Water isotope spectroscopy near 2.6μm for D/H, 18O/16O, and 17O/16O analysis in planetary bodies

Abstract

A laser absorption spectroscopy method was developed for in situ extraterrestrial sensing of four distinct water isotopologues near $2.64\mu m$. Infrared rovibrational transitions for $H_2^{16}O$, $H_2^{18}O$, $H_2^{17}O$, and HDO were selected within the overlapping ${\nu }_1$ and ${\nu }_3$ fundamental bands to maximize relative absorption intensity and reduce spectral interference with neighboring water lines and other species, such as ${\mathrm{CO}}_2$, while being positioned within the injection current tuning range of a single distributed-feedback diode laser held at one temperature. Temperature-dependent self-broadening parameters were measured from 296–375 K for the target lines via Voigt lineshape fitting of the transitions in the range of 3788.2–3789.9 cm⁻¹ using vaporized water samples in a temperature- and pressure-controlled gas cell. Based on the novel wavelength selection and spectroscopic measurements, quantitative, calibration-free sensing of isotope abundance ratios of D/H, ${}^{18}O{/}^{16}O$, and ${}^{17}O{/}^{16}O$ in water vapor was demonstrated in a portable form-factor device using a diode laser, compact Herriott cell, and a liquid water injection system. Accuracy and detection limits using scanned-wavelength direct absorption were estimated by comparison to water samples of known isotopic abundance.