Dielectric polarization invariance in dissociative electron attachment from condensed saturated hydrocarbons

Reactive scattering damage to DNA components by hyperthermal secondary ions

We have observed reactive scattering damage to fundamental DNA building blocks by the type of hyperthermal secondary ions that are produced along heavy ion tracks in biological media. Reactions include carbon abstraction by N+, and hydrogen abstraction by O- and N+, at collision energies down to 1 eV. Our results show that localized reactive scattering by hyperthermal secondary fragments can lead to important physicochemical damage to DNA in cells irradiated by heavy ions. This suggests a fundamentally different picture of nascent DNA damage induced by heavy ion tracks, compared to conventional (x or gamma) radiation tracks.

10–100 eV Ar+ ion induced damage to d-ribose and 2-deoxy-d-ribose molecules in condensed phase

We report that 10-100 eV Ar+ ion irradiation induces severe damage to the biologically relevant sugar molecules D-ribose and 2-deoxy-D-ribose in the condensed phase on a polycrystalline Pt substrate. Ar+ ions with kinetic energies down to 15 eV induce effective decomposition of both sugar molecules, leading to the desorption of abundant cation and anion fragments, including CH3+, C2H3+, C3H3+, H3O+, CHO+, CH3O+, C2H3O+, H-, O-, and OH-, etc. Use of isotopically labelled molecules (5- 13C D-ribose and 1-D D-ribose) reveals the site specificity for some of the fragment origins, and thus the nature of the chemical bond breaking. It is found that all of the chemical bonds in both molecules are vulnerable to ion impact at energies down to 15 eV, particularly both the endo- and exocyclic C-O bonds. In addition to molecular fragmentation, several chemical reactions are also observed. A small amount of O-/O fragments abstract hydrogen to form OH-. It is found that the formation of the H3O+ ion is related to the hydroxyl groups of the sugar molecules, and is associated with additional hydrogen loss from the parent or adjacent molecules via hydrogen abstraction or proton transfer. The formation of several other cation fragments also requires hydrogen abstraction from its parent or an adjacent molecule. These fragmentations and reactions are likely to occur in a real biomedium during ionizing radiation treatment of tumors and thus bear significant radiobiological relevance.

Fragmentation dynamics of condensed phase thymine by low-energy (10–200eV) heavy-ion impact

We report measurements of the formation and desorption of ionic fragments induced by very low-energy (10-200 eV) Ar(+) irradiation of thymine (T) films, deposited on a polycrystalline Pt substrate. A multitude of dissociation channels is observed, among which the major cation species are identified as HNCH(+), HNC(3)H(4) (+), C(3)H(3) (+), OCNH(2) (+), [T-OCN](+), [T-OCNH(2)](+), [T-O](+), and [T+H](+) and the major anions as H(-), O(-), CN(-),and OCN(-). Cation fragment desorption appears at much lower threshold energies (near 15 eV) than anion fragment desorption, where the latter depends strongly on the film thickness. It is proposed that anion fragment formation and desorption results from projectile impact-induced excitation of either (1) a neutral thymine molecule, followed by fragmentation and charge exchange between the energetic neutral fragment and the substrate (or film) and/or (2) a deprotonated monoanionic thymine molecule to a dissociative state, followed by a unimolecular fragmentation of the excited thymine anion. The H(-) and O(-) fragment formations may have a further contribution from dipolar dissociation, e.g., formation of electronically excited neutral thymine, followed by dissociation into O(-)+[T-O](+), due to their reduced sensitivity to the film thickness. Positive-ion fragment desorption exhibits no significant dependence on film thickness before the emergence of surface charging, and originates from a kinetically assisted charge-transfer excitation. The results suggest that the potential energy of the incident ion plays a significant role in lowering the threshold energy of kinetic fragmentation of thymine. Measurements of the time-dependent film degradation yields for 100-eV Ar(+) suggest a quantum efficiency for degradation of about six thymine molecules per incident ion.

Dissociative electron attachment to DNA

Electron-stimulated desorption of anions from thin films of linear and supercoiled DNA is investigated in the range 3-20 eV. Resonant structures are observed with maxima at 9.4+/-0.3, 9.2+/-0.3, and 9.2+/-0.3 eV, respectively, in the yield dependence of H-, O-, and OH- on the incident electron energy. Their formation is attributed to dissociative electron attachment.