Electron stimulated desorption of H− from thin films of 5-halouracils

Abstract

We present measurements of low-energy (1–20 eV) electron stimulated desorption (ESD) of H⁻ from thin films of 5-halouracils (5-XU, X = F, Cl, Br and I) condensed on polycrystalline Pt. The onset for H⁻ desorption is ∼5.0 eV. For all 5-XU a single dissociative electron attachment (DEA) H⁻ peak is observed, which is attributed to at least two dissociation channels at approximately 7.1 and 8.6 eV, resulting from N–H and C–H bond cleavage, respectively. Their relative contribution is parent-molecule dependent. Also, for a given molecule, these dissociation channels are affected differently by the metal, and the DEA maximum shifts by ∼0.5 eV as the film thickness increases. Above 10 eV, dipolar dissociation (DD) is the dominant H⁻ formation mechanism. The yields of H⁻ vary in the following relative order (T = thymine, U = uracil): T > U > 5-FU > 5-ClU > 5-BrU > 5-IU. A model is developed to explain the film thickness dependence of the desorbed anion signal: above a coverage of ∼1 monolayer (ML), the film thickness dependence of the H⁻ ESD yield is interpreted in terms of incident electron and desorbing ion transmission within the films. The main difference in the thickness dependence of H⁻ produced via DEA and DD is attributed to the fact that electron transfer to the metal from the transient molecular anion leads to dramatic decrease in the H⁻ ESD signal below ∼1 ML, whereas it does not affect significantly DD, since its intermediate state is neutral.