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

Using the laser photoelectron attachment method with an energy width less than or equal to1 meV at low energies (less than or equal to 172 meV) and an electron beam method (energy width about 70 meV) at higher energies (up to 2 eV), we have determined absolute cross sections or, (E) for dissociative electron attachment to the dipolar molecules CH2Br2 and CCl3Br 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 coupling of the attachment process with scattering channels; below the v(3) = I onset for the symmetric CBr2 stretch vibration, a clear vibrational Feshbach resonance is observed. At low energies the cross sections or, (E) show an energy dependence between E-1/2 and E-1, in essential agreement with predictions of an extended Vogt-Wannier (EVW) capture model which includes the long-range electron-dipole interaction in addition to the polarization force. The determined absolute values are, however, substantially smaller than those predicted by the EVW model. Semiempirical R-matrix calculations have been carried out which show that at low electron energies (E < 0.15 eV) Br formation from CCl3 Br proceeds by s-wave attachment to the anion ground state while a, broad peak, observed around 0.6 eV and evolving predominantly into the Cl- channel, is due to an excited anion state. Comparisons are made with cross sections and rate coefficients obtained in previous photoelectron attachment work (TPSA) and in electron beam as well as swarm experiments. Based on our joint experimental results for or, (E), we report the electron temperature dependence of the rate coefficients k(e) (T-e) for free electron attachment involving a Maxwellian electron ensemble and a gas at room temperature (T-G = 300 K).