Low energy (0-10 eV) electron driven reactions in the halogenated organic acids CCl_3COOH, CClF_2COOH, and CF_3CHNH _2COOH (trifluoroalanine)

摘要

Negative ion formation following resonant electron attachment to the three title molecules is studied by means of a beam experiment with mass spectrometric detection of the anions. All three molecules exhibit a pronounced resonance in the energy range around 1 eV which decomposes by the loss of a neutral hydrogen atom thereby generating the closed shell anion (M-H)- (or RCOO -), a reaction which is also a common feature in the non-substituted organic acids. The two chlorine containing molecules CCl_3COOH and CClF_2COOH exhibit an additional strong and narrow resonance at very low energy (close to 0 eV) which decomposes by the cleavage of the C-Cl bond with the excess charge finally localised on either of the two fragments Cl ~- and (M-Cl)~-. This reaction is by two to three orders of magnitude more effective than hydrogen loss. Apart from these direct bond cleavages (C-Cl, O-H) resonant attachment of subexcitation electrons trigger additional remarkably complex unimolecular decompositions leading, e.g., to the formation of the bihalide ions ClHCl~- and ClHF~- from CCl_3COOH and CClF_2COOH, respectively, or the loss of a neutral CF_2 unit from trifluoroalanine thereby generating the fluoroglycine radical anion. These reactions require substantial rearrangement in the transitory negative ion, i.e., the cleavage of different bonds and formation of new bonds. F~- from both chlorodifluoroacetic acid and trifluoroalanine is formed at comparatively low intensity (more than three orders of magnitude less than Cl~- from the chlorine containing molecules) and predominantly within a broad resonant feature around 7-8 eV characterised as core excited resonance.