Calibration of the SHERLOC Deep Ultraviolet Fluorescence–Raman Spectrometer on the Perseverance Rover

Abstract

We describe the wavelength calibration of the spectrometer for the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument onboard NASA’s Perseverance Rover. SHERLOC utilizes deep-UV Raman and fluorescence (DUV R/F) spectroscopy to enable analysis of samples from the martian surface. SHERLOC employs a 248.6 nm deep UV laser to generate Raman scattered photons and native fluorescence emission photons from near-surface material to detect and classify chemical and mineralogical compositions. The collected photons are focused on a charge-coupled device (CCD) and the data are returned to Earth for analysis. The compact DUV R/F spectrometer has a spectral range from 249.9 nm to 353.6 nm (~200 cm-1 to 12000 cm<sup>−1</sup>) (with a spectral resolution of 0.296 nm (∼40 cm<sup>−1</sup>)). The compact spectrometer uses a custom design to project a high-resolution Raman spectrum and a low-resolution fluorescence spectrum on a single CCD. The natural spectral separation enabled by deep UV excitation enables wavelength separation of the Raman/fluorescence spectra. The SHERLOC spectrometer was designed to optimize the resolution of the Raman spectral region and the wavelength range of the fluorescence region. The resulting illumination on the CCD is curved, requiring a segmented, non-linear wavelength calibration in order to understand the mineralogy and chemistry of martian materials.

SHERLOC Raman Mineral Class Detections of the Mars 2020 Crater Floor Campaign

“…To reduce noise, the CCD readout is binned into three regions, a Raman region ∼800–4000 cm −1 (250–273 nm) and two fluorescence regions (274–337 and 338 to ∼370 nm). Each region has a separate wavenumber calibration, previously discussed in detail (Uckert et al., 2021).…”

“…The potential to observe Raman bands in the spectral range below 800 cm −1 is limited because SHERLOC's laser‐injection filter has reduced transmission in this region and the 252.9 nm laser plasma line is observed at ∼650 cm −1 . However, very strong bands below 800 cm −1 can be observed (Bhartia et al., 2021; Razzell Hollis, Abbey, et al., 2021; Uckert et al., 2021).…”

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

“…The off-axis optical design of both instruments causes the projection of the spectrum to curve across the charge-coupled device (CCD), referred to as a “spectral smile” (Uckert et al, 2021 ). To avoid disruptive levels of dark noise in the Raman region, which would result from integrating over the full height of the CCD, both instruments must read out data for the Raman and fluorescence regions separately using different vertical binning ranges.…”

SHERLOC Raman Mineral Detections of the Mars 2020 Crater Floor Campaign

“…To reduce noise, the CCD readout is binned into 3 regions, a Raman region ∼ 800 to 4000 cm−1 (250 to 273 nm) and two fluorescence regions (274 to 337 and 338 to ∼370 nm). Each region has a separate wavenumber calibration, previously discussed in detail (Uckert et al, 2021).…”

“…The potential to observe Raman bands in the spectral range below 800 cm-1 is limited because SHERLOC's laser-injection filter has reduced transmission in this region and the 252.9 nm laser plasma line is observed at ~650 cm-1. However, very strong bands below 800 cm-1 can be observed (Bhartia et al, 2021;Uckert et al, 2021;Razzell Hollis et al, 2021a).…”