Rotational and vibrational effects on the energy loss of hydrogen colliding on glycine at low irradiation energies

Abstract The implications of the energy deposition of neutral hydrogen irradiation on biomolecules is important to understand cell damage by heavy ions in radiotherapy and dosimetry. In this work, we study the energy loss of hydrogen atoms incidents on the glycine molecules in gas phase at low collision energy, based on the Density-Functional Tight-Binding (DFTB) and the Electron-Nuclear Dynamics (END) theories. DFTB is a quantum-classical molecular dynamics approach on a ground state surface, valid at low collision energy while END is an ab initio quantum chemistry approach that includes electronic excitations at the random phase approximation level and accounts for electron and nuclear couplings. These approaches have an overlap collision energy region from 10 to 100 eV, where our results complement fairly well to each other. We find that the electronic energy loss (stopping cross-section) shows a linear velocity dependence with a threshold at a hydrogen velocity of 0.01414 a. u. Corresponding to around 5 eV and 1 eV for the END and DFTB results, respectively. The ro-vibrational and nuclear energy loss become significant producing the main contributions to the energy loss. Finally, we compare to SRIM results showing that it neglects chemical bonding, as well as the ro-vibrational contributions at low collision energy such that it underestimates the energy loss and does not account for threshold effects. Our results shed light on the importance of ro-vibrational chemical processes at low collision energies when describing the energy loss of heavy ions when interacting with molecules.