Infrared spectroscopy has been coupled with the matrix isolation technique. First, to identify the ozone dimer isolated in solid argon, and second, to investigate the mechanism of its photodissociation at 11 K using the 266 nm laser line. Identification of dimers, which are trapped in two different sites, was performed through the observation of the nu (3) asymmetric stretching modes. By absorption of one photon, dimers are photodissociated with a high photolysis rate when compared with that of ozone monomers, suggesting the absence of cage recombination of the photofragments. Experimental observations suggest that the major pathway after photolysis is the formation of an excited transient species O-4*, which fragments into O+O-3. A two-step mechanism, involving the subsequent reaction of O(D-1) with O-3 formed in a first step appears unlikely. (C) 2001 American Institute of Physics.