Dissociative electron attachment to dipolar molecules at low energies with meV resolution: CFCl3, 1,1,1-C2Cl3F3, and HI

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Abstract
Using the laser photoelectron attachment method we investigated dissociative electron attachment e-(E) + XY→X + Y- (DEA, short notation Y-/XY) to three molecules XY with a permanent electric dipole moment in the electron energy range 0<E/meV ≤ 173 with very high resolution (energy width ΔE≤1 meV). With reference to reliable thermal attachment rate coefficients absolute DEA cross sections σe(E) have been derived for the processes Cl-/CFCl3, Cl-/1,1,1-C2Cl3F3 and I-/HI. At thresholds for vibrational excitation of the neutral molecule, the cross sections exhibit a pronounced cusp structure (in most cases of a downward step character) due to coupling of the attachment process with scattering channels. At low energies (from about 0.8 meV to the first vibrational cusp structure) the cross sections are well described by the simple analytical expression σe(E) = (σ0/E)[1-exp (-βE1/2)] (with β falling in the range from 0.88 to 1.6 (meV)-1/2), indicating an energy dependence of 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. Comparisons are made with cross sections derived from previous photoelectron attachment work (TPSA) and from experiments involving electron beams, electron swarms and Rydberg atoms. Based on our experimental results for σe(E), we calculate and report for all investigated processes the energy dependence of the rate coefficients ke(E) for monoenergetic free-electron attachment as well as for Rydberg electron attachment knl (n≈30-150,l<<n) and 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).