UV-photoprocessing of acetic acid (CH3COOH)-bearing interstellar ice analogs
arxiv(2023)
摘要
Acetic acid (CH3COOH) was detected in the gas toward interstellar clouds, hot
cores, protostars and comets. Its formation in ice mantles was proposed and
acetic acid awaits detection in the infrared spectra of the ice as most other
COMs except methanol. The thermal annealing and UV-irradiation of acetic acid
in the ice was simulated experimentally in this work under astrophysically
relevant conditions. The experiments were performed under ultra-high vacuum
conditions. An ice layer was formed by vapor deposition onto a cold substrate,
and was warmed up or exposed to UV photons. The ice was monitored by infrared
spectroscopy while the molecules desorbing to the gas phase were measured using
a quadrupole mass spectrometer. The transformation of the CH3COOH monomers to
cyclic dimers occurs at 120 K and the crystal form composed of chain polymers
was observed above 160 K during warm-up of the ice. Ice sublimation proceeds at
189 K in our experiments. Upon UV-irradiation simpler species and radicals are
formed, which lead to a residue made of complex molecules after warm-up to room
temperature. The possible formation of oxalic acid needs to be confirmed. The
photodestruction of acetic acid molecules is reduced when mixed with water in
the ice. This work may serve to search for the acetic acid photoproducts in
lines of sight where this species is detected. A comparison of the reported
laboratory infrared spectra with current JWST observations allows to detect or
set upper imits on the CH3COOH abundances in interstellar and circumstellar ice
mantles.
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