Absorption and dispersion spectroscopy in the far infra-red region

Abstract

The work described in this thesis can be divided into two main parts. The first part describes the modifications of the instrument (a Beckman-RIIC FS720) to enable it to be operated in the polarising mode with a liquid helium cooled detector. Several problems were encountered during the testing of this equipment particularly when operating the polarising system. Initially the spectra recorded with this system showed considerable distortion in the high and low frequency regions. The low frequency distortion was found to arise from electrical non-linearity (due to overload) within the electronics (RIIC FS200) and modifications to the amplifier have, eliminated this problem. The high frequency distortion, which appears as a ≈120 cm(^-1)limit, has not been corrected and is now thought to arise from a 'software' (computing) problem. The signal-to-noise ratio tests carried out on the cooled detsctor indicate that the bolometer element may not be operating at its most sensitive temperature due to overheating by the hot source. However, when operating with heavy optical filtering (therefore reducing the radiation reaching the bolometer) it has been possible to record spectra in the very far-infrared region (20 -» 5 cm(^-1)).The second part of the thesis describes the practical operation and tests carried out on the previously constructed cell designed for conversion of the instrument to operate in the dispersive mode. To enable this cell to be mounted vertically on the instrument a top window has been included to hold a liquid sample. This leads to the appearance of reflections and multiple reflections not present when studying a gravity held liquid. These reflection fringes have been fully assigned and therefore it has been possible to obtain the continuous dispersion (refractive index) curve of a liquid in the far- infrared region. The overall aim of this work has been to test the limitations and accuracy of the new equipment and make original studies in the far-infrared region.