Swift heavy ion irradiation of thymine at cryogenic temperature

Thymine (C5H6N2O2) is a basic N-heterocyclic nucleobase in all known organisms, and this molecule is also found in meteoritic materials. This study aims to investigate thymine's physical and chemical modifications under ion irradiation in cryogenic conditions. Space radiation was simulated by exposing thymine at 27 K to 230 MeV 48Ca10+ ions. Fourier transform infrared spectroscopy (FTIR) was employed to monitor the degradation of a 2.8 μm thick sample film under irradiation. From the intensity decrease of the infrared absorptions as a function of ion fluence, the destruction cross-section (σ), required to dissociate or eject a thymine molecule, is deduced by an exponential function. The physical and chemical modifications induced by energetic projectiles can be related to the electronic stopping power Se as σ=Se/D0, where D0=9.6±0.4 eV/molecule is the effective mean dose needed to destroy the thymine molecule at 27 K. Also, new molecular species formed under irradiation are observed and, based on infrared spectra, identified as CN−, OCN−, HCNO, and CO.