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

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 similar to5.0 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 similar to0.5 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 similar to1 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 similar to1 ML, whereas it does not affect significantly DD, since its intermediate state is neutral.