SIZE EFFECTS IN CLUSTER INFRARED-SPECTRA - THE V(1) BAND OF AR-N-HCO-13)( (N=1)

Spectroscopic and dynamical properties of the Ar-n-HCO+ (n = 1-13) clu ster series have been explored by exciting the chromophore HCO+ in the vicinity of its nu(1) C-H stretch transition. Spectra were obtained b y mass selecting the clusters, exposing them to tunable, pulsed LR lig ht (0.02 cm(-1) bandwidth), and monitoring the fragment intensity as a function of laser frequency. The yl band of the Ar-HCO+ dimer is rota tionally resolved and has a form consistent with a linear proton-bound complex. Analysis in terms of a pseudodiatomic Hamiltonian yields the following parameters: nu(0) = 2815.063 +/- 0.020 cm(-1), B'' = 0.0664 6 +/- 0.000 08 cm(-1), D'' = (7 +/- 4) x 10(-8) cm(-1), B' = 0.06829 /- 0.000 08 cm(-1), D' = (17 +/- 4) x 10(-8) cm(-1). The rotational li ne widths indicate a lifetime of more than 250 ps for the optically pr epared state. The nu(1) vibrational bands of the larger Ar-n-HCO+ clus ters, while lacking resolved rotational structure, are still reasonabl y narrow (<10 cm(-1)) and decrease in width as the cluster size increa ses. Excitation of the yl transition in Ar-n-HCO+ (n = 2-13) results i n the production of a relatively narrow range of daughter ions. Increm ental Ar binding energies are extracted from the branching ratio data using a statistical model which takes into account the kinetic energy of the departing Ar atoms. The variation with cluster size of the bind ing energies, vibrational band shifts, and combination band spacings a re argued to be evidence for Ar-n-HCO+ structures where A atoms form p rimary and secondary solvation rings about a linear Ar-HCO+ core with shell completion at n = 12. This view is consistent with simple empiri cal potential energy calculations.