Fragmentation of condensed-phase DNA components by hyperthermal He+ impact

We have observed severe damage to films of DNA components (thymine, D-ribose, 2-deoxy-D-ribose, and thymidine) induced by 10 to 100 eV He+ ions (2.5-25 eV/amu). The damage is attributed to the kinetic and potential energies, as well as the chemical reactivity of the He+ projectiles. Hyperthermal He+ ion impact on these films results in the complete destruction of the molecules via fragmentation, and direct and indirect (secondary fragment) reactive scattering, all of which leads to the desorption of abundant cation and anion fragments. The chemical composition of the fragments is identified, and the fragmentation patterns are compared to those produced by Ar+ irradiation. While the lower mass of He+ ions causes less efficient desorption of very heavy fragments, several reactive collisions are also observed, including hydrogen abstraction by incident He+ from any of the molecules studied to yield desorbing HeH+. This process likely occurs via the formation of an intermediate molecular ion (He-H-R)*+, which decays to HeH++R . Compared to Ar+, here a significant (x23) enhancement in H+ desorption is observed during He+ ion irradiation, which likely involves (a) the decay of the intermediate (He-H-R)*+, or desorbing HeH+, and (b) Auger or quasiresonant excitations of C, N, or O atom centers (or C-H, N-H, or O-H bonds) by the incident He+ ion. The formation of several molecular cations, e.g., H3O+, also requires hydrogen abstraction from its parent or adjacent molecules by initial cation fragments prior to desorption