X-ray induced irradiation effects in glycine thin films: A time-dependent XPS and TPD study

Abstract

The radiation-induced chemistry in ultrahigh vacuum (UHV)-sublimated thin glycine films during continuous soft X-ray irradiation was investigated by X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). The glycine films were irradiated with an Al Kα X-ray source (1486.6 eV) for over 360 min and XPS spectra were taken repeatedly every 60 min. TPD measurements of pristine and X-ray exposed glycine layers were also conducted. The results show that the glycine films are very sensitive to damage by X-ray illumination. The analysis of C 1s, N 1s and O 1s spectral regions together with TPD changes indicate that amino acid degradation occurs mainly via dehydrogenation, decarboxylation, dehydration and deamination of pristine molecules. Among the decomposition products, H 2, CO 2, H 2O and NH 2 are identified. Enrichment of the X-ray exposed surface with hydrocarbon fragments is detected as well. The observed effects must be taken into account in studies of amino acids and peptides utilizing X-rays.