Anisotropic Interaction Potentials between Helium and Linear Molecules from Crossed Beam Experiments

Citation

Winniczek, Jaroslaw W. (1985) Anisotropic Interaction Potentials between Helium and Linear Molecules from Crossed Beam Experiments. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/5nvw-ym37. https://resolver.caltech.edu/CaltechTHESIS:01222019-123545387

Abstract

Anisotropic interaction potentials between helium and linear molecules have been studied experimentally and theoretically to examine the relationship between the potential and the scattering differential cross-section, and to extract these potentials from crossed-molecular beam data.

Chapter 2 presents the measurement of total (elastic and inelastic) differential scattering cross sections for He + CO₂, CS₂, OCS at a relative collision energy of about 65 meV with a crossed molecular beam apparatus. Anisotropic interaction potentials were extracted from these data, by way of an infinite order sudden approximation analysis. Several different anisotropic potential models were used in this analysis. The necessity for considering the anisotropy in the position of the well minimum as well as of its depth is demonstrated. A potential is proposed for He + OCS that reflects the symmetry of OCS, with a minimum number of modeling parameters.

In Chapter 3 a detailed sensitivity analysis of the total and rotational state-to-state differential cross-section (DCS) is performed on an empirical potential energy surface for He + CO₂. The infinite order sudden approximation is used to calculate the cross-sections. The sensitivity analysis consists of: 1) a large scale modification of the parameters that determine the anisotropic potential, and 2) an infinitesimal variation of these parameters to obtain a relative sensitivity function for the DCS. From these we demonstrate the effect each potential parameter has on the cross-sections. Despite the fact that this highly quantum system displays no classical effects such as rainbow scattering, we have shown that the quantal DCS oscillations contain significant information regarding the depth and width of the potential well and its anisotropy. Much of this information can be extracted from total-DCS scattering data. However the rotationally inelastic DCS contain a substantial amount of additional information regarding the shape of the potential energy surface.

Some of the conclusions reached in Chapter 3 indicate that the measuring of rotationally inelastic differential cross-sections can provide significantly greater insight into the nature of the potential than does the total DCS. In Chapter 4 we develop the means for simulating experimentally observable data from the potential for a given set of apparatus conditions. In Chapter 5 we describe modifications made to the crossed-beam apparatus in order to observe this inelasticity and present preliminary results for He + CO₂.

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