Abstract
Secondary ions produced by 400 eV He+ ion impact on Ar, Kr, and Xe thin films deposited on a silicon substrate at 8 K were measured as a function of film thickness using a time-of-flight secondary-ion mass spectrometer. For Ar, the cluster ions Arn+ up to n=4 as well as the monomer ion (n=1) were observed and they showed characteristic film-thickness dependence with increase of the film thickness up to ~200 monolayers. This is due to the momentum transfer in the elastic collision between the incident He+ ion and the matrix Ar atoms and also to the relaxation of electronic excitations (e.g., holes and excitons) to phonons resulting in the film erosion. In contrast, neither dimer nor cluster ions were detected for solid Kr and Xe films. This is due to the less efficient momentum transfer in the elastic collision between He+ and Rg's (Rg=Kr and Xe) and also to the efficient electronic energy migration in solid Kr and Xe. The C+ ions originating from the carbonic contaminants on the silicon substrate were found to be sensitized by the deposition of rare gas films. The penetration depths of the primary ion He+ through the rare gas films increase in the order of Ar<Kr<Xe.
