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

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