The H fragment produced in the photodissocn. of water (H2O -> H + OH) is lighter than the lattice atoms and a local heating occurs over long time scales and large spatial regions. Therefore, use of a continuum model for the lattice is justified. The local heating is promoted by a decreasing heat cond. with increasing temp. Solns. of a nonlinear heat cond. equation show that the heat release inside the cage leads to a temp. increase in its nearest surroundings up to the m.p. in argon. Melting enables the light fragment to overcome the cage barrier. In the case of krypton, the thermal effect is less pronounced, while in Xe, there is none. The above results are in qual. agreement with exptl. data. [on SciFinder (R)]