Resonant infrared irradiation of CO and CH₃OH interstellar ices

Abstract

Context. Solid-phase photo-processes involving icy dust grains greatly affect the chemical evolution of the interstellar medium by leading to the formation of complex organic molecules and by inducing photodesorption. So far, the focus of laboratory studies has mainly been on the impact of energetic ultraviolet (UV) photons on ices, but direct vibrational excitation by infrared (IR) photons is expected to influence the morphology and content of interstellar ices as well. However, little is still known about the mechanisms through which this excess vibrational energy is dissipated, as well as its implications for the structure and ice photochemistry. Aims. In this work, we present a systematic investigation of the behavior of interstellar relevant CO and CH3OH ice analogs following the resonant excitation of vibrational modes using tunable IR radiation. We seek to quantify the IR-induced photodesorption and gain insights into the impact of vibrational energy dissipation on ice morphology. Methods. We utilized an ultrahigh vacuum setup at cryogenic temperatures to grow pure CO and CH3OH ices, as well as mixtures of the two. We exposed the ices to intense, near-monochromatic mid-IR (MIR) free-electron-laser radiation using the LISA end-station at the FELIX free electron laser facility to selectively excite the species. Changes to the ice are monitored by means of reflection-absorption IR spectroscopy combined with quadrupole mass-spectrometry. These methods also allowed us to characterize the photodesorption efficiency. Results. The dissipation of vibrational energy is observed to be highly dependent on the excited mode and the chemical environment of the ice. All amorphous ices undergo some degree of restructuring towards a more organized configuration upon on-resonance irradiation. Moreover, IR-induced photodesorption is observed to occur for both pure CO and CH3OH ices, with interstellar photodesorption efficiencies on the order of 10 molecules cm−2 s−1. This result is comparable to or higher than what is found for UV-induced counterparts. An indirect photodesorption of CO upon vibrational excitation of CH3OH in ice mixtures is also observed to occur, particularly in environments that are rich in methanol. Here, we discuss the astrochemical implications of these IR-induced phenomena.

Infrared photodesorption of CO from astrophysically relevant ices studied with a free-electron laser

“…They observed the loss of CO signal in RAIRS following irradiation at both the methanol and CO (4.67 µm) wavelengths. The lack of any detectable effect due to irradiation at 4.67 µm in our experiments is therefore in contrast to observations made by Santos et al 45 .…”

“…Hence it is expected that the small effects seen in RAIRS for their thicker ices, following irradiation at 4.67 µm, would not be detectable for our much thinner ices. We also note that Santos et al 45 did not observe a CO PID signal during irradiation at 4.67 µm, in agreement with our observations. It is also likely that CO-water ices show different behaviour when compared to CO-methanol ices due to the different interactions between CO and methanol compared to those between CO and water.…”

“…However, the ices used here are considerably thinner than those studied by Santos et al 45 . Hence it is expected that the small effects seen in RAIRS for their thicker ices, following irradiation at 4.67 µm, would not be detectable for our much thinner ices.…”

“…Although no previous work has investigated the resonant infrared irradiation of mixed CO and water-containing ices to the best of our knowledge, Santos et al 45 have studied the irradiation of mixed CO and methanol-containing ices. They observed the loss of CO signal in RAIRS following irradiation at both the methanol and CO (4.67 μm) wavelengths.…”

“…A more recent study of the infrared free-electron laser irradiation of CO 2 ice 44 under astrophysically relevant conditions has also shown that restructuring of the ice, as well as photodesorption, occurs following irradiation with infrared light. Infrared free-electron laser studies of CO irradiation in the presence of methanol ice have also been undertaken very recently and show the photodesorption of CO 45 .…”