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Broadband infrared spectroscopy of methanol isotopologues in pure, H2O-rich, and CO-rich ice analogues

Adam Vyjidak, Barbara Michela Giuliano, Pavol Jusko, Heidy M. Quitian-Lara, Felipe Fantuzzi, Giuseppe A. Baratta, Maria Elisabetta Palumbo, Paola Caselli

TL;DR

This study provides a broadband infrared reference for methanol and its five deuterated isotopologues in solid ice analogues, combining pure, H2O-rich, and CO-rich matrices at 10 K with annealing to 120 K. The authors integrate high-resolution FTIR spectroscopy with anharmonic vibrational calculations to assign fundamentals, overtones, and combination bands, revealing robust isotopologue-specific fingerprints such as the CH2DOH doublet at 1293 and 1326 cm$^{-1}$ and the CHD2OH doublet at 1301 and 1329 cm$^{-1}$ that persist across matrices. These signatures enable reliable identification of deuterated methanol in JWST observations and provide stringent constraints for gas-grain astrochemical models of deuterium enrichment in the early phases of star and planet formation. The data, including matrix effects and temperature-dependent crystallisation, form a valuable reference for interpreting interstellar ices and accompanying theoretical work on methanol deuteration pathways; all data are openly available at Zenodo.

Abstract

Deuterium fractionation is highly efficient during the early stages of star formation, particularly in starless and prestellar cores where temperatures are low (<10 K) and molecular freeze-out onto dust grains is significant. Methanol forms early in these environments following CO freeze-out via successive hydrogenation reactions on grain surfaces, while the production of deuterated methanol requires elevated gas-phase D/H ratios generated through dissociative recombination of deuterated H3+. Consequently, large abundances of deuterated methanol are observed toward young stellar objects where prestellar ices have recently sublimated. Here, we present laboratory infrared spectra of methanol and its deuterated isotopologues in astrophysical ice analogues, complemented by anharmonic vibrational calculations used to guide band assignments. Experiments were performed at the CASICE laboratory using a Bruker Vertex 70v spectrometer coupled to a closed-cycle helium cryostat, with isotopologue ices deposited at 10 K under high-vacuum conditions. Infrared transmission spectra were recorded over 6000-30 cm-1 (1.67-333 um) and compared with spectra of pure isotopologue ices. Distinctive mid-infrared band patterns are identified for each deuterated species. In particular, CH2DOH exhibits a characteristic doublet at 1293 and 1326 cm-1 (7.73 and 7.54 um), while CHD2OH shows a similar doublet at 1301 and 1329 cm-1 (7.69 and 7.52 um), both remaining largely invariant across all studied ice mixtures. These robust spectral signatures provide reliable tracers for identifying deuterated methanol in JWST observations and for constraining astrochemical gas-grain models of deuterium enrichment prior to star and planet formation.

Broadband infrared spectroscopy of methanol isotopologues in pure, H2O-rich, and CO-rich ice analogues

TL;DR

This study provides a broadband infrared reference for methanol and its five deuterated isotopologues in solid ice analogues, combining pure, H2O-rich, and CO-rich matrices at 10 K with annealing to 120 K. The authors integrate high-resolution FTIR spectroscopy with anharmonic vibrational calculations to assign fundamentals, overtones, and combination bands, revealing robust isotopologue-specific fingerprints such as the CH2DOH doublet at 1293 and 1326 cm and the CHD2OH doublet at 1301 and 1329 cm that persist across matrices. These signatures enable reliable identification of deuterated methanol in JWST observations and provide stringent constraints for gas-grain astrochemical models of deuterium enrichment in the early phases of star and planet formation. The data, including matrix effects and temperature-dependent crystallisation, form a valuable reference for interpreting interstellar ices and accompanying theoretical work on methanol deuteration pathways; all data are openly available at Zenodo.

Abstract

Deuterium fractionation is highly efficient during the early stages of star formation, particularly in starless and prestellar cores where temperatures are low (<10 K) and molecular freeze-out onto dust grains is significant. Methanol forms early in these environments following CO freeze-out via successive hydrogenation reactions on grain surfaces, while the production of deuterated methanol requires elevated gas-phase D/H ratios generated through dissociative recombination of deuterated H3+. Consequently, large abundances of deuterated methanol are observed toward young stellar objects where prestellar ices have recently sublimated. Here, we present laboratory infrared spectra of methanol and its deuterated isotopologues in astrophysical ice analogues, complemented by anharmonic vibrational calculations used to guide band assignments. Experiments were performed at the CASICE laboratory using a Bruker Vertex 70v spectrometer coupled to a closed-cycle helium cryostat, with isotopologue ices deposited at 10 K under high-vacuum conditions. Infrared transmission spectra were recorded over 6000-30 cm-1 (1.67-333 um) and compared with spectra of pure isotopologue ices. Distinctive mid-infrared band patterns are identified for each deuterated species. In particular, CH2DOH exhibits a characteristic doublet at 1293 and 1326 cm-1 (7.73 and 7.54 um), while CHD2OH shows a similar doublet at 1301 and 1329 cm-1 (7.69 and 7.52 um), both remaining largely invariant across all studied ice mixtures. These robust spectral signatures provide reliable tracers for identifying deuterated methanol in JWST observations and for constraining astrochemical gas-grain models of deuterium enrichment prior to star and planet formation.
Paper Structure (30 sections, 2 equations, 19 figures, 18 tables)

This paper contains 30 sections, 2 equations, 19 figures, 18 tables.

Table of Contents

  1. Introduction
  2. Experimental methods
  3. Experimental setup
  4. Ice preparation
  5. Film thickness
  6. Spectral data fitting
  7. Computational details
  8. Results
  9. Pure methanol ices
  10. O-H stretch
  11. C-H stretches
  12. CH3 deformations
  13. O-H bending
  14. CH3 rocking
  15. C-O stretch
  16. ...and 15 more sections

Figures (19)

  • Figure 1: Schematic representation of the cryostat's vacuum chamber, coupled to the Bruker Vertex 70v spectrometer at CAS. Adapted from Giuliano2019.
  • Figure 2: Transmission spectra of solid methanol and its five deuterated isotopologues at 10 K. The region between 650590$\text{cm}^{-1}$ (15.416.9µm), affected by increased noise from the beamsplitter, has been removed. The main CH3OH vibrational modes are indicated at the top. Spectra are vertically offset for clarity.
  • Figure 3: Transmission spectra of solid methanol and its deuterated isotopologues recorded at 10K, highlighting the O--H/O--D stretching region. For CH3OH, the dashed rectangle marks the O--H stretching band. The labels $\nu(\text{O--H})$ and $\nu(\text{O--D})$ denote the fundamental O--H and O--D stretching modes, respectively. Corresponding peak positions are listed in Table \ref{['tab:mode1_OH_stretch']}. Spectra are vertically offset for clarity.
  • Figure 4: Transmission spectra of solid methanol and its deuterated isotopologues recorded at 10K, focusing on the C--H/C--D stretching region. For CH3OH, the dashed rectangle marks the C--H stretching bands. The stretching fundamentals are labelled $\nu$as and $\nu$s, with the vibrating group indicated in parentheses. The corresponding peak positions are listed in Table \ref{['tab:mode2_CH_stretches']}. Spectra are vertically offset for clarity.
  • Figure 5: Transmission spectra of solid methanol and its deuterated isotopologues recorded at 10K, highlighting the CH3 deformation region. Modes are labelled $\delta$ for the in-plane bend. $\delta$as and $\delta$s denote the asymmetric and symmetric components, respectively, with the vibrating group given in parentheses. For CH3OH, the dashed rectangle marks the CH3 deformation region. Corresponding peak positions are listed in Table \ref{['tab:mode3_CH3_deformations']}. Spectra are vertically offset for clarity.
  • ...and 14 more figures