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

Abstract

Using the laser photoelectron attachment method we investigateddissociative electron attachment e-(E) + XY→X + Y-(DEA, shortnotation Y-/XY) to three molecules XY with a permanent electricdipolemoment in the electron energy range 0<E/meV ⩽ 173 with veryhighresolution (energy width ΔE⩽1 meV). With reference toreliable thermal attachment rate coefficients absolute DEA cross sectionsσe(E) have been derived for the processes Cl-/CFCl3,Cl-/1,1,1-C2Cl3F3 and I-/HI. Atthresholdsfor vibrational excitation of the neutral molecule, the cross sectionsexhibit a pronounced cusp structure (in most cases of a downward stepcharacter) due to coupling of the attachment process with scatteringchannels. At low energies (from about 0.8 meV to the first vibrationalcuspstructure) the cross sections are well described by the simple analyticalexpressionσe(E) = (σ0/E)[1-exp (-βE1/2)] (withβ 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, inessential agreement with predictions of an extended Vogt-Wannier (EVW)capture model which includes the long-range electron-dipole interactionin addition to the polarization force. Comparisons are made with crosssections derived from previous photoelectron attachment work (TPSA) andfrom experiments involving electron beams, electron swarms and Rydbergatoms. Based on our experimental results for σe(E), wecalculate and report for all investigated processes the energy dependenceof the rate coefficients ke(E) for monoenergetic free-electronattachment as well as for Rydberg electron attachment knl (n≈30-150,l<<n) and the electron temperature dependenceof the rate coefficients ke(Te) for free-electronattachmentinvolving a Maxwellian electron ensemble and a gas at room temperature(TG = 300 K).