Charge-state dependence of image-charge acceleration of convoy electrons in fast, grazing collisions of carbon ions with a silicon (100) surface

Electron emission distributions, triply differential in emission energy and angles, are presented for collisions of singly through quadruply charged C ions impinging on a Si(100) surface, with ion energies between 3.6 and 6.0 MeV and grazing angles of incidence between 0.1{degree} and 1.0{degree}, respectively. The electron emission spectra measured under these conditions confirm previous reports of a shift of the convoy electron peak toward higher energy due to surface wake or image acceleration. Detailed angular distributions demonstrate a shift of convoy emission towards larger angles with respect to the surface plane, which is further evidence of the acceleration of emitted electrons by the surface wake potential. In addition, we observe the peak energy shift to depend upon the incident ion charge state for the higher projectile velocity ({approximately}4.5 a.u.). This dependence may be understood as a consequence of incomplete projectile charge equilibration at the time of convoy electron production. The charge-state dependence is not observed at the lower collision velocity ({approximately}3.5 a.u.). {copyright} {ital 1996 The American Physical Society.}