The wavelength dependence of photoinduced dissociation of CH3Br via dissociative electron attachment (DEA) of ＇＇hot＇＇ electrons for one monolayer CH3Br adsorbed on GaAs(110) has been measured. The cross section for dissociation is found to decrease monotonically by two orders of magnitude as the incident wavelength is varied from 308 to 550 nm. There is an apparent threshold near 490 nm (2.5 eV), well below the gas phase photodissociation threshold near 250 nm (5.0 eV), but in good agreement with a simple estimate based on expected values for the decrease in the photoemission threshold and the lowering of the molecular affinity level upon adsorption of CH3Br on a semiconductor surface. The observed threshold is found to move to higher energy as dissociation of the monolayer proceeds. Based on the work of Hasselbrink and co-workers [F. Weik, A. de Meijere, and E. Hasselbrink, J. Chem. Phys. 99, 682 (1993)], a simple theoretical model is developed which considers the tunneling of hot electrons through the interfacial barrier between the physisorbed CH3Br and the GaAs. The results of our theoretical model in conjunction with those of earlier ab initio calculations [S. Black, R. Friesner, P. H. Lu, and R. M. Osgood, Jr., Surf. Sci. 382, 154 (1997)] suggest that the adsorbate affinity level is centered at similar to 0.6 eV above the (adsorbate-modified) vacuum level of the substrate. This value corresponds to a similar to 1.8 eV stabilization of the negative ion resonance upon adsorption.