The UV photodissociation dynamics of CH3I multilayers on GaAs(1 1 0) are studied using 2 + 1 resonant enhanced multiphoton ionization (REMPI) detection of the CH3 nu(2) umbrella vibrational mode. The vibrational distribution formed appears much colder than the distribution resulting from gas-phase photodissociation of CH3I. Measurements of the surface reaction cross-section versus photon exposure and time-of-flight (TOF) studies of the CH3 translational energy suggest that the colder temperature is attributable to dissociative electron attachment (DEA) by photoemitted substrate electrons. (C) 2002 Published by Elsevier Science B.V.