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 (2) Sigma state leading to direct electronic dissociation and channel b to a (2) Pi. 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.