Invited Talk

Laboratory Simulations of Chemical Processes under Interstellar Conditions

Guillermo Manuel Muñoz Caro (Center of Astrobiology)

The accretion and desorption processes of gas molecules on cold grains play an important role in the evolution of dense clouds and circumstellar regions around YSOs. Given the low temperatures of 10-20 K in dark cloud interiors, thermal desorption is negligible and most molecules are expected to stick to grains leading to depletion in the gas phase. Laboratory simulations of these processes under astrophysically relevant conditions are required for their understanding. Several studies were conducted to study thermal desorption and more recently UV-photodesorption of ices, the later requiring optimum vacuum conditions. Reactions in the ice matrix are driven by energetic processing such as photon and ion irradiation. Cosmic rays penetrate inside dense clouds and process the ice either directly or by their induced secondary UV flux of 10³-10⁴ photons cm^-2 s^-1. In circumstellar regions the photon flux (UV and X-rays) is expected to be significantly higher and icy grain mantles present in the outer parts experience significant irradiation. The produced radicals lead to the formation of new species in the ice, some of them of prebiotic interest.

Ultra-high-vacuum (UHV) set-ups, with base pressures in the 10^-10 to 10^-11 mbar range, are used to study ice surface processes like photodesorption and temperature programmed desorption with negligible background H₂O contamination. More traditional set-ups operating at 10^-7 to 10^-8 mbar remain useful to study bulk ice processes. The evolution of the ice sample is usually monitored by in situ infrared spectroscopy, while the volatile species in the gas phase are monitored by mass spectroscopy. Different devices are used to calibrate the photon flux of the lamp used for irradiation. In this presentation, we will outline the improvements introduced by UHV set-ups for a number of selected experiments. In this context, UV-photoproduct formation and UV-photodesorption of H₂O, CH₃OH, CO, and other abundant molecular ice components in interstellar and circumstellar regions deserve extra attention. Also the effects of ion irradiation and photoprocessing of ice using different photon wavelengths (e.g. X-rays) should be considered. The new UHV experiments are revisiting previous results and opening new research topics in the field of solid state astrochemistry. The majority of the ice absorptions detected in the infrared have an assigned carrier molecule. We are now able to study ices at the monolayer level and therefore current research should include the study of aspects that are not well known such us ice surface processes, characterization of the precise ice morphology, and a better determination of the reaction rates in ice experiments where the presence of water contamination was a serious obstacle.

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