High-resolution study of dissociative electron attachment to dipolar molecules atlow energies: CH2Br2 and CCl3Br

Abstract

Using the laser photoelectron attachment method with an energy width ≤ 1 meV at lowenergies (≤ 172meV) and an electron beam method (energy width about 70 meV) at higherenergies (up to 2 eV), we have determined absolute cross sections σe (E)for dissociative electron attachment to the dipolar molecules CH2Br2 andCCl3Br by normalization to known electron attachment rate coefficients.At thresholds for vibrational excitation of the CH2Br2 molecule,the DA cross section exhibits pronounced structure due to couplingof the attachment process with scattering channels; below the ν3 = 1onset for the symmetric CBr2 stretch vibration, a clear vibrationalFeshbach resonance is observed. At low energies the cross sections σe (E)show an energy dependence between E−1/2 andE−1,in essential agreement with predictions of an extended Vogt–Wannier (EVW)capture model which includes the long-range electron–dipole interaction inaddition to the polarization force. The determined absolute values are, however,substantially smaller than those predicted by the EVW model. SemiempiricalR-matrixcalculations have been carried out which show that at low electron energies(E < 0.15eV) Br− formation from CCl3Br proceeds by s-wave attachment to the anionground state while a broad peak, observed around 0.6 eV and evolvingpredominantly into the Cl− channel, is due to an excited anion state. Comparisonsare made with cross sections and rate coefficients obtained in previousphotoelectron attachment work (TPSA) and in electron beam as well asswarm experiments. Based on our joint experimental results for σe (E),we report the electron temperature dependence of the rate coefficientske (Te) forfree electron attachment involving a Maxwellian electron ensemble and a gas at room temperature(TG = 300K).