Low-Energy Electron Damage to Condensed-Phase Deoxyribose Analogues Investigated by Electron Stimulated Desorption of H- and Electron Energy Loss Spectroscopy

Abstract

We report the 5−20 eV electron-stimulated desorption (ESD) yields of H- produced by dissociative electron attachment (DEA) to the DNA backbone sugar-like analogues tetrahydrofuran (I), 3-hydroxytetrahydrofuran (II), and α-tetrahydrofurfuryl alcohol (III) physisorbed on a polycrystalline Pt substrate. For the pure disordered solid films, we observe one peak in the H- yield function at an incident electron energy, Ei, of ∼10 eV, which is attributed to selective dissociation of endocyclic α-CH bonds via the formation of a core-excited shape resonance; no desorbing polyatomic fragments were detected. A second peak is also observed in the H- ESD yield function of II; it appears as a weak shoulder superimposed on the low-energy side of the 10 eV structure displaying a sharp vertical onset near 6.7 eV and a peak maximum around 7.3 eV. The sharp onset and narrow energy width are characteristic of a core-excited Feshbach resonance; it is attributed to H- produced via DEA to the OH substituent whose corresponding parent state may be similar to that observed in CH3OH, where the 7.3 eV resonance originating from the hydroxyl group was assigned to a 2A‘ ‘ Feshbach resonance. High-resolution electron energy loss (HREEL) spectra recorded with 11 and 14 eV incident electrons are also reported for solid I. They show that, with increasing electron dose, degradation of the solid leads to formation of a C−O π-bonded product believed to arise from fragmentation of an α-cleaved transient intermediate following direct electronic excitation of the parent molecule. The HREEL spectra suggest a rather complex fragmentation pathway following low-energy electron bombardment. In vacuo kinetic energy (Ek) distribution measurements of desorbed H- in the 10 eV resonance suggest that for I, II, and III a dissociation mechanism similar to that proposed for the 14 eV HREELS spectra occurs, whereby α-cleavage of the C−O bond occurs within the lifetime of the dissociative negative ion state.