Empty Level Structure and Dissociative Electron Attachment Cross Sections in Bromo and Chloro Dihaloalkanes

The energies of electron attachment associated with temporary occupation of the C-Br and/or C-Cl virtual sigma* orbitals of dichloro, bromochloro, and dibromo alkanes are measured in the gas phase with electron transmission spectroscopy (ETS). The corresponding orbital energies of the neutral molecules, supplied by HF/6-31G(d)//MP2/6-31G(d) calculations and scaled using an empirically calibrated linear equation, are compared with the experimental vertical attachment energies (VAEs). The largest energy splittings between the first two sigma* anion states are found in the methane derivatives, but two distinct resonances are also observed in the bromochloro and dibromo ethane derivatives. The temporary anions observed in ETS may dissociate, producing Br(-) and Cl(-) negative fragments with widely varying cross sections over the dihaloalkanes studied. The absolute total dissociative electron attachment (DEA) cross sections are evaluated, and their peak values are compared with the corresponding VAEs. In the mixed bromochloro alkanes, the relative intensities of the Br(-) and Cl(-) anion currents detected with a mass filter are also evaluated.