Internal energy of sputtered metal clusters

The internal excitation of silver clusters sputtered from a polycrystalline silver surface by Ar⁺ ions of 5 keV was investigated by laser postionization of the ejected neutral species and time-of-flight mass spectrometry. For sputtered dimers, resonant two-photon ionization spectroscopy was employed using either a one- or a two-color scheme. From the resulting spectra, vibrational and rotational population temperatures around 2700 and 6700 K are determined which agree well with corresponding predictions obtained from molecular-dynamics simulations of the molecule sputtering process. For larger clusters, single-photon ionization by a fixed frequency laser was used for cluster detection and the spectra obtained with different laser wavelengths were compared. The results show that around 50% of the sputtered Ag₆ clusters are formed with internal energies in excess of 0.75 eV.