Electronic excitation in sputtered atoms and the oxygen effect

Abstract

Bombardment of surfaces by ions gives rise to a variety of inelastic collision events leading to the ejection of excited atoms and ions. Such excited sputtered particles have been studied since more than 80 years through their optical emission, when they decay in front of the target to the electronic ground state, having lifetimes of 10 −9 to 10 −7 s, typically. Information on the energy distribution of such excited states can be obtained by two different techniques: light vs distance measurements (LvD) and by studying line profile broadening in light emission due to the Doppler effect. Only recently it has become possible to study in addition metastable excited atoms using laser induced fluorescence spectroscopy (LIF). Relative sputtering yields and energy distributions have been measured for such metastable states and two types can be distinguished. States with a very low excitation energy (0–0.3 eV), being sublevels of the electronic ground state, were found to have yields and energy distributions comparable to the electronic ground state, while metastable states at higher excitation energies (above 1 eV) seem to behave similar to short lived excited states, typically observed in secondary photon emission (BLE) with excitation energies in the range of 2–6 eV. This behaviour is also clearly visible with respect to oxygen surface coverage or increased near surface oxygen concentration where, similar to secondary ion emission, drastic changes in the yield by orders of magnitude have been found for excited atoms as well as for ions. In addition, under the same conditions a strong decrease in the sputtering yield of neutral ground state atoms has been observed for a number of metals. LIF results for highly excited metastable states are compared with recent results obtained by studying line profile broadening in light emission for Ca, Al and Cr targets. Different mechanisms that have been proposed to account for the observations will be discussed.