Low-energy electron damage to condensed-phase deoxyribose analogues investigated by electron stimulated desorption of H- and electron energy loss spectroscopy

We report the 5-20 eV electron-stimulated desorption (ESD) yields of H- pro duced by dissociative electron attachment (DEA) to the DNA backbone sugar-l ike analogues tetrahydrofuran (I), 3-hydroxytetrahydrofuan (II), and alpha- tetrahydrofurfuryl alcohol (III) physisorbed on a polycrystalline Pt substr ate. For the pure disordered solid films, we observe one peak in the H- yie ld function at an incident electron energy, E-i, of similar to 10 eV, which is attributed to selective dissociation of endocyclic alpha-CH bonds via t he formation of a core-excited shape resonance; no desorbing polyatomic fra gments were detected. A second peak is also observed in the H- ESD yield fu nction 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 a nd a peak maximum around 7.3 eV. The sharp onset and narrow energy width ar e 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 origin ating from the hydroxyl group was assigned to a (2)A " Feshbach resonance. High-resolution electron energy loss (HREEL) spectra recorded with 11 and 1 4 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 pi-bonded product believed to arise from fragmentation of an cr-cleave d transient intermediate following direct electronic excitation of the pare nt molecule. The HREEL spectra suggest a rather complex fragmentation pathw ay following low-energy electron bombardment. In vacuo kinetic energy (E-k) distribution measurements of desorbed H- in the 10 eV resonance suggest th at for I, II, and III a dissociation mechanism similar to that proposed for the 14 eV HREELS spectra occurs, whereby a-cleavage of the C-O bond occurs within the lifetime of the dissociative negative ion state.