Calibration of Raman Bandwidths on the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) Deep Ultraviolet Raman and Fluorescence Instrument Aboard the Perseverance Rover.

In this work, we derive a simple method for calibrating Raman bandwidths for the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument onboard NASA's Perseverance rover. Raman bandwidths and shapes reported by an instrument contain contributions from both the intrinsic Raman band (IRB) and instrumental artifacts. To directly correlate bandwidth to sample properties and to compare bandwidths across instruments, the IRB width needs to be separated from instrumental effects. Here, we use the ubiquitous bandwidth calibration method of modeling the observed Raman bands as a convolution of a Lorentzian IRB and a Gaussian instrument slit function. Using calibration target data, we calculate that SHERLOC has a slit function width of 34.1 cm-1. With a measure of the instrument slit function, we can deconvolve the IRB from the observed band, providing the width of the Raman band unobscured by instrumental artifact. We present the correlation between observed Raman bandwidth and intrinsic Raman bandwidth in table form for the quick estimation of SHERLOC Raman intrinsic bandwidths. We discuss the limitations of using this model to calibrate Raman bandwidth and derive a quantitative method for calculating the errors associated with the calibration. We demonstrate the utility of this method of bandwidth calibration by examining the intrinsic bandwidths of SHERLOC sulfate spectra and by modeling the SHERLOC spectrum of olivine.

A Deep Ultraviolet Raman and Fluorescence Spectral Library of 51 Organic Compounds for the SHERLOC Instrument Onboard Mars 2020.

We report deep ultraviolet (DUV) Raman and Fluorescence spectra obtained on a SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals) analog instrument for 51 pure organic compounds, including 5 carboxylic acids, 10 polycyclic aromatic hydrocarbons, 24 amino acids, 6 nucleobases, and 6 different grades of macromolecular carbon from humic acid to graphite. Organic mixtures were not investigated. We discuss how the DUV fluorescence and Raman spectra exhibited by different organic compounds allow for detection, classification, and identification of organics by SHERLOC. We find that 1- and 2-ring aromatic compounds produce detectable fluorescence within SHERLOC's spectral range (250-355 nm), but fluorescence spectra are not unique enough to enable easy identification of particular compounds. However, both aromatic and aliphatic compounds can be identified by their Raman spectra, with the number of Raman peaks and their positions being highly specific to chemical structure, within SHERLOC's reported spectral uncertainty of ±5 cm-1. For compounds that are not in the Library, classification is possible by comparing the general number and position of dominant Raman peaks with trends for different kinds of organic compounds.

Aqueous alteration processes in Jezero crater, Mars-implications for organic geochemistry.

The Perseverance rover landed in Jezero crater, Mars, in February 2021. We used the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument to perform deep-ultraviolet Raman and fluorescence spectroscopy of three rocks within the crater. We identify evidence for two distinct ancient aqueous environments at different times. Reactions with liquid water formed carbonates in an olivine-rich igneous rock. A sulfate-perchlorate mixture is present in the rocks, which probably formed by later modifications of the rocks by brine. Fluorescence signatures consistent with aromatic organic compounds occur throughout these rocks and are preserved in minerals related to both aqueous environments.

Detection and Degradation of Adenosine Monophosphate in Perchlorate-Spiked Martian Regolith Analog, by Deep-Ultraviolet Spectroscopy.

The search for organic biosignatures on Mars will depend on finding material protected from the destructive ambient radiation. Solar ultraviolet can induce photochemical degradation of organic compounds, but certain clays have been shown to preserve organic material. We examine how the SHERLOC instrument on the upcoming Mars 2020 mission will use deep-ultraviolet (UV) (248.6 nm) Raman and fluorescence spectroscopy to detect a plausible biosignature of adenosine 5'-monophosphate (AMP) adsorbed onto Ca-montmorillonite clay. We found that the spectral signature of AMP is not altered by adsorption in the clay matrix but does change with prolonged exposure to the UV laser over dosages equivalent to 0.2-6 sols of ambient martian UV. For pure AMP, UV exposure leads to breaking of the aromatic adenine unit, but in the presence of clay the degradation is limited to minor alteration with new Raman peaks and increased fluorescence consistent with formation of 2-hydroxyadenosine, while 1 wt % Mg perchlorate increases the rate of degradation. Our results confirm that clays are effective preservers of organic material and should be considered high-value targets, but that pristine biosignatures may be altered within 1 sol of martian UV exposure, with implications for Mars 2020 science operations and sample caching.

An Optical Model for Quantitative Raman Microspectroscopy.

Raman spectroscopy is an invaluable technique for identifying compounds by the unique pattern of their molecular vibrations and is capable of quantifying the individual concentrations of those compounds provided that certain parameters about the sample and instrument are known. We demonstrate the development of an optical model to describe the intensity distribution of incident laser photons as they pass through the sample volume, determine the limitations of that volume that may be detected by the spectrometer optics, and account for light absorption by molecules within the sample in order to predict the total Raman intensity that would be obtained from a given, uniform sample such as an aqueous solution. We show that the interplay between the shape and divergence of the laser beam, the position of the focal plane, and the dimensions of the spectrometer slit are essential to explaining experimentally observed trends in deep ultraviolet Raman intensities obtained from both planar and volumetric samples, including highly oriented pyrolytic graphite and binary mixtures of organic nucleotides. This model offers the capability to predict detection limits for organic compounds in different matrices based on the parameters of the spectrometer, and to define the upper/lower limits within which concentration can be reliably determined from Raman intensity for such samples. We discuss the potential to quantify more complex samples, including as solid phase mixtures of organics and minerals, that are investigated by the unique instrument parameters of the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) investigation on the upcoming Mars 2020 rover mission.

The Cell and the Sum of Its Parts: Patterns of Complexity in Biosignatures as Revealed by Deep UV Raman Spectroscopy.

The next NASA-led Mars mission (Mars 2020) will carry a suite of instrumentation dedicated to investigating Martian history and the in situ detection of potential biosignatures. SHERLOC, a deep UV Raman/Fluorescence spectrometer has the ability to detect and map the distribution of many organic compounds, including the aromatic molecules that are fundamental building blocks of life on Earth, at concentrations down to 1 ppm. The mere presence of organic compounds is not a biosignature: there is widespread distribution of reduced organic molecules in the Solar System. Life utilizes a select few of these molecules creating conspicuous enrichments of specific molecules that deviate from the distribution expected from purely abiotic processes. The detection of far from equilibrium concentrations of a specific subset of organic molecules, such as those uniquely enriched by biological processes, would comprise a universal biosignature independent of specific terrestrial biochemistry. The detectability and suitability of a small subset of organic molecules to adequately describe a living system is explored using the bacterium Escherichia coli as a model organism. The DUV Raman spectra of E. coli cells are dominated by the vibrational modes of the nucleobases adenine, guanine, cytosine, and thymine, and the aromatic amino acids tyrosine, tryptophan, and phenylalanine. We demonstrate that not only does the deep ultraviolet (DUV) Raman spectrum of E. coli reflect a distinct concentration of specific organic molecules, but that a sufficient molecular complexity is required to deconvolute the cellular spectrum. Furthermore, a linear combination of the DUV resonant compounds is insufficient to fully describe the cellular spectrum. The residual in the cellular spectrum indicates that DUV Raman spectroscopy enables differentiating between the presence of biomolecules and the complex uniquely biological organization and arrangements of these molecules in living systems. This study demonstrates the ability of DUV Raman spectroscopy to interrogate a complex biological system represented in a living cell, and differentiate between organic detection and a series of Raman features that derive from the molecular complexity inherent to life constituting a biosignature.

Interfacial Chemical Composition and Molecular Order in Organic Photovoltaic Blend Thin Films Probed by Surface-Enhanced Raman Spectroscopy.

Organic electronic devices invariably involve transfer of charge carriers between the organic layer and at least one metal electrode, and they are sensitive to the local properties of the organic film at those interfaces. Here, we demonstrate a new approach for using an advanced technique called surface-enhanced raman spectroscopy (SERS) to quantitatively probe interfacial properties relevant to charge injection/extraction. Exploiting the evanescent electric field generated by a ∼7 nm thick layer of evaporated silver, Raman scattering from nearby molecules is enhanced by factors of 10-1000× and limited by a distance dependence with a measured decay length of only 7.6 nm. When applied to the study of an all-polymer 1:1 blend of P3HT and F8TBT used in organic solar cells, we find that the as-cast film is morphologically suited to charge extraction in inverted devices, with a top (anode) interface very rich in hole-transporting P3HT (74.5%) and a bottom (cathode) interface slightly rich in electron-transporting F8TBT (55%). While conventional, uninverted P3HT:F8TBT devices are reported to perform poorly compared to inverted devices, their efficiency can be improved by thermal annealing but only after evaporation of a metallic top electrode. This is explained by changes in composition at the top interface: annealing prior to silver evaporation leads to a greater P3HT concentration at the top interface to 83.3%, exaggerating the original distribution that favored inverted devices, while postevaporation annealing increases the concentration of F8TBT at the top interface to 34.8%, aiding the extraction of electrons in a conventional device. By nondestructively probing buried interfaces, SERS is a powerful tool for understanding the performance of organic electronic devices.