For 3He–CO and 4He–CO van der Waals bimers, fully resolved infrared spectra in the 4.7 μm region near the fundamental band origin of the CO monomer have been measured for the first time. Only a small fraction of the observed lines could be assigned using conventional empirical spectroscopic techniques, and little additional insight was gained from synthetic spectra generated from a published ab initio potential for this system. However, a complete set of unique assignments was made on the basis of comparisons with synthetic spectra generated from a variety of trial potential energy surfaces, and least-squares fits to the observed transition frequencies were used to determine a new anisotropic potential energy surface for this system. This new surface is much deeper and has a much stronger well depth anisotropy than the best previous one, and its predictions of very low temperature microwave line broadening cross sections raise serious questions regarding the methodology for calculating that property.

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.

A sensitive capacitance thermometer at low temperature for use in magnetic fields up to 20 T Rev.The pressure broadening and shifting coefficients for pure rotational transitions of CO in a He bath gas at very low temperatures are calculated from the best available potential energy surfaces, and compared with very recent measurements by M. M. Beaky, T. M. Goyette, and F. C. De Lucia ͓J. Chem. Phys. 105, 3994 ͑1996͔͒. The results obtained for two recent empirical potentials determined from fits to Van der Waals spectra, and for a recent high quality purely ab initio surface, are consistent with one another. The best of the spectroscopic potentials also yields good agreement with high temperature virial coefficients and transport properties. Predictions from all three of these potentials agree with linebroadening and shifting measurements at temperatures above ϳ20 K, but are in substantial disagreement with the measurements at temperatures below 4 K. At present, the source of this discrepancy is not known.

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Accurate calculations of the energies of all bound rovibrational states, of the energies and lifetimes of some metastable states, and of the resulting rovibrational spectrum are performed for the helium-acetylene Van der Waals complex using an iterative secular equation method. The calculations are carried out both on an empirical potential surface which has a single 'T shaped' minimum, and on a potential surface based on ab initio electronic structure calculations which has a single minimum at a linear geometry. The two surfaces differ in the strength of the anisotropy, as well as its sign. A comparison of the spectra generated by the two surfaces allows spectroscopic manifestations of the shape of the potential energy surface to be discerned, results which should help guide analyses of the spectra of similar very weakly bound complexes.