Experimental investigation of electric-field-induced birefringence in fluids.
Measurements of the quadratic electro-optical (Kerr) effect of uids can provide knowledge of fundamental molecular properties, such as (hyper)polarizabilities. They also provide valuable information about intermolecular forces through the measured Kerr-effect virial coefficients. These properties play a considerable role in aspects of physics, chemistry and biology, and so are of inherent value. They also provide benchmarks against which to evaluate ab inito quantum mechanical calculations of the properties, especially for larger atoms and molecules, where the need for large basis sets and adequate account of electron correlation places onerous demands on computational resources. This thesis reports the development of an apparatus to measure the electric-field-induced birefringence (Kerr effect) in a fluid. The apparatus has been designed in an attempt to increase the precision and absolute accuracy of the measured Kerr effect, with a long-term view to obtaining precise new data for a range of molecular species. The apparatus has been fully automated, using a personal computer containing an IEEE interfacing card to communicate with a data-acquisition and control unit, which in turn controls the experiment and collects the data from which the Kerr constant can be determined. The apparatus has been used to measure the Kerr constant of gaseous helium at two different temperatures, namely 399.4 K and 445.7 K. Helium was chosen because extremely precise and accurate ab initio calculations of this two-electron system have yielded very precise knowledge of the Kerr constant (to within 0.1%), so that it provides an ideal benchmark against which to assess the performance of the apparatus. Once the Kerr apparatus is yielding precise data for helium, it will be possible to measure the Kerr effect of other gaseous species with confidence. A full Jones calculus analysis of the optical cascade is presented, providing useful insights into the best experimental procedure to be employed in the gathering of data. In addition, the molecular-tensor theory of the second Kerr-effect virial coefficient BK is reviewed. Measurements of the Kerr-effect of helium gas have yielded the second Kerr hyperpolarizability at the experimental wavelength of 632.8 nm.