Abstract
The modification of photodissociation mechanisms of small molecules by adsorption on a solid or by condensation is briefly reviewed. The coupling to the surroundings leads to quenching of easily delocalizable primary excitations so that only strongly localized channels show up in dissociation (detected by photodesorption, i.e. by the appearance of fragments in the gas phase). In reverse, fast direct dissociation channels will be less in need of such localization and will be preferred over slow channels. Investigation of dissociation induced by core excitations or core shake-ups introduces an internal time mark—core life time—to which reaction time can be compared. The example of small molecules, such as CO or NO, adsorbed on transition metal surfaces is surveyed, in which complex multiple excitations predominate in dissociation. Recent results on the core-induced dissociation of condensed hydrogenic molecules such as water, ammonia, and benzene have shown the existence of preferential dissociation channels in the core-to-bound region. It is argued that these dissociation processes are at least competitive with core decay; they are termed ultrafast. It appears that they should also exist in the free molecules.