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

Abstract

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-, CH2-, SH- and SCH2-. 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 DEA 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 C4H3S- (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 DEA 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 approximately 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 ESD of anions.

Electron stimulated desorption of anions containing Oxygen and Nitrogen from self-assembled monolayers of DNA

“…Anions desorbed by LEE impact were measured by a time-of-flight (TOF) mass analyzer. The ESD system and its specifications have been discussed in detail previously [19]. Recently, a newly designed load-lock has been coupled to the TOF mass analyzer, and permits us to introduce and to study samples prepared under ambient conditions.…”

Revisiting the electron stimulated desorption of anions from thin films of CF2Cl2

“…The TOF mass analyzer has been described earlier [11]. In the present experiments, sample films were vapor deposited onto a platinum foil substrate, the temperature of which can be stabilized between 10 and 300 K using a closed cycle He cryostat and resistive heating elements.…”

“…We have attempted to improve on these earlier measurements using a method previously employed to study electron-induced degradation of thiophene films [11]. The data in Fig.…”

“…4b cannot be attributed to the combined effects of Cl 2 formation and loss. Indeed as discussed elsewhere [11], the linear slopes of each data set presented in Fig. 4a are proportional to the cross-section for Cl 2 production at the relevant energy.…”

“…Moreover, the sensitivity of the present TOF mass analyzer is dependent on the initial kinetic energy of desorbed ions, and faster (usually less massive) ions are detected less efficiently than the slower (typically more massive) ones [11]. However, since the mass ratio between the lightest (F − ) and heaviest fragment (Cl 2 − ) is only ∼3.5 this effect is unlikely as pronounced as in our earlier measurements involving the detection of H − [11]. Below 12 eV, the present F − and Cl − yield functions are in broad agreement with those published earlier [6,7].…”