Infrared analysis of Valine degradation by keV electrons. Measurements and CASINO-extended model predictions

Abstract

ABSTRACTDegradation of L-valine by 0.06–1.0-keV electron beams is analysed in laboratory, at 10, 150, and 300 K. Valine film thicknesses are measured by profilometry, permitting band strength determination for selected valine bands. The column density evolutions during the irradiation are measured via infrared spectroscopy and destruction cross-sections are extracted; the latter range from 1–100 × 10−16 cm2. Data show that, in general, destruction cross-sections depend not only on projectile energy and sample temperature but also on sample thickness and beam fluence. In order to understand these findings, a statistical model is proposed for describing the radiolysis of organic materials. Comparing predictions with experimental results for valine, the main trends are reproduced. The quantitative disagreement indicates that it is necessary to include sputtering in the model. A major contribution of the model is to permit to simulate, layer by layer, the sample degradation rate as a function of fluence. The model assumes that the destruction cross-section of precursor molecules is proportional to the local stopping power and uses the Monte Carlo CASINO code to determine the deposited energy distribution in the bulk. As astrophysical implications, the radiolysis of valine dissolved in H2O ice and shielded by a CO2 layer is predicted, as an attempt to analyse the degradation of realistic cosmic materials by keV electrons.