Ionization and fragmentation modes of nucleobases after collisions with multiply charged ions

We studied multiply charged ion (MCI) induced ionization, excitation and fragmentation of the nucleobases uracil and thymine. Ions of different charge state at velocities between 0.2 and 0.4 atomic units were used as projectiles. By means of time-of-flight spectrometry of the collision products in a statistically representative mode as well as in triple coincidence mode, three different interaction types could be identified. Large impact parameters lead to "gentle'' ionization of the molecules, whereas in close collisions mainly multifragmentation occurs. This gives rise to a bimodal mass-spectrum as found in fullerene fragmentation studies. An additional two-body breakup channel leading to intermediate fragment sizes was found for both nucleobases. From the coincidence data, the kinetic energy release of this channel could be obtained.