Dependence of ion-electron emission from clean metals on the incidence angle of the projectile

We have studied the dependence of electron yields γ from clean Cu and Au surfaces on the incidence angle theta of 5--50-keV He+, Ar+, and Xe+ projectiles, in the angular range 0--80 0, and under ultrahigh-vacuum conditions. We have found that, at small angles, γproportionalsec/sup f/theta, with f generally different from unity. For Xe+ on Cu, γ(theta) presents an energy-dependence maximum, similar to that obtained for sputtering. The results are explained in terms of the anisotropy of the electron cascade in the solid, the depth distribution of the inelastic energy deposited by the projectile and by rapidly recoiling target atoms in the near-surface region of the solid