A Schramm et al 2002 J. Phys. B: At. Mol. Opt. Phys. 35 4179 doi:10.1088/0953-4075/35/20/306
A Schramm1, M-W Ruf1, M Stano2, S Matejcik2, I I Fabrikant1,3 and H Hotop1
Show affiliationsUsing the laser photoelectron attachment method with an energy width ≤ 1 meV at low energies (≤ 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 σe (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 ν3 = 1 onset for the symmetric CBr2 stretch vibration, a clear vibrational Feshbach resonance is observed. At low energies the cross sections σe (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 CCl3Br 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 σe (E), we report the electron temperature dependence of the rate coefficients ke (Te) for free electron attachment involving a Maxwellian electron ensemble and a gas at room temperature (TG = 300 K).
34.80.Lx Recombination, attachment, and positronium formation
31.15.bu Semi-empirical and empirical calculations (differential overlap, Hückel, PPP methods, etc.)
33.60.+q Photoelectron spectra
34.80.Ht Dissociation and dissociative attachment
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
Issue 20 (28 October 2002)
Received 14 June 2002, in final form 30 August 2002
Published 1 October 2002
A Schramm et al 2002 J. Phys. B: At. Mol. Opt. Phys. 35 4179
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