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 energ y and angles, are presented for collisions. of singly through quadrupl y charged C ions impinging on a Si(100) surface, with ion energies bet ween 3.6 and 6.0 MeV and grazing angles of incidence between 0.1 degre es and 1.0 degrees, respectively. The electron emission spectra measur ed under these conditions confirm previous reports of a shift of the c onvoy electron peak toward higher energy due to surface wake or image acceleration. Detailed angular distributions demonstrate a shift of co nvoy 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 s hift to depend upon the incident ion charge state for the higher proje ctile velocity (similar to 4.5 a.u.). This dependence may be understoo d as a consequence of incomplete projectile charge equilibration at th e time of convoy electron production. The charge-state dependence is n ot observed at the lower collision velocity (similar to 3.5 a.u.).