the barrier heights of the cage and the dependence of the dissocn. efficiency on excess energy, deuteration, guest-matrix pair potentials and temp. A long range migration of F fragments is demonstrated and applied to produce XeF mols. in rare gas crystals. The concept of solid state excimer lasers and the achieved large gain coeffs. and stored energy densities in the UV spectral range are discussed. 61 Refs. [on SciFinder (R)]