INFRARED-SPECTROSCOPY OF AR2CO2 TRIMER - VIBRATIONALLY AVERAGED STRUCTURES, SOLVENT SHIFTS, AND 3-BODY EFFECTS

Ar2CO2 is studied using direct absorption infrared spectroscopy. The v an der Waals molecules are formed when a mixture of CO2 and Ar gases i s expanded in a supersonic slit jet. To probe the clusters, the nu(3) asymmetric stretch of the CO2 monomer is then monitored in absorption. Sixty-one trimer transitions are assigned and fit to a Watson asymmet ric top Hamiltonian. Rotational constants for the upper and lower vibr ational states permit determination of vibrationally averaged molecula r structures, which indicate that the Ar atoms lie in the plane that b isects CO2 and is perpendicular to the CO2 intramolecular axis. These geometries are consistent with an equivalent ''T-shaped'' ArCO2 geomet ry for each Ar atom. Vibrational origins for the nu(3) CO2 asymmetric stretch frequency in ArnCO2 are found to shift approximately linearly for zero, one, and two Ar atoms. Calculations using pair potentials ar e used to extrapolate these red shifts out to the bulk phase and to co mpare the results to experimental matrix data. Finally, the slight non linearity in the red shift between ArCO2 dimer and Ar2CO2 trimers is i nterpreted in the context of three-body forces. (C) 1996 American Inst itute of Physics.