We demonstrate that electrons at energies below the threshold for electronic excitation (<3 eV) effectively decompose gas phase uracil generating a mobile hydrogen radical and the corresponding closed shell uracil fragment anion (U-H)(-). The reaction is energetically driven by the large electron affinity of the (U-H) radical. This observation has significant consequences for the molecular picture of radiation damage, i.e., genotoxic effects or damage of living cells due to the secondary component of high energy radiation.

We present experimental results for 80 keV proton impact ionization of nucleobases (adenine, cytosine, thymine and uracil) based on an event by event analysis of the different ions produced combined with an absolute target density determination. We are able to disentangle in detail the various proton ionization channels from mass analyzed product ion signals in coincidence with the charge-analyzed projectile. Thus, for the first time, cross sections and fragmentation patterns are compared for direct ionization (with no charge transfer between the target molecule and the projectile) and for single electron capture (with projectile neutralization) in proton-nucleobase collisions. In addition we are able to determine a complete set of cross sections for the ionization of uracil by 20-150 keV proton