Steady-state and time-resolved emission spectroscopy of the A-X system of Cl2 is used to distinguish molecular chlorine isolated as monomers and clusters in solid argon samples. The lifetime of the metastable A' 3-PI-2u state is measured to be an order of magnitude less, when chlorine is present as clusters, than when it is truly isolated. Photodissociation of molecular chlorine clusters was found to be insignificant as monitored by the emission of Ar2Cl at 260 nm. Measurement of the dissociation threshold of molecular chlorine in the 9 eV region as a function of temperature showed little variation. Using spectroscopic data it is concluded that dissociation is occurring by an impulsive mechanism involving curve-crossing from the initially populated Ar+ (Cl2)- charge transfer state to repulsive potentials correlating with ground state atomic chlorine and not via a harpooning mechanism. A simple microscopic model, drawn from experimental data and pairwise addition of ArCl potential terms, is constructed to describe the steps involved in this dissociation process in the solid lattice.