Photoionization studies of chromium clusters: Ionization energies of Cr-4 to Cr-25

The photoionization spectra of isolated chromium clusters from Cr-4 to Cr-25 have been recorded. The vertical ionization energies (E-i(v)) derived from the spectra decrease discontinuously with increasing cluster size. The E-i(v) values determined for Cr-n species were, in most cases, within 0.2 eV of those of the corresponding CrnO and CrnO2 species, recently reported by Wang [Appl. Phys. B 73, 417 (2001)], with the largest differences observed for several of the smaller clusters. The E-i(v) determined for Cr-4 was 0.7 ev larger than that found previously for Cr4O, suggesting that in this case the bare cluster and oxide have different structures.