NEGATIVE-ION RESONANCES IN CF4 PROBED BY DISSOCIATIVE ELECTRON-ATTACHMENT

An electrostatic impact spectrometer is used to measure, between 3 and 10 eV, the kinetic energy, the angular distributions and the yield of negative fragment ions formed by dissociative attachment in CF4. The energy range and the symmetry of the resonances are determined as well as the dissociation processes and their dynamics. The first resonance state, observed around 6.8 eV, is attributed to the CF4-2T2 shape res onance adiabatically dissociating in the two channels F- + CF3 and CF3 - + F. The excess energy is shared between the fragment kinetic energi es and the internal (vibrational) excitation of the trifluoromethyl fr agment. The next resonance, located around 8 eV, is assumed to be of T 1 symmetry. We suggest that this Feshbach resonance dissociates in F i ons by the sequential breaking of two C-F bonds through an intermediat e CF3 electronically excited negative molecular ion. The 2A1 shape r esonance has not been observed in the dissociative channel.