The adsorbate- and the substrate-mediated chemical processes induced by ultraviolet (UV) laser pulses have been identified for the CH2I 2/Ag system. In contrast to CH2I2 on Al 2O3, little photodissociation or desorption is observed for the molecules adsorbed on an Ag surface at submonolayer coverages. Photochemical effect is evident at coverages higher than a monolayer. Explosive desorption dominates at high surface coverages and high laser fluences. The explosive behavior is elucidated and its implications relevant to material ablation by lasers is discussed. © 1989 American Institute of Physics.