Effect of projectile parameters on charge state formation of sputtered atoms

In the majority of the electron-exchange models, it is assumed that the charge state formation processes of atoms sputtered (scattered) from a solid do not depend on projectile parameters. This means that an electronic subsystem excitation produced by projectiles in the impact region does not affect on the ionization probability P+ of ejected atoms. In the present work this basic assumption has been subjected to experimental examination. To study it the clean surface of a silicon sample was bombarded by the following atomic and molecular projectiles: Al- ions with the energy of E-0=9 and 18 keV; Au- ions with E-0=18 keV; Al-2(-) ions with E-0=4.5 and 9 keV/atom. The kinetic energy distribution f(E) of sputtered Si+ ions were used to obtain information on P+. It was found that the ionization probability P+ depends on the projectile parameters increasing with the rise of energy E-0, mass M-0, and the number m of atoms in projectile. The results obtained are discussed in the framework of the simple model in which the charge state formation occurs in the electron exchange between the sputtered atom and a local surface area where the relaxation of the electronic subsystem excited by the projectile impact takes place.