Dissociative electron attachment to CCl 4 molecules at low electron energies with meV resolution

Abstract

Using the laser photoelectron attachment (LPA) method we investigated dissociative electron attachment to carbon tetrachloride [ e −( E) + CCl 4 → Cl − + CCl 3] in the electron energy range 0 < E ≤ 173 meV with very high resolution (energy width 1 meV). With reference to a reliable thermal electron attachment rate coefficient k e ( T = 300 K) absolute values for the attachment cross section σ e ( E) are derived. The σ e ( E) ∝ E −1/2 behaviour of the cross section for E → 0, theoretically predicted for s-wave attachment to molecules without permanent electric dipole moment, is approximately reached at very low energies ( E ≲ 0.3 meV). Toward higher energies the cross section decreases more rapidly. At thresholds for vibrational excitation of the neutral molecule, the cross section exhibits pronounced cusp structure of downward step character due to coupling of the attachment process with scattering channels. Comparisons are made with cross sections derived from previous photoelectron attachment work and from experiments involving electron beams, electron swarms and Rydberg atoms. Combination of our LPA cross section with beam data results in a recommended total DA cross section for CCl 4 over the energy range 0–2 eV. Based on this cross section, we calculate and report the energy dependence of the rate coefficient k e ( E) for monoenergetic free electron attachment and the electron temperature dependence of the rate coefficient k e ( T e ) for free electron attachment involving a Maxwellian electron ensemble and CCl 4 gas at room temperature ( T G = 300 K). The effects of electron energy resolution on measured low energy electron attachment yields, in particular on the effective location and energy width of the “zero energy peak,” are discussed in some detail.