Infrared spectra of complex organic molecules in astronomically relevant ice mixtures

Abstract

Context. In the near future, high spatial and spectral infrared (IR) data of star-forming regions obtained by the James Webb Space Telescope (JWST) may reveal new solid-state features of various species, including more intriguing classes of chemical compounds. The identification of Complex organic molecules (COMs) in the upcoming data will only be possible when laboratory IR ice spectra of these species under astronomically relevant conditions are available for comparison. For this purpose, systematic series of laboratory measurements are performed, providing high-resolution IR spectra of COMs. Here, spectra of pure methylamine (CH3NH2) and methylamine-containing ices are discussed. Aims. The work is aimed at characterizing the mid-IR (500 -4000 cm −1 , 20 -2.5 µm) spectra of methylamine in pure and mixed ices to provide accurate spectroscopic data of vibrational bands that are most suited to trace this species in interstellar ices. Methods. Fourier transform infrared (FTIR) spectroscopy is used to record spectra of CH3NH2 in the pure form and mixed with H2O, CH4, and NH3, for temperatures ranging from 15 to 160 K. The IR spectra in combination with HeNe laser (632.8 nm) interference data of pure CH3NH2 ice was used to derive the IR band strengths of methylamine in pure and mixed ices. Results. The refractive index of amorphous methylamine ice at 15 K was determined as being 1.30 ± 0.01. Accurate spectroscopic information and band strength values are systematically presented for a large set of methylamine-containing ices and different temperatures. Selected bands are characterized and their use as methylamine tracers is discussed. The selected bands include the following: the CH3 antisymmetric stretch band at 2881.3 cm −1 (3.471 µm), the CH3 symmetric stretch band at 2791.9 cm −1 (3.582 µm), the CH3 antisymmetric deformation bands, at 1455.0 and 1478.6 cm −1 (6.873 µm and 6.761 µm), the CH3 symmetric deformation band at 1420.3 cm −1 (7.042 µm), and the CH3 rock at 1159.2 cm −1 (8.621 µm). Using the laboratory data recorded in this work and ground-based spectra of ices toward YSOs (Young Stellar Objects), upper-limits for the methylamine ice abundances are derived. In some of these YSOs, the methylamine abundance is less than 4% relative to H2O.

JWST Observations of Young protoStars (JOYS+): Detecting icy complex organic molecules and ions

“…In addition, because of the ice matrix in which the molecules are embedded, the shape of the absorption bands change, with mixing ratio and temperature (e.g., Öberg et al 2007;Bouwman et al 2007). While ice COMs show peculiar band shapes at high temperatures (>70 K), they are very similar at lower temperatures (see Terwisscha van Scheltinga et al 2018Rachid et al 2020Rachid et al , 2021Rachid et al , 2022Slavicinska et al 2023). In this sense, we add to the criteria presented by Boogert et al (2015) and Jørgensen et al (2020) that a degeneracy analysis of molecules sharing similar functional groups is needed in order to claim a firm ice COM detection.…”

“…These two COMs were also tentatively identified in the JWST high spectral resolution and sensitivity spectrum of IRAS 15398-3359 (Yang et al 2022) and toward two background stars (McClure et al 2023), albeit at lower spectral resolution. Other upper limits have been determined for methyl formate (CH 3 OCHO; Terwisscha van Scheltinga et al 2021), methylamine (CH 3 NH 2 ; Rachid et al 2021), methyl cyanide (CH 3 CN; Rachid et al 2022), and formamide (NH 2 CHO; Schutte et al 1999;Slavicinska et al 2023). Boudin et al (1998) also estimated upper limits for molecules such as ethane (C 2 H 6 ), acetylene (C 2 H 2 ), hydrazine (N 2 H 4 ), hydrogen peroxide (H 2 O 2 ) and the hydrozonium ion (N 2 H + 5 ).…”

LIDA: The Leiden Ice Database for Astrochemistry

“…All these data are recorded in transmission. Details concerning IRASIS are available in Rachid et al (2021). In the near future, a quartz crystal micro-balance will also be incorporated.…”

“…Infrared (IR) spectroscopy is a diagnostic tool used to characterize chemical structures of molecules and distinguish their functional groups (e.g., Coblentz 1905;Balkanski 1989). For this reason, a number of laboratories around the world have been focusing on providing laboratory-based IR data of interstellar ice analogs for a range of different ice compositions and temperatures (e.g., Hagen et al 1979;Strazzulla et al 1984;Schmitt et al 1989;Grim et al 1989;Hudgins et al 1993;Boudin et al 1998;Palumbo et al 1998;Schutte 1999;Muñoz Caro et al 2002;Öberg et al 2009;Pilling et al 2010;Vinogradoff et al 2015;Terwisscha van Scheltinga et al 2018;Urso et al 2020;Terwisscha van Scheltinga et al 2021;Rachid et al 2020Rachid et al , 2021Potapov et al 2021). IR spectra directly represent the molecular geometry of a molecule and as such can act as a molecular fingerprint.…”

Methoxymethanol formation starting from CO hydrogenation

“…However, as this is a gas-phase observation, we cannot make a direct comparison with this study to investigate the formation route because many of the precursors such as H 2 CO are known to have rich gas-phase chemistry as well. Scheduled observations with JWST will allow us to elaborate more accurately on the grain-surface chemistry, although recently it was found that it is exceptionally hard to pre-deposit pure MM ice and record high-resolution IR spectra as for other frozen COMs (Rachid et al 2021). Solid MM is clearly much harder to tackle than its gas phase equivalent.…”