The quantum efficiency value is determined by studying the surface potential development in chalcogenide vitreous semiconductors. Sensitization by a corona discharge in response to exposure to radiation from a Q-switched nanosecond-pulse ruby laser is achieved. A method to calculate the quantum efficiency is presented, and its dependence on parameters such as the exposure moment switch, temperature, charging electrode potential, and radiation energy density is studied. The mobility of the holes is determined by the time of flight of carrier packets. The observed effects are explained using the residual conduction mechanism.