Minimum in the distribution of electrons emitted from solids in the backward direction

We have investigated the electron emission from carbon and aluminum foils in the backward direction induced by proton impact with energies in the range 50-250 keV. Doubly differential angular emission spectra were obtained for several electron energies covering the angular range extended from 160 deg. to 200 deg. A minimum in the electron angular distribution centred at 180 deg. was observed for slow electrons and it disappears with increasing electron energies. We have interpreted these results as a combination of three effects: the excitation of single conduction electrons in the target, the transmission of the excited electrons across the surface barrier and the effect of the image force acting on the emitted electrons when they are in the vacuum