The cluster represents the region of a solid close to the emitter. The solid is assumed to extend towards negative values of z. The electronic edge is taken as the value of z below which the photoelectron can suffer inelastic losses. Therefore, the electron inelastic mean free path will apply while the photoelectron is moving inside the solid, that is, for z below the electronic edge. Besides, the photoelectron suffers refraction when it crosses the electronic edge, and this is accounted for by a transmission factor and a change of angles of emission as described by a step potential of height V0 (see inner potential).