SIGNATURES OF LARGE-AMPLITUDE MOTION IN A WEAKLY-BOUND COMPLEX - HIGH-RESOLUTION IR SPECTROSCOPY AND QUANTUM CALCULATIONS FOR HECO2

The infrared spectrum of the HeCO2 van der Waals molecule is recorded in the region of the CO2 ν3 asymmetric stretch via direct absorption of a tunable Pb-salt diode laser. HeCO2 is formed in a slit jet supersonic expansion; the slit valve and the stagnation gas must be precooled to -35 °C before substantial formation of the complex is observed. Sixty-six rovibrational transitions are recorded by exciting the ν3 asymmetric stretch of the CO2 monomer within the complex. Forty-three of these transitions can be assigned using internally consistent combination differences as a b-type band of a T-shaped asymmetric rotor. There are several indications that large amplitude motion is significant in HeCO2, including the poor quality of the fit to an asymmetric rotor model and the large positive inertial defects of Δ=8.54 and 10.98 uÅ2 in the ground and excited states, respectively. However, a hindered rotor analysis based on these inertial defects demonstrates that the CO2 motion within the complex is far from the free rotor limit. No evidence of predissociation broadening is observed, indicating a lifetime for the complex of τ>6 ns. Quantum close-coupling calculations which correctly treat both angular and radial degrees of freedom are carried out on the full 2D HeCO2 potential energy surface of Beneventi et al. [J. Chem. Phys. 89, 4671 (1988)]. Comparison of this analysis with the experimental results demonstrates that the theoretical potential is too isotropic in the region of the potential minimum. Predicted spectra from this model potential, however, indicate that the remaining 17 much weaker HeCO2 transitions are due to a "hot band" excitation out of the first intermolecular bending level, lying 9±2 cm-1 above the ground state. In sharp contrast to the ground vibrational state of HeCO2, an asymmetric rotor model fails qualitatively to characterize the rotational structure for the lowest excited bend. The simple physical reason for this is confirmed by inspection of the quantum wave functions; in the ground state the He atom is localized near the C atom in a T-shaped geometry, whereas in any of the excited bending states the He atom is largely delocalized around the CO2 molecular framework. © 1994 American Institute of Physics.