Bond and site selectivity in dissociative electron attachment to gas phase and condensed phase ethanol and trifluoroethanol

Abstract

The formation of negative ions following electron impact to ethanol (CH₃CH₂OH) and trifluoroethanol (CF₃CH₂OH) is studied in the gas phase by means of a crossed electron–molecular beam experiment and in the condensed phase via Electron Stimulated Desorption (ESD) of fragment ions from the corresponding molecular films under UHV conditions. Gas phase ethanol exhibits two pronounced resonances, located at 5.5 eV and 8.2 eV, associated with a remarkable selectivity in the decomposition of the precursor ion. While the low energy resonance exclusively decomposes into O⁻, that at higher energy generates OH⁻ and a comparatively small signal of [CH₃CH₂O]⁻ due to the loss of a neutral hydrogen. CF₃CH₂OH shows a completely different behaviour, as now an intense feature at 1.7 eV appears associated with the loss of a neutral hydrogen atom exclusively occurring at the O site. The H⁻ formation from the gas phase compounds is below the detection limit of the present experiment, while in ESD from 3 MonoLayer (ML) films of CH₃CH₂OH and CF₃CH₂OH the most intense fragment is H⁻, appearing from a broad resonant feature between 7 and 12 eV. With CF₃CH₂OH, by using the isotopically-labelled analogues CF₃CD₂OH and CF₃CH₂OD it can be shown that this feature consists of two resonances, one located at 8 eV leading to H⁻/D⁻ loss from the O site and a second resonance located at 10 eV leading to the loss of H⁻/D⁻ from the CH₂ site.