Ion spectra from slow collisions of He+with He, Ne, Ar, and of Ne+with Ne

An ion spectrometer is described, which allows the doubly differential analysis of all ions emerging from the scattering centre of a projectile ion beam and a thermal target gas beam. The transition of the spectrometer is high enough as to make possible also the analysis of multiply charged ions formed by ionization of the projectile, as well as of singly and multiply charge recoil-ions formed from the target gas. The energy resolution obtained is Delta E/E=5%. For collisions of He + with He, Ne, Ar, and of Ne + with Ne, at 1 keV collision energy and a detection angle of theta =8 degrees (with respect to the projectile beam direction) total ion spectra are shown, and most probable energy losses-corresponding to peaks or structures of the ion spectra-are given and discussed. It is found that all dominant excitations and ionizations of the final collision system can be explained by initial excitation of one or two electrons of the outer shell of the molecule formed during collision, where one- and two-electron excitations are of the same order of importance. This result is in qualitative agreement with predictions of the Fano- Lichten electron promotion model for outer shell excitations. In the case of the He + /Ar collision system the ion spectrum shows that charge exchange processes leading to neutral He-atoms dominate the elastic and inelastic scattering of He + .