Dissociative electron attachment in cyclopentanone, γ-butyrolactone, ethylene carbonate, and ethylene carbonate-d4:: Role of dipole-bound resonances

The title compounds allow the study of the effect of the dipole moment and the energy of the lowest shape resonance on dissociative electron attachment, since both the dipole moments (2.9, 4.5, and 5.3 Debye) and the pi* attachment energies (1.15, 1.98, and 2.94 eV) increase progressively along the series. An unexpected observation was made in ethylene carbonate, the molecule with the largest dipole moment, where two fragments (CO3- and C2H3O-) are formed at low energies (1-1.5 eV), well below the first p* attachment energy. We assign these bands to dissociation of a vibrationally excited dipole bound anion formed upon electron attachment. Furthermore, the number of fragments at low energies (below 5 eV) was generally found to increase with the number of oxygen atoms in the molecules, presumably because of the larger number of possible fragments with large electron affinity. Finally, "scrambling'' of atoms was found in the fragmentation of ethylene carbonate even at low energies, indicating that the initially formed autodetaching anion rapidly stabilizes by sliding to sections of the potential surface where autodetachment is slow or not possible, allowing more time for chemical rearrangement. Even more "scrambling'' and more fragments are found at higher energies, 6-9 eV, for all three compounds, where dissociative attachment is assigned to doubly excited Feshbach resonances. (C) 1999 American Institute of Physics. [S0021-9606(99)00423-7].