Pulsed 266 and 355 nm ultraviolet laser irradiation of monolayer vinyl chloride physisorbed on Ag(111) results in molecular dissociation leading to C2H3 and Cl, much of which is adsorbed to the surface. On the basis of observations made on dissociation dependences on chlorine isotope and photon energy, it is deduced that upon excitation vinyl chloride forms a transient negative ion through a substrate mediated, vertical electron attachment mechanism. The anion either dissociates or relaxes through energy transfer to the neutral state causing the neutral molecule to desorb. The threshold for vertical attachment of substrate electron is estimated to be 0.8 eV below the vacuum level, in agreement with the experimentally observed wavelength dependence in photoinduced dissociation. Chemisorbed Cl on the Ag( 111) surface inhibits the photodissociation process by increasing the substrate work function and consequently the energy threshold for electron vertical attachment. Upon heating the Ag( 111) surface, adsorbed vinyl combines to produce 1,3-butadiene in a first order, diffusion limited, process with an activation energy of 10.4 kcal/mol.