ABSORPTION-SPECTRA OF SIZE-SELECTED SOLVATED METAL-CATIONS - ELECTRONIC STATES, SYMMETRIES, AND ORBITALS IN SR+(NH3)1,2 AND SR+(H2O)1,2

This paper presents photodissociation spectra for the solvated metal ion clusters Sr+ (NH3)1,2 and Sr+ (H2O)1,2 from 420 to 740 nm in the visible region of the spectrum. The spectra have a banded structure, corresponding to transitions from ground electronic states based upon the S2 configuration of the Sr+ ion to excited states based primarily upon p-orbitals of the excited Sr+. The photodissociation cross sections are large, approximately 10(-17)-10(-16) cm2. For the same solvent, spectral band positions are only weakly dependent upon the degree os solvation. We show that a dramatic reduction in intensity of the second excited state band in the Sr+ (NH3)2 spectrum suggests that this state has strong atomic ion d-orbital parentage and that the molecule is centrosymmetric. Photodissociation of the H2O solvated species proceeds through three excited electronic states corresponding principally to three different orientations of the metal p orbitals with respect to solvent symmetry axis. Absorption band positions for Sr+ (H2O)2 are shifted slightly from those of Sr+ (H2O) and the presence of a substantial unstructured continuum appears in the doubly solvated ion. The absorption spectra for the Sr+ (H2O)1,2 species are significantly blue-shifted and narrowed relative to those of Sr+ (NH3)1,2, an observation that is understood through simple molecular orbital diagrams incorporating the fact that the ionization potential of H2O is 2.4 eV larger than that of NH3.