Infrared spectra of solid-state ethanolamine: Laboratory data in support of JWST observations

¹ Max Planck Institute für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
² Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, 07743 Jena, Germany

^⋆ Corresponding author; suhasaria@mpia.de

Received: 22 March 2024
Accepted: 14 October 2024

Abstract

Context. Ethanolamine (NH₂CH₂CH₂OH; EA) has been identified in the gas phase of the interstellar medium within molecular clouds. Although EA has not been directly observed in the molecular ice phase, a solid-state formation mechanism has been proposed. However, the current literature lacks an estimation of the infrared band strengths of EA ices, which are crucial data for quantifying potential astronomical observations and laboratory findings related to their formation or destruction via energetic processing.

Aims. We conducted an experimental investigation of solid EA ice at low temperatures to ascertain its infrared band strengths, phase transition temperature, and multilayer binding energy. Since the refractive index and the density of EA ice are unknown, the commonly used laser interferometry method was not applied. Infrared band strengths were determined using three distinct methods. In addition to evaluating EA band strengths, we also tested the advantages and disadvantages of different approaches used for this purpose. The obtained lab spectrum of EA was compared with the publicly available MIRI MRS James Webb Space Telescope observations towards a low-mass protostar.

Methods. We used a combination of Fourier-transform transmission infrared spectroscopy and quadrupole mass spectrometry.

Results. The phase transition temperature for EA ice falls within the range of 175 to 185 K. Among the discussed methods, the simple pressure gauge method provides a reasonable estimate of band strength. We derived a band strength value of about 1 × 10⁻¹⁷ cm molecule⁻¹ for the NH₂ bending mode in the EA molecules. Additionally, temperature-programmed desorption analysis yielded a multilayer desorption energy of 0.61±0.01 eV. By comparing the laboratory data documented in this study with the JWST spectrum of the low-mass protostar IRAS 2A, an upper-limit for the EA ice abundances was derived.

Conclusions. This study addresses the lack of quantitative infrared measurements of EA at low temperatures, crucial for understanding EA’s astronomical and laboratory presence and formation routes. Our approach presents a simple yet effective method for determining the infrared band strengths of molecules with a reasonable level of accuracy.