Competition between nondissociative and dissociative electron attachment to halogenated cyclic alkenes in the gas phase

A flowing-afterglow Langmuir probe apparatus with mass spectral analysis wa r used to measure rate constants, identify reaction products, and measure b ranching fractions for electron attachment to 1,2-dichlorohexafluorocyclope ntene (c-C5F6Cl2) and 1,2-dichlorooctafluorocyclohexene (c-C(6)F(8)Ci(2)) o ver the temperature range 295-555 K and buffer gas density range of 1.3-2.1 x 10(16) cm(-3). Electron attachment to both of these compounds is efficie nt over this temperature range and the rate coefficient for attachment is r elatively independent of temperature. At 295 K the electron attachment rate coefficient for c-C5F6Cl2 is 2.5 +/- 0.6 x 10(-7) cm(3) s(-1) and for c-C6 F8Cl2 is 3.5 +/- 0.9 x 10(-7) cm(3) s(-1). At 300 K, electron attachment to both neutral reactants is predominantly nondissociative. At higher tempera tures, a dissociative reaction channel forming Cl- is observed for both rea ction systems. Significant branching fractions For Cl- (greater than or equ al to 0.05) are observed at temperatures greater than 425 K for c-C5F6Cl2 a nd at temperatures greater than 375 K for c-C6F8Cl2. Over the temperature r ange of 300-550 K the branching fractions for formation of the nondissociat ive and dissociative products were not dependent on buffer gas density in t he range of 1.3-2.1 x 10(16) cm(-3). (C) 2000 Elsevier Science B.V.