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 lambda = 230 nm. The par tides 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.