Attachment of low-energy electrons (O-10 eV) to polyatomic molecules

Attachment of low-energy electrons (0-10 eV) to six normal perfluoroalkanes [n-C_NF_2N+2(N = 1-6)]. three isomeric perfluoroalkanes (i-C₄F₁₀, i-C₅F₁₂, neo-C₅F₁₂), one cyclic perfluorocarbon (c-C₅F₁₀), four fluoroethers (CF₃OCF₃, CF₃OCF₂H, CF₂HOCF₂H, and CF₃OCH₃), two fluorosulphides (CF₃SCF₃ and CF₃SCH₃), and the freon CC1F₃ has been studied using a time-of-flight mass spectrometer. The molecules n-C_NF_2N+24F₁₀, i-C₅F₁₂, neo-C₅F₁₂, and c-C₅F₁₀ were found to attach electrons dissociatively and/or nondissociatively. The autodetachment lifetimes, τ_a of the long-lived metastable parent a and fragment anions of all these molecules were measured. Additionally, for those long-lived anions whose cross section was large, the variation of τ_a with incident electron energy was measured. On the other hand, for the lower normal perfluoroalkanes (N=l-3), the fluoroethers, the fluorosulphides, and the CC1F₃ molecules studied, only fragment negative ions were observed. The relative cross sections for all observed negative ions have been measured and corrected for the finite width of the electron pulse using an unfolding procedure. Possible fragmentation mechanisms of the dissociative negative ion states (NISs) leading to the production of the observed fragment anions have been suggested and discussed. From the appearance onsets of a number of fragment negative ions, various bond dissociation energies, heats of formation, and electron affinities of certain fragments have been determined and are reported. The separation times of the dissociating fragments and the autodetachment lifetimes of the extremely short-lived (~10^-15 s) and dissociating NISs of CF₄ and C₂F₆were estimated. The effect of molecular size and geometry and the effect of atomic substitution in a molecule on its electron attaching properties are discussed in relation to the position of the observed NISs and to the relative intensities of the parent and fragment anions.

A high temperature (300-900 K) electron swarm apparatus was constructed and used to study the effect of temperature on the electron attachment to the molecule CC1F₃. The electron attachment rate constant, k_a(<ε>), of this molecule was measured as a function of temperature. The swarm-unfolding technique was used to determine the attachment cross section, σ(ε), from the measured k_a(<ε>) at all the temperatures a d studied. The enhancement of k_a(<ε>) with T is discussed and a rationalized.