Energy-and angle-dependent excitation probability of sputtered metastable silver atoms

We have investigated neutral silver atoms sputtered from a polycrystalline silver surface under bombardment with keV rare-gas ions by resonant multiphoton ionization in combination with time-of-flight mass spectrometry. Using state-selective ionization schemes, metastable excited atoms in the 4d(9)5s(2)(D-2(5/2)) State (excitation energy 3.75 eV) were detected as well as ground-state atoms. The total population and the velocity distribution of atoms sputtered in both states were determined from the saturation behavior of the photoionization process and the flight time distribution of the ejected neutral species between the ion-bombarded surface and the ionization volume. We find total excitation fractions of several percent, which are only weakly dependent on the sputtering conditions such as the nature or energy of the primary ions. Furthermore, we show that metastable atoms are ejected with lower average velocity than those emitted in the electronic ground state, thus indicating an excitation mechanism populating this state which becomes less efficient with increasing emission velocity. From the polar emission angle dependence of the measured energy spectra, it is inferred that the. excitation probability is not simply related to the normal component of the emission velocity.