Dissociative electron attachment near threshold, thermal attachment rates, and vertical attachment energies of chloroalkanes

The peaks appearing near zero energy in the dissociative electron attachment cross section of 18 chloroalkanes are studied by electron beam methods. Fits to the experimental data are made using model cross sections having appropriate energy dependences and inclusion of the broadening due to the electron energy distribution. The magnitudes of the zero peaks are found to be well correlated with the vertical attachment energies (VAE) associated with occupation of the lowest empty orbitals of the compounds. The magnitudes rise exponentially by more than five orders of magnitude as VAE decreases from 2 eV to a slightly negative value. This dependence is a consequence not only of the thermal population of vibrational levels, but also of an approximately linear relationship between VAE and the energy of the crossing between the neutral and anion potential curves. Franck–Condon factors for the transition to the anion curve are computed for model potential curves, and the nature of the attachment from vibrational levels with energies near that of the crossing point is explored in a local potential resonance picture. A substantial contribution arises from tunneling to the anion state from vibrational levels below the barrier. Thermal attachment rate constants are also computed from our data. These are also shown to vary exponentially with VAE.