Multiphoton Ionization Spectroscopy of AlAr_N Clusters

Abstract

Experimental and theoretical studies are reported of the multiphoton ionization spectroscopy of selected AlAr_N clusters (N = 2−54). Resonantly enhanced 1_(uv) + 1_(vis) and 2_(vis) + 1_(vis) ionization spectra are recorded of neutral clusters employing a laser-ablation/pulsed supersonic expansion source and time-of-flight mass spectrometric cluster-ion detection. The spectra are dominated by broad red- and blue-shifted asymmetric bands in the neighborhood of the 308 and 303 nm atomic Al 3p → 3d and 4p lines. The detailed structures of these bands and the observed degree of their spectral shifts with increasing cluster size are attributed on the basis of concomitant ab initio theoretical calculations to interplay among a number of factors, including (i) the comparable strengths of spin−orbit-split anisotropic (^2P_(1/2))Al−(^1S_0)Ar interactions and Ar−Ar mutual attractions, responsible for predicted external-site Al atom locations on distorted icosahedral Ar_N structures, (ii) avoided crossings in the nearly degenerate AlAr_N potential energy surfaces accessed by one- and two-photon atomic Al 3p → 3d and 4p excitations, giving rise to the red- and blue-shifted spectral profiles, and (iii) significant dynamical rearrangement and parent cluster-ion fragmentation following ionization, resulting in Al+Ar_M signals that generally reflect the absorption cross sections of an ensemble of larger prior clusters (AlAr_N, N > M). Additionally, nonuniformity in the cluster-size distribution of the incident molecular beam is inferred from the calculated and measured spectra and must be incorporated in the development for a completely satisfactory accounting between theory and experiment. Comparisons with the results of earlier experimental studies of the ionization potentials of AlAr_N clusters also underscore the importance of dynamical parent-ion rearrangement and fragmentation, consequent of the increased Ar solvation of the Al+ radical in the equilibrium Al+Ar_M cluster-ion structures. The reported multiphoton ionization cluster-ion spectra are evidently highly sensitive to the details of the atomic Ar arrangements around the Al chromophore and accordingly provide a spectroscopic probe of the nature and evolution of the Al trapping sites and cluster geometries with increasing cluster size when the complex electronic and vibrational phenomena underlying the measurements are appropriately interpreted.