Date of Award

1998

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

First Advisor

Sean P. McGlynn

Abstract

A new device, a selective digital integrator (SDI), for the acquisition of modulated polarization spectroscopy (MPS) signals is described. Special attention is given to the accurate measurement of very small (AC component of interest $<$10$\sp{-3}$ x DC component), rapidly modulated ($\sim$50 kHz) signals at or below noise levels. Various data acquisition methods and problems associated with the collection of modulated signals are discussed. The SDI solves most of these problems and has the following advantages: it provides the average-time resolved profile of a modulated signal; it eliminates errors if the modulation is not sinusoidal; it enables separate measurements of the various phases of the signal modulation cycle; it permits simultaneous measurement of absorption, circular dichroism (CD) and linear dichroism (LD) spectra; it facilitates 3-D absorbance measurements; it has a wide gain-switching-free dynamic range (10 orders of magnitude or more); it offers a constant S/N ratio mode of operation; it eliminates the need for photomultiplier voltage feedback, and it has faster scanning speeds. The time-resolution, selectivity, wide dynamic range, and low-overhead on-the-fly data processing are useful for other modulated spectroscopy (MS) and non-MS experiments such as pulse height distribution and time-resolved pulse counting measurements. The advantages of the MPS-SDI method are tested on the first Rydberg electronic transitions of (+)-3-methylcyclopentanone. The experimental results validate the predicted SDI capabilities. However, they also point to two difficulties that had not been noted previously: the presence of LD in a gaseous sample and a pressure-dependence of the relative peak heights of the CD spectrum. Models for these anomalies are proposed. The presence of the oscillatory LD (but not an LD background) is explained with a sample cell model based on the observed polarization-dependent time-resolved profiles of transmitted light intensity. To obtain expressions for these intensities, a theoretical background, which provides a new approach to the treatment of light/matter interaction, is included as an Appendix. To explain the second anomaly, present only at high optical densities, a model based on the presence of scattered light is introduced and verified. The mode of correction for the scattering problem is outlined.

ISBN

9780591905014

Pages

164

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