Femtosecond multi-photon photoemission of small transition metal cluster anions

Photoelectron spectra have been measured for small Pt-n(-) and Pd-n(-) clus ters (n=1-7) by using femtosecond single laser pulses of 3 eV and 1.5 eV ph oton energy, respectively. The spectra reveal multi-photon processes which result in a continuously distributed electron signal at binding energies be low the electron affinity of the clusters. The intensity of the multi-photo n electron signal normalized to the one-photon signal in the spectra record ed with 3 eV increases Linearly with the laser peak power indicating a seco nd order process. Using 1.5 eV photons, photoelectrons from two-, three- an d four-photon processes have been observed for Pt-3(-). (C) 2000 Elsevier S cience BN. All rights reserved.