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Time-Resolved Raman Spectroscopy for Planetary Surface Exploration on Rovers and Landers
Raman spectroscopy has long been a candidate for the next generation of in situ planetary science instruments, and several instruments are under development for both NASA and ESA targeting this purpose. Raman can be performed in concert with microscopic imaging, preserving the geological context of mineral phases. In addition to minerals, Raman can identify organic materials, and has been used in the laboratory to analyze organics, for example those present in Martian meteorites. Even with these clear advantages of the Raman technique, several challenges still exist when targeting mixed phase materials with Raman spectroscopy, particularly where organics are present. Of primary concern is fluorescence, which makes conventional 532 nm Raman spectroscopy of many of these samples challenging. Fortunately, for the next generation of Raman instrumentation under development, it has become possible to take advantage of the fact that fluorescence can be distinguished from Raman processes in the time domain. Raman scattering is an instantaneous process while fluorescence processes are associated with decay times which vary from ps to ms. In natural mixed-phase samples, there can be several fluorescent phases, leading to both long lifetime (mineral) and short lifetime (organic) fluorescence. We present a time-resolved Raman spectrometer that builds on the widely used 532 nm Raman technique to provide a means for performing Raman spectroscopy while minimizing the background noise that is often generated by fluorescence.
We will discuss significant advances leading to the feasibility of a compact time-resolved spectrometer, including the development of a new solid-state Single Photon Avalanche Diode detector array based on Complementary Metal-Oxide Semiconductor (CMOS) technology capable of sub-ns time resolution, and promising new technology for high repetition rate pulsed lasers. We will present results on Mars analog samples characteristic of habitable environments like Gale Crater on Mars to demonstrate instrument performance.