Femtosecond multi-photon photoemission of small transition metal cluster anions

Photoelectron spectra have been measured for small Ptn- and Pdn- clusters (n=1-7) by using femtosecond single laser pulses of 3 eV and 1.5 eV photon energy, respectively. The spectra reveal multi-photon processes which result in a continuously distributed electron signal at binding energies below the electron affinity of the clusters. The intensity of the multi-photon electron signal normalized to the one-photon signal in the spectra recorded with 3 eV increases linearly with the laser peak power indicating a second order process. Using 1.5 eV photons, photoelectrons from two-, three- and four-photon processes have been observed for Pt3-.