Vacuum ultraviolet single photon versus femtosecond multiphoton ionization of sputtered germanium clusters.

Neutral atoms and clusters desorbed from a solid germanium surface by ion bombardment are detected by laser postionization and time-of-flight mass spectrometry. Two different photoionization schemes are compared which are generally believed to be candidates for the 'soft' ionization of polyatomic species without significant photon induced fragmentation. First, a single photon ionization process is employed using an F2 laser as an intense VUV source with a photon energy in excess of all relevant ionization potentials. It is shown that the available laser pulse energy is sufficient to saturate the ionization of Ge atoms and all detected Ge(n) clusters. The resulting mass spectra are compared to those obtained with a non-resonant multiphoton ionization process using a high intensity laser delivering pulses of 250 femtoseconds duration at a wavelength of 267 nm. Also in this case, the ionization process can apparently be driven into saturation. The mass spectra measured under these conditions are found to be almost identical to those obtained using single photon ionization. We take this as an indication that the results obtained with both postionization techniques closely reflect the true cluster sputtering yields and, in particular, are not dominated by photon induced fragmentation.