PAIRWISE AND NONPAIRWISE ADDITIVE FORCES IN WEAKLY-BOUND COMPLEXES - HIGH-RESOLUTION INFRARED-SPECTROSCOPY OF ARNDF (N=1,2,3)

High resolution infrared spectra of the v(DF)=1 <-- 0 stretch in ArnDF (n = 1-3) have been recorded using a slit-jet infrared spectrometer. Analysis of the rotationally resolved spectra provides vibrationally a veraged geometries and vibrational origins for a DF chromophore sequen tially ''solvated'' by Ar atoms. Calculations using pairwise additive Ar-Ar and Ar-DF potentials predict lowest energy equilibrium structure s consistent with the vibrationally averaged geometries inferred spect roscopically. Variational calculations by Ernesti and Hutson [A. Ernes ti and J. M. Hutson, Faraday Discuss. Chem. Soc. (1994)] using pairwis e additive potentials predict rotational constants which are in qualit ative agreement with, but consistently larger than, the experimental v alues. The inclusion of nonpairwise additive (three-body) terms improv es the agreement, though still not to within the uncertainty of the pa ir potentials. The vibrational redshifts of 8.696, 11.677, and 14.461 cm(-1) for n=1-3, respectively, reflect a nonlinear dependence of the redshift on the number of Ar atoms. Both the variational calculations of Ernesti and Hutson and diffusion quantum Monte Carlo calculations [ M. Lewerenz, J. Chem. Phys. (in press)] using pairwise additive potent ials systematically overpredict the magnitude of these redshifts, furt her signifying the need for corrective three-body terms. Analysis of t he ArnDF (n=2,3) rovibrational line shapes reveals an upper limit to h omogeneous broadening on the order of 2-3 MHz, consistent with vibrati onal predissociation lifetimes in excess of 50 ns. (C) 1995 American I nstitute of Physics.