Investigation of time-of-flight and energy distributions of atoms and molecules sputtered from oxygen-covered metal surfaces by laser-aided techniques

Time-of-flight measurements of atoms and molecules sputtered from oxygen-covered metal surfaces (titanium, tungsten and tantalum) by 6-keV argon ions are reported. Two-color resonant multiphoton ionization is applied for the detection of titanium, tungsten and tantalum atoms, using a resonant excitation step around λ=230 nm. The particles are ionized by visible radiation. Atomic oxygen is detected with the aid of a single-color ionization scheme by application of a two-photon resonant excitation step. Metal oxide molecules are detected by non-resonant ionization processes. While the energy distribution of atoms sputtered from ion-beam-cleaned surfaces is found to be close to the prediction of the Sigmund–Thompson model, strong changes of the energy spectra are observed for the case of oxygen coverage. The energy distribution of titanium atoms sputtered from an oxygen-covered surface is reasonably well described by a Thompson energy distribution but with a higher value for the surface binding energy as compared to a sputter-cleaned surface (in agreement with previous observations). However, the description by a Thompson-type energy distribution fails for oxygen-covered tungsten and tantalum surfaces. The energy distributions are strongly depleted in the low-energy part in this case and do not resemble a Thompson spectrum any more.