Electron attachment to the perfluoroalkanes n-CNF2N+2 (N=1–6) using high pressure swarm techniques

Abstract

The electron attachment rate constants and negative ion formation mechanisms for six perfluoroalkanes [n‐C_NF_2N+2(N=1–6)] have been studied in a high pressure swarm experiment within the mean electron energy range from thermal energy (≊0.04 eV) to ≊4.9 eV. These experiments were performed over a total gas number density range of 3.2×10¹⁹ to 3.9×10²⁰ cm⁻³ using N₂ and argon as buffer gases. Dissociative electron attachment was found to be the only negative ion formation process for CF₄ and C₂F₆. For C₃F₈, n‐C₄F₁₀, and n‐C₅F₁₂ the electron attachment rate constant measurements exhibited a large total pressure dependence which was strongest for C₃F₈ and decreased with increasing size of the perfluoroalkane molecule. These measurements have been interpreted as electron attachment by parent negative ion formation due to three‐body stabilization processes of the initially excited, short‐lived (5×10⁻¹¹ s <τ−⁸ s) parent anion. The lifetimes of these transient parent anions have been found to depend on the nature of the parent ion and on the electron energy. The electron attachment rate constants are largest for the n‐C₆F₁₄ molecule and decrease with decreasing size of the perfluoroalkane molecule. Furthermore, the peak in the attachment rate constants occurs at the lowest mean energy (∼1.1 eV) for n‐C₆F₁₄ and shifts to higher mean energy with decreasing size of the molecule (to ≥5 eV for CF₄). The electron attachment cross sections for all of these molecules have been calculated using the swarm‐unfolding method and calculated electron energy distribution functions in argon. These measurements are compared with relative negative ion cross sections, determined previously using mass spectrometric techniques. The negative ion formation mechanisms are discussed and the effect of molecular size on the electron attachment properties of these molecules is indicated using the present measurements in conjuction with the results of a recent single collision negative ion study.