A theoretical model is presented to examine recently reported steric effects in electron-adsorbate interactions. The model is based on a short-lived anionic excited state formed by vertical photoelectron transition. An initial wave packet propagates on the excited-state potential and is quenched back to the ground state after a short residence time. The acquired momentum is the origin for desorption and dissociation. It is shown by quantum time-dependent modeling of the process that the orientation of the molecule has a profound effect on the kinematics of the photodesorption. The Br-up configuration is calculated to have a much larger desorption cross section. It is predicted that the adsorbed molecule tilt angle with respect to the surface normal is larger for the Br-up configuration.