Probing the stopping power near the surface by specular reflection of protons from graphite

The energy loss of fast (v ≅ vBohr) protons specularly reflected from a graphite-surface is measured as a function of incidence angle and projectile velocity. The data show that two contributions to the energy loss and, thus, two electronic stopping powers S1 and S2 near the surface may be distinguished. The predominant stopping power S1 is roughly proportional to the velocity, in agreement with theoretical predictions; it decreases with increasing distance from the atomic surface with a decay length of 0.17 nm. The stopping power S2 is proportional to the reciprocal of the velocity; it is only effective within a small interval of distances outside the atomic surface, namely between about 0.2 and 0.4 nm. The energy loss does not depend noticeably on the charge state of the incident projectiles. (orig.)