Comparative studies of dissociative electron attachment to methyl halides

The dissociative electron attachment cross sections for the methyl halides vary in an enormous range from the virtually unmeasurable 10(-23) cm(2) for CH3Cl at room temperature to 10(-14) cm(2) for CH3I. In this paper we supp lement our previous studies by calculations of dissociative electron attach ment to CH3Br and compare results for all methyl halides studied so far. Th e rate as a function of temperature for CH3Cl and CH3Br exhibits an exponen tial dependence on 1/T (Arrhenius law) with the activation energy lower for CH3Br. CH3I does nor obey the Arrhenius law since the crossing point of th e neutral and anion potential curves occurs near the lowest vibrational lev els. The cross section as a function of electron energy for all of the meth yl halides studied here exhibits structure at the vibrational excitation th resholds that is associated with a vibrational Feshbach resonance like that observed recently in CH3I by Schramm et al(Schramm A, Fabrikant I I, Weber J M, Leber E, Ruf M-W and Hotop H 1999 J. Phys. B: Al. Mel. Opt. Phys. 32 2153).