Electron stimulated desorption of anionic fragments from films of pure and electron-irradiated thiophene

The electron stimulated desorption (ESD) of anions is used to explore the effects of electron irradiation on a thiophene film and we report measurements for electron impact on multilayer thiophene condensed on a polycrystalline platinum substrate. Below 22 eV and at low electron dose, desorbed anions include H ^- (the dominant signal) as well as S ^-, CH ₂ ^-, SH ^- and SCH ₂ ^-. Yield functions show that anions are desorbed both by dissociative electron attachment (DEA) with resonances observed at 9.5, 11, and 16 eV, and for energies >13 eV, by dipolar dissociation (DD). An increase in the S ^- signal from electron irradiated (beam-damaged) thiophene films and the appearance of a new DBA resonance in the S ^- yield function at 6 eV are linked to rupture of the thiophene ring and the formation of sulfur-terminated products within the film. The threshold energy for ring rupture is 5 eV. The desorption of new anions such as C ₄H ₃S ^- (Thiophene-H) ^-is also observed from electron irradiated films and these likely arise from the decomposition of large radiation product molecules synthesized in the film. The yield functions of H ^-, S ^-, SH ^-, (Thiophene-H) ^-, and (Thiophene+H) ^- anions from irradiated thiophene films that have been annealed to 300 K, each exhibit a single resonant feature centered around 5.1 eV, suggesting that all signals derive from DBA to the same molecular radiation product. In contrast, only H ^- and S ^- are observed to desorb from films of 2-2-bithiophene and no resonance is seen below ∼10 eV in the anion yield functions. These data suggest that electron irradiation causes formation of ring-opened oligomers, and that closed-ring or "classical" oligomers, (similar to bithiophene) if formed, contribute little to the BSD of anions.