Effect of hydrogen chemisorption on the photoionization threshold of isolated transition metal clusters

Abstract

Large increases in the photoionization threshold energies of small V x , NB x , and Fe x clusters ( x = 3–25) induced by chemisorption of H 2 have been observed using photoionization time-of-flight mass spectrometry of a molecular beam. These shifts exhibit a definite dependence both on the number of atoms constituting the bare metal cluster and on the number of chemisorbed hydrogens, and are particularly large (≳ 0.8 eV) for multiple-H 2 chemisorption on small clusters. A simple frontier orbital model for the chemisorption process predicts the direction of adsorbate-induced shifts in cluster ionization threshold for both H 2 and NH 3 as adsorbates.