Ground state and excited state sputtering Doppler-shift laser-fluorescence studies of Cr and Ca targets

The recently much debated problem of excited state sputtering has been investigated for Cr and Ca targets using Doppler-shift laser-fluorescence spectroscopy (DSLFS). Using this sensitive and level selective method, sputtered densities for Ca and Cr atoms in the electronic ground and excited metastable states have been investigated as a function of oxygen background pressure. Furthermore, their velocity distributions have been measured for increasing oxygen coverage. In general the population of the ground state during sputtering of Cr and Ca decreases drastically by a factor greater than 50 for Cr or 20 for Ca, respectively, when the clean target is exposed to an oxygen partial pressure between 5×10−8 and 1×10−6 Torr. At the same time the maximum of the velocity distribution shifts from about 2 km/s to values between 3 and 5 km/s for Cr as well as for Ca. Thus, the velocity distribution of the Ca ground state becomes similar to the one obtained for metastable states of Ca. In addition, the light of short-lived excited states produced during the sputtering process has been monitored for different transition lines of Ca and Cr for different oxygen pressures. A quartz microbalance was used to monitor relative sputtering yields as a function of the oxygen partial pressure. For high oxygen coverage the total sputtering yield for Ca and Cr decreases by a factor of about 3. The contribution of charged particles remains below 20%. The majority of sputtered products are molecules or clusters.