The need for better potential-energy models for atom-molecule and molecule-molecule interactions is discussed and the utility of the exchangecoulomb (XC) model is critically examined, by fitting a potential based on it to new high-resolution discrete infrared data for the He-CO Van der Waals molecule. In addition to explaining the observed spectrum as well as does an optimized empirical potential previously determined from the same data, the resulting XC surface is expected to be more realistic in regions not directly sampled by the fitted data.

A reliable new three-dimensional potential energy surface is obtained for the H 2 -Ar system using an exchange-coulomb potential model with five parameters determined empirically from a least-squares fit to experimental data. This surface fully accounts for new high resolution IR data, virial coefficients, and vibrational transition pressure-shifting coefficients used in the analysis, and yields excellent predictions of elastic and inelastic scattering cross sections and hyperfine transition intensities not included in the analysis. Quantitative comparisons with the best previous empirical potential and a high quality fully ab initio potential are also presented.

The pure rotational transition (J,j,l)=(101)←(000) of the weakly bound CO-He complex has been observed in the 17 GHz region for five isotopomers, namely C12O-16He4, C13O-16He4, C12O-18He4, C13O-18He4, and C13O-17He4, using a pulsed-jet cavity Fourier-transform microwave spectrometer. Hyperfine structure due to the quadrupolar O17 (I=5/2) nucleus has been observed and analyzed to yield the quadrupole coupling parameters. A new microwave-terahertz double resonance spectrometer has been used to carry out an experiment on C13O-16He4 in which a submillimeter-wave pump transition (J,j,l)=(110)←(000) and a microwave signal transition (211)←(110) were detected. Infrared spectra of C13O-16He4 and C12O-18He4, have been obtained in the 2100 cm−1 region of the C-O stretch using a tunable diode laser spectrometer and a long-path (200 m), low-temperature (46 K) equilibrium gas cell. The combined data are sufficient to construct essentially complete experimental energy level schemes for the bound states of the C12O-16He4, C13O-16He4, and C12O-18He4 forms of the complex. These energies are compared with calculated levels derived from two intermolecular potential models, V(3,3,3) and XC (fit). It is shown that the new spectra, in particular those of C13O-He16, discriminate between the two models and indicate that V(3,3,3) provides a better representation of the potential in the region of the attractive well probed by the bound states. A quasibound state of CO-He was observed for the first time, and its width due to predissociation was approximately determined, providing a sensitive measure of the depth of the potential.

Pure rotational spectra of three isotopomers of the Van der Waals complex Ar-N, have been investigated in the frequency range 3.5-20 GHz, using a pulsed molecular beam cavity microwave Fourier-transform spectrometer. Rotational constants and quartic and sextic centrifugal distortion constants have been obtained, along with N hyperfine constants. The spectra of Ar-I4N, and Ar-"N, indicate equivalence of the nitrogen nuclei, and thus confirm C,, symmetry for the complexes. The measured transition frequencies and the derived constants have been used to test the best available literature potential-energy surfaces for the Ar-N, interaction. For this purpose rotational transition frequencies and expectation values of other properties were calculated and compared with the corresponding values from the microwave experiments. A refined version of one of the surfaces has been generated by inclusion of the microwave results.

Accurate close-coupling calculations are used to investigate the vibrational predissociation of ArH 2 0 as a function of the overall rotation J of the van der Waals complex. A full vibrational and rotational basis of H 2 0 states is used in the calculation. The potential energy surface is of a form due to Cohen and Saykally and derived from far-infrared spectra, with an additional term to introduce the dependence on the vibrations of H 2 0. The linewidths calculated in this work show a maximum at J = 6 and it is found that Fermi resonances affect dramatically the magnitude of the calculated linewidths. Good agreement with experimentally measured linewidths of Nesbitt and Lascola is achieved and the calculations provide a simple picture for the J dependence of the linewidths.