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

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, H2, CO2, H2O and NH2 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.