The 248 nm initiated dissociation dynamics of methyl iodide physisorbed on GaAs (110) is investigated using 2+1 REMPI detection of the CH3 umbrella mode. The velocity and vibrational distributions of the photofragment are dependent on the adsorbate coverage and surface termination. Two translational energy distributions are measured for the CH3 fragment at high coverage, i.e., 25 ML, with maxima at 1 and 1.9 eV. These maxima are within 0.5-0.7 eV of those obtained in direct dissociation of gas-phase CH3I. At 1 ML one distribution is measured with its maxima at 0.3 eV. A modification of the surface with chemisorbed iodine reduces the yield of the CH3 at 1 ML but not at 25 ML. The dissociation dynamics observed at 1 ML is attributed to bond scission following electron attachment while that at 25 ML to direct photodissociation. At 3 ML coverage both the 1 and 25 ML components are observed. The measured fragment vibrational distribution at 1 ML has a 4 times lower v"=1/v"=0 ratio compared with that at 25 ML. Differences in the translational and vibrational distributions for the two dissociation mechanisms are attributed to the available energy and molecular geometry of photoexcited CH3I. (C) 2003 American Institute of Physics.