Photoionization of Na, Cs, Ca and Ba oxide clusters

Abstract

Unusually high ionization energies have been observed for Cs–O clusters having certain sizes and composition, namely for Cs_{2n + z}O_n with z= 8, 18, 34, 58 and 92. The anomalies are well defined for clusters containing from 1 to 7 oxygen atoms. The indicated values of z are identical to the number of electrons in closed shells of angular momentum. Mass spectra of Na_z(Na₅O₂)⁺ clusters drop sharply at z= 34, 40, 58, 92 and 138. Na₁₃₈(Na₅O₂)⁺, for example, apparently has the electron configuration 1s, 1p, 1d, 2s, 1f, 2p, 1g, 2d, 1h, 3s, 2f, 1i, 3p. Mass spectra of metal-rich, singly and doubly ionized Ba_nO_x clusters are characterized by high peak intensities for n= 13, 19, 23, 26, 29, 32 and 35 independent of the number of oxygen atoms (x= 1–4). These values are identical to the magic numbers found for Ar clusters. Finally, mass spectra of stoichiometric, doubly ionized [Ca(CaO)_n]²⁺ clusters are characterized by low peak intensities for n= 14, 24, 32, 38, 40, 50 and 64. These values correspond to unstable clusters formed by the addition of one Ca atom or one CaO molecule to a closed rectangular block of atoms with the rock-salt structure.