Temperature dependence of electron attachment processes

Abstract

The temperature dependence of associative attachment (AA) and dissociative attachment (DA) is studied in the temperature range 300–800 K for the systems CF 3I, CF 3Cl, CF 2Cl 2, C 6F 5Cl and C 6F 5CN in a beam experiment. The cross-section for the nearly thermoneutral DA reactions Cl − CF 3 Cl and Cl − CF 2 Cl 2 increases strongly with temperature associated with a shift of the resonance position to lower energies and the appearance of a threshold peak at 0 eV. The shift of the resonance position is interpreted by the interplay between dissociation and autodetachment (AD) in the transient negative ion (TNI) while the threshold signal is due to transitions from vibrationally excited molecules close to the crossing between the neutral and anionic potential energy surface which represents the activation energy for electron attachment. The exothermic DA reaction I − CF 3 I has no activation barrier and is insensitive on temperature. In C 6F 5X (X = Cl, CN) both DA reactions X − C 6 F 5 X are endothermic. While in the case of X = Cl the cross-section increases with temperature as expected from thermodynamics, that for for CN − decreases with temperature. This last effect is interpreted by the comparatively slow mechanism for CN − formation (vibrational predissociation) in relation to the temperature dependence of the autodetachment rate. Both C 6F 5X molecules form long lived parent anions at 0 eV. Their intensity decreases with temperature owing to a decrease in the autodetachment lifetime.