Electron Attachment to XCN (X = Br, Cl). Competition between X- and CN- Formation

Electron attachment to BrCN and ClCN is studied in a crossed beam experiment. Relative cross sections for the formation of negative ions in the energy range 0?15 eV are reported. The kinetic energy release of fragment ions is studied by means of a time-of-flight (TOF) analysis. Both target molecules effectively capture low-energy electrons (<0.5 eV), leading to the complementary dissociative attachment (DA) channels X^- + CN (a) and X + CN^- (b). From the shape of the ion yield curve, the temperature behavior, and ab initio calculations, it is concluded that in both BrCN and ClCN the DA channels a and b originate from precursor ions with a different electronic configuration:? channel a correlates with a ²Σ state leading to direct electronic dissociation and channel b to a ²Π state associated with vibrational predissociation. The ions X^- and CN^- also appear from further, comparatively weak resonances at higher energies. The TOF analysis reveals that only the products Br^- + CN appear with appreciable kinetic energy (2.25 eV) from a resonance between 4.5 and 8.5 eV. For the three other channels (Br + CN^-, Cl^- + CN, Cl + CN^-), however, most of the total excess energy (amounting up to 7?8 eV) appears as internal energy of CN or CN^-. In the system BrCN, the Br^- intensity strongly increases with the gas temperature while the CN^- intensity strongly decreases as expected from the endothermicity of reaction a and the exothermicity of reaction b. In ClCN both DA channels are endothermic. While the intensity of Cl^- increases with the gas temperature, the CN^- intensity decreases above 600 K. This behavior is explained by the (temperature-dependent) competition between autodetachment and the comparatively slow vibrational predissociation process in the transient molecular anion.