VUV and mid-UV photoabsorption cross sections of thin films of glycine and alanine: Application to their photochemistry in the solar system.

Experiments conducted in Low Earth Orbit (LEO) since the 1990s have investigated the evolution of organic matter in solid-phase (including amino acids) under solar photon irradiation to improve our understanding of its stability and reactivity on Solar System bodies. Interpreting results of such irradiation experiments requires detailed knowledge of the absorption properties of the exposed compounds, which are essential inputs for modeling their photochemical evolution in LEO. This study examines the photostability of two amino acids, glycine and alanine, which are prevalent in known life forms. The objective is to measure their photoabsorption cross sections and estimate photolysis rates within the VUV/UV range (50-300 nm). To achieve this, thin films of glycine and alanine (thickness from 10 to 100 nm) were prepared on MgF2 windows using a sublimation/recondensation method. The thickness of these deposits was accurately measured using interferometric microscopy and infrared spectroscopy. Subsequently, VUV/UV spectra of the samples were recorded, followed by calculation of the absorption cross sections and estimation of upper limits for photolysis rates and corresponding lower limits for half-lives of glycine and alanine in solid phase. These data provide quantitative constraints on amino acids photostability in extraterrestrial environments, improving the interpretation of irradiation experiments and photochemical models.