This thesis is concerned with the study of van der Waals interactions between He atoms and various atomic and molecular partners. These studies involve measuring differential cross sections (DCS) for scattering of thermal-energy crossed atomic and molecular beams. Although this crossed-beams technique has been used extensively in the past ten or so years, the accuracy with which it may be used to obtain van der Waals potentials for highly quantum systems heretofore has not been examined quantitatively. Under typical experimental conditions, we show in Chapter 1 that the attractive minimum region of the potential is determined to within about ± 10%. However, this accuracy is attained only when appropriately flexible interatomic potentials are used in the iterative inversion procedures needed to obtain these potentials from the DCS scattering measurements. Suitable parametric representations for the potential are those capable of effectively decoupling the attractive minimum region from the adjacent regions of the potential. This is particularly important for highly quantum systems having shallow attractive minima, since this is the region most sensitively probed by the elastic DCS. The related difficulty of non-uniqueness for iteratively derived potential energy curves is circumvented by introducing these flexible parametric representations. The range of interatomic distances actually sampled in the experiment is found to be significantly wider than would be expected classically. In Chapter 2, the above results find practical application in the analysis of our measurements for He-Ar scattering. Using two very different mathematical parameterizations having the appropriate flexibility to represent the He-Ar interaction, an accurate and substantially model-independent potential is determined. This potential is significantly more accurate than previously proposed ones. By further using measurements of properties which are sensitive primarily to repulsive interactions of He-Ar, its potential may now be considered as well known for interatomic separations >2.4Å. (potential energies < 0.15 eV). This potential provides an accurate description of all available gas-phase data for the He-Ar system. The techniques used to obtain it can easily be extended to the other He-rare gas pair potentials. Later chapters in this thesis emphasize the van der Waals interactions of He atoms with molecular partners, whose intermolecular potentials are anisotropic. To be accurately represented, they should therefore be parameterized in a dimensionality higher than one . The importance of additional degrees of freedom is demonstrated to be slight for small diatomic partners (N2, O2, CO and NO), for hydrides (CH4, NH3, H2O), and even for large but highly symmetric molecules (SF6). In Chapter 3, the small anisotropic contributions to the He-diatomic intermolecular potentials are determined, using a model in which the locus of the attractive minimum position is represented by an ellipse. In Chapter 4, anisotropies in the He-hydrides and He-SF6 potentials are ignored, maintaining nonetheless a good description of the DCS's for these scattering partners. Chapter 5 presents a brief study of the He-CO2 van der Waals potential, which is sufficiently anisotropic to result in a scattering pattern which cannot be fit with central-field models, even very flexible ones. The anisotropic potential extracted from the experiment may be used to predict rotationally inelastic DCS's for thermal-energy He-CO2 collisions.
Mark Keil (Thu,) studied this question.