Effects of electron-impact ionization on the damage to biomolecules irradiated by XFEL

The damage to biomolecules irradiated by an X-ray free electron laser is investigated with time-dependent rate equations, considering photoionization, Compton scattering, Auger decay and electron-impact ionization of the C atom and its ions. Averaged energies of secondary electrons, which are produced by electron-impact ionization, are calculated with the binary encounter dipole model (Kim Y-K. and Rudd M. E., 1994 Phys. Rev. A, 50 3954) to study electron-energy distribution in the biomolecule. We found that those energies for the C atom, C1+ and C2+ ions are about 20, 60 and 150 eV, respectively, at an incident electron energy of 12keV, while those energies are in the order of tens of electronvolts when the incident electron energy is about 250 eV. We also found that the importance of electron-impact ionization increases as the X-ray flux decreases.