Measurements of the population partitions and state-selected flight-time distributions of keV ion-beam-sputtered metastable atoms - art. no. 195417

A sensitive and generally applicable experimental procedure to determine th e relative population on different quantum states of free metastable atoms is presented. The atoms are both element and state-selectively ionized usin g highly efficient double-resonant two-color two-step laser ionization. Pop ulation partitions can be quantitatively measured, by extending the procedu re to include all metastable states. In combination with a pulsed atom sour ce, state-selective flight-time distributions can be measured. Such experim ents allow one to evaluate the kinetic energy of the released atoms. The me thodology is demonstrated and fully commented with the measurement of the p opulation partition of gas-phase atoms released from a hot Ni filament. A s ystematic study of the emission of neutral atoms during 15 keV Ar+ sputteri ng of clean polycrystalline Co and Ni foils is reported, illustrating that sets of population partitions and flight-time distributions can be obtained with the presented experimental procedure. The combined interpretation of the experimental evidence allows one to qualitatively model the emission of excited atoms during sputtering of metals as the result of a multichannel electron transfer process. In this model, a sputtered particle escapes from the surface as a positive ion and becomes neutralized into an atomic state by resonant transfer of an electron from the valence band of the metal. Th e velocity-dependent population of a particular state is governed by the co rrespondence of the electronic configuration of the atomic state with the b ulk electronic configuration.