Photofragmentation spectra and structures of Sr+Arn, n=2-8 clusters:: Experiment and theory

The total photofragmentation cross sections of size selected Sr+ Ar-n, n = 2-8, clusters have been recorded in a time of flight (TOF) mass spectrometer, In the energy range of 21 000-27 000 cm(-1) three absorption bands are observed that are attributed to transitions from the ground to the three excited stales that correlate to the 5p orbitals of strontium. No vibrational structure in the spectral bands is observed, even in regions where the spectrum is recorded with a resolution of 1 cm(-1). The absorption spectra are calculated within a semiclassical approximation. Accurate ab initio calculations for the ground X (2)Sigma and the A (2)Pi and B (2)Sigma excited slates of Sr+Ar are combined with a perturbative one-electron model, which includes the spin-orbit coupling, to construct potential energy surfaces for the excited states of all clusters. The theoretical spectra obtained without using any adjustable parameters reproduce the patterns and trends of the experimental spectra, but they are shifted to higher frequencies. A better agreement between theoretical and experimental spectra is obtained by adjusting the depth of the potential wells of the ground X (2)Sigma and excited A (2)Pi states of the Sr+Ar dimer to the experimentally estimated values. From both calculations we conclude that Sr+Ar3 is a trigonal pyramid of C-3v Symmetry and Sr+Ar6 has a C-5v symmetry with five argon atoms forming a regular pentagon, one argon atom below and the cation above the pentagon plane. For these clusters, theory reproduces the characteristic blue shifts found in the absorption spectra and the splittings of the doubly degenerate states encountered in these symmetric complexes. (C) 1998 American Institute of Physics. [S0021-9606(98)02125-4].