Effect of the electronic subsystem excitation on the ionisation probability of atoms sputtered from metals by atomic and molecular projectiles

In the present work an effect of excitation of the metal electronic subsystem on the ionisation probability of atoms sputtered under fast ion bombardment has been studied. Atomic and molecular primary ions with the same velocity were used to produce different degrees of the electronic excitations. Information on the ionisation probability was obtained from the kinetic energy distributions of Nb+ and Ta+ ions sputtered from the respective clean Nb and Ta targets by Aum− projectiles (1⩽m⩽3) with the energy of E0=6 keV per atom. It was found that, as compared with the atomic ion bombardment (m =1), the molecular one (m=2,3) leads to the increase of the ionisation probability Pm+ (P1+<P2+<P3+). Such an effect depends on the kinetic energy E of the secondary ions, increasing with decreasing E. It was shown that the bombardment of metals by the molecular projectiles produces non-additive sputtering of atomic ions, which is determined by the joint action of such two factors as non-additive sputtering of atoms and non-additive process of their charge state formation. The results obtained are discussed in the framework of the model where the charge state formation occurs in electron exchange between sputtered atoms and a local surface area excited by the impact of the projectile.