Multiphoton Ionization Studies of Metal Atom-Solvent Interactions from the Van der Waals Dimer to the Mesoscopic Scale

Author: Spotts, James Michael

Year: 1998

Degree: Dissertation (Ph.D.)

Advisor: Okumura, Mitchio

Committee Members: McKoy, Basil Vincent; Okumura, Mitchio; Bercaw, John E.; Zewail, Ahmed H.

Option: Chemistry

DOI: 10.7907/n39e-bw61

Abstract

A unified molecular-level description of solute-solvent interactions within condensed-phase systems has remained elusive despite recent advances towards the detailed understanding of the fundamental electrostatic interactions between the atomic and/or molecular moieties that compose a bulk system. Through the use of simplified cluster model systems such as those employing metal atoms solvated by a rare gas solvent, however, microscopic details pertaining to the intrinsic composite nature of the atomic orbital interactions can be effectively dissected away from the complexity inherent to condensed-phase systems.

This thesis work presents a systematic spectroscopic investigation of Al·Arn (n ≤ 60) clusters generated by standard laser ablation techniques using 1'uv + 1vis two-color and 2vis + 1vis single-color resonance-enhanced multiphoton ionization (REMPI) in the vicinity of the Al(3d) ← Al(3p) atomic transitions. UV absorption spectra were collected for Al·Arn cluster sizes corresponding to solvation within the first icosahedral salvation shell (1 ≤ n ≤ 12) as well as the second icosahedral solvation shell (13 ≤ n ≤ 54). These spectra exhibited significant red-shifted absorption features whose absolute peak positions were noted to be highly sensitive to the degree of solvation. Such behavior was strongly suggestive of an aluminum atom surface binding site. Nevertheless, classical Monte Carlo simulations failed to reproduce the observed magnitude of the spectral shifts. Consequently, the underlying origin of this anomalous spectral behavior was believed to derive from electronic state mixing interactions between those states originating from the Al(3d) and Al(4p) electronic manifolds.

Evidence for such a coupling was sought at the level of the diatomic Al·Ar molecule using both 1'uv + 1vis and 2vis + 1vis REMPI to characterize all electronic states arising from the Al(3d)·Ar and Al(4p)·Ar asymptotes. Strong evidence for state mixing was observed among the 2∏(4p), 2∏(3d), and 2Σ+1/2(3d) diatomic electronic states. With increasing solvation, this coupling was believed to result in the formation of aluminum 3d/4p hybrid orbitals that permitted aluminum electron density to be drawn away from the cluster surface. The concomitant introduction of partial ionic character directly adjacent to the cluster surface provided a net stabilizing effect that increased upon further solvation.

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