Electron attachment to perfluorocarbon compounds. I. c-C4F6, 2-C4F6, 1,3-C4F6, c-C4F8 and 2-C4F8

Electron attachment rates αw, as a function of the pressure-reduced electric field E/P298 and mean electron energy <ε≳ have been measured for trace amounts (<10−3 Torr) each of c-C4F6 (perfluorocyclobutene), 2-C4F6 (perfluoro-2-butyne), 1,3-C4F6 (perfluoro-1,3-butadiene), c-C4F8 (perfluorocyclobutane), and 2-C4F8 (perfluoro-2-butene) in mixtures with N2 (pressures 500–2000 Torr) and Ar (pressures 500–1500 Torr) at T=298°K. The thermal attachment rates for these molecules were found to be 4.89×109, 1.77×109, 4.26×109, 4.05×108, and 1.56×109 sec−1 Torr−1, respectively. From the αw (<ε≳) data, the attachment cross sections σa(ε) as a function of electron energy ε have been determined using the swarm-unfolding technique and are reported. For all five perfluorocarbon molecules, σa(ε) are very large. They exhibit three distinct negative ion resonances with maxima at: ∼0.0, 0.19, and 0.80 eV for 2-C4F6; ∼0.0, 0.17, and 1.04 eV for 1,3-C4F6; ∼0.0, 0.22, and 0.48 eV for c-C4F8; and ∼0.0, 0.18, and 0.59 for 2-C4F8. For c-C4F6, only a shoulder with a possible maximum at ∼0.14 eV was observed, in addition to the maxima at ∼0.0 and 0.71 eV. Although the positions of the maxima in σa(ε) vary only slightly among these compounds, the magnitude of σa(ε) depends strongly on structure.