The development of a high resolution deep-UV spatial heterodyne Raman spectrometer (2016)

Raman spectroscopy is a light scattering technique that has a huge potential for standoff measurements in applications such as planetary exploration because a Raman spectrum provides a unique molecular fingerprint that can be used for unambiguous identification of target molecules. For this reason, NASA has selected a Raman spectrometer as one of the major instruments for its new Mars lander mission, Mars 2020, in the search for biomarkers that would be the indicators of past or present life. Raman scattering is strongest at UV wavelengths because of the inherent increase in the Raman cross section at shorter wavelengths and because of the possibility of UV resonance enhancement. Thus, a Raman spectrometer for planetary exploration would ideally be a UV instrument. However, existing UV Raman spectrometers are not optimal to integrate for planetary exploration because they are large and heavy. Existing UV Raman spectrometers also offer very low light throughput due to the need for narrow entrance slits to provide high spectral resolution. This thesis discusses the development of a new type of Fourier transform (FT) Raman spectrometer; a spatial heterodyne Raman spectrometer (SHRS), which offers several advantages for field-based UV Raman applications. The SHRS generates a spatial interferogram using stationary diffraction gratings and an imaging detector. The SHRS is lightweight, contains no moving parts, and allows very high spectral resolution Raman measurements to be made in an exceptionally small package, even in the deep UV. In this study, for the first time, we developed a SHRS system for deep UV applications using 244 nm excitation that has a spectral resolution less than 5 cm-1 and a spectral bandpass of 2600 cm-1. Raman spectra of several liquid and solid compounds were measured using a 244 nm laser to demonstrate the spectral resolution and range of the system. The SHRS has a large entrance aperture and wide collection angle, which was shown to be beneficial for the deep UV measurements of photosensitive materials like NH4NO3 by using a large laser spot size with low laser irradiance on the sample. This is not possible using conventional UV Raman systems where the need to focus the laser on sample often leads to photodecomposition. In addition, the use of deep-UV excitation to mitigate fluorescence was demonstrated by measuring Rh6G, a highly fluorescent compound, in acetonitrile solution. We also evaluated the performance of the SHRS for standoff Raman measurements in ambient light conditions using pulsed lasers and a gated ICCD detector. Standoff UV and visible Raman spectra of a wide variety of materials were measured at distances of 3-18 m, using 266 nm and 532 nm pulsed lasers, with 12.4” and 3.8” aperture telescopes, respectively. We observed that the wide acceptance angle of the SHRS simplifies optical coupling of the spectrometer to the telescope and makes alignment of the laser on the sample easier. More recently, we improved the SHRS design by replacing the cube beamsplitter with a custom-built higher quality plate beamsplitter, designed to operate in the range of 240-300 nm, with higher transmission, higher surface flatness and better refractive index homogeneity. The new design addresses two major issues of the previous UV SHRS design, namely, optical losses and poor fringe visibility; as a result, the Raman spectra obtained with new design have much higher signal to noise ratio than the measurements made using previous design.