Date of Award

2016

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

Sub-Department

College of Arts and Sciences

First Advisor

S. Michael Angel

Abstract

Our group recently developed a new type of Fourier transform Raman spectrometer, the spatial heterodyne Raman spectrometer (SHRS), for planetary exploration. The SHRS is a high spectral resolution, high throughput, compact dispersive interferometer, similar in design to a Michelson interferometer, where the mirrors are replaced by stationary diffraction gratings. The SHRS has no moving parts and does not require an entrance slit, making the throughput of the system orders of magnitude larger than a typical dispersive spectrometer. The wide field-of-view (FOV) of the SHRS enables wide area measurements without loss of sensitivity or spectral resolution and offers advantages such as minimizing sample photodegradation by using large laser spots on the sample and easy coupling to a telescope for standoff Raman. The lack of moving parts and compactness of the SHRS makes the system lightweight and robust, which is ideal for a deployable standoff detection instrument intended for planetary exploration. Our group has demonstrated visible and UV measurements using the SHRS in bench-top and standoff configurations; however, there are no published quantitative studies measuring the FOV of the SHRS for standoff Raman or the laser irradiance necessary to reduce photodegradation. The work presented quantifies the FOV, beam diameter and irradiance required to reduce photodegradation, and demonstrates 1D imaging using wide area illumination. A prism-based SHRS was also investigated and preliminary results are discussed.

The wide FOV and high throughput of the SHRS makes the system well suited for transmission Raman spectroscopy (TRS). In TRS, the laser is brought in from the backside of the sample, and Raman photons are collected on the opposite side after diffusely scattering through the sample. The diffuse scattering of light increases interactions within the sample leading to spectra that are more representative of the bulk. However, the Raman scattered photons exit the sample over a very large area, and the dispersive Raman spectrometers typically used in TRS suffer sensitivity losses, in comparison to backscatter Raman, due to low collection efficiency resulting from the narrow spectrometer slit. TRS measurements using the SHRS are demonstrated and comparisons to a more conventional, high throughput dispersive spectrometer are discussed.

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Chemistry Commons

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