Abstract:
The molecular theory of the second light-scattering virial coefficient Bp describing the effects
of interacting pairs of molecules on the depolarization ratio p of Rayleigh-scattered light is
reviewed, both for interacting linear and nonlinear molecules. The molecular tensor theory
of Bp for nonlinear molecules is extended for the first time to include in the scattered intensity p
those contributions arising from field gradient effects and induced quadrupole moments in
the molecular interactions. The expressions for contributions to Bp are evaluated numerically
for the nonlinear polar molecule dimethyl ether.
We have used an existing light-scattering apparatus to investigate the pressure-dependence
of the depolarization ratio p for dimethyl ether, allowing Bp to be extracted. The measured
value is compared with the calculated value, theory and experiment being found to agree to
within 9%.
This success in modelling Bp for dimethyl ether spurred us on to extend our investigation to
the second Kerr-effect virial coefficient BK • The molecular-tensor theory of BK for nonlinear
molecules is reviewed, and is applied in this work to dimethyl ether. The calculated BK values
generally lie within the uncertainty limits of the available measured data over their full range
of temperatures. We have used a recently-commissioned Kerr cell to undertake our own
measurement of BK for dimethyl ether at room temperature. This value is in good agreement
with the findings of our molecular model, and is in reasonable agreement with the other
measured data.
This thesis serves to reaffirm recent claims that comprehensive dipole-induced-dipole
theories of molecular interaction effects explain the observed phenomena adequately
provided one works to higher orders in the molecular tensors so that the series of contributing
terms has converged to a meaningful numerical result, and provided the full symmetry of the
molecules is allowed for.
