SEQUENTIAL SOLVATION OF HCL IN ARGON - HIGH-RESOLUTION INFRARED-SPECTROSCOPY OF ARNHCL (N=1, 2, 3)

High-resolution near-infrared spectra of the nu(HCl)=1<--0 fundamental stretch in Ar2HCl and Ar3HCl have been characterized using a slit-jet infrared spectrometer. Analysis of the jet-cooled, rotationally resol ved spectra (i) permits unambiguous identification of the cluster size , (ii) provides vibrationally averaged geometries in the nu(HCl)=1 exc ited state, and (iii) allows the vibrational shift of the HCl chromoph ore to be measured as a function of the number of Ar atoms in the comp lex. The equilibrium structures of ArnHCl (n=1-3) clusters calculated using accurate Ar-Ar and Ar-HCl pair potentials are consistent with th e vibrationally averaged structures inferred spectroscopically. The vi brational red-shifts for ArnHCl (n=1-3) reflect a near-linear dependen ce on the number of Ar atoms, which is qualitatively reproduced by sim ple classical calculations on nu(HCl)=0 and 1 pairwise additive potent ial surfaces. Theoretical predictions of the ArnHCl red-shifts in a fe e lattice indicate good agreement with experimental matrix results. Ho wever, to achieve this asymptotic limit requires up to n approximate t o 54 Ar atoms; this underscores a clear sensitivity to non-nearest nei ghbor Ar-HCl interactions significantly outside the first solvation sh ell. Finally, for smaller ArnHCl clusters with only one solvation shel l (n=12), the potentials predict an energetic preference for HCl in su rface vs interior sites. Analysis indicates that this effect is predom inantly due to Ar/HCl size mismatch, which destabilizes the nearest ne ighbor Ar shell for HCl solvated in the center of the cluster. (C) 199 7 American Institute of Physics.