Photoionization studies of chromium clusters: Ionization energies of Cr₄ to Cr₂₅

The photoionization spectra of isolated chromium clusters from Cr₄ to Cr₂₅ 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 Cr_nO and Cr_nO₂ species, recently reported by Wang et al. [Appl. Phys. B 73, 417 (2001)], with the largest differences observed for several of the smaller clusters. The E_i^v determined for Cr₄ was 0.7 ev larger than that found previously for Cr₄O, suggesting that in this case the bare cluster and oxide have different structures.