Pressure and temperature dependence of dissociative and non-dissociative electron attachment to CF3: Experiments and kinetic modeling

Abstract

The kinetics of electron attachment to CF(3) as a function of temperature (300-600 K) and pressure (0.75-2.5 Torr) were studied by variable electron and neutral density attachment mass spectrometry exploiting dissociative electron attachment to CF(3)Br as a radical source. Attachment occurs through competing dissociative (CF(3) + e(-) → CF(2) + F(-)) and non-dissociative channels (CF(3) + e(-) → CF(3)(-)). The rate constant of the dissociative channel increases strongly with temperature, while that of the non-dissociative channel decreases. The rate constant of the non-dissociative channel increases strongly with pressure, while that of the dissociative channel shows little dependence. The total rate constant of electron attachment increases with temperature and with pressure. The system is analyzed by kinetic modeling in terms of statistical theory in order to understand its properties and to extrapolate to conditions beyond those accessible in the experiment.

Dissociative electron attachment to C2F5 radicals

“…44,45 Finally, competition between (8)- (10) is determined by explicitly accounting for collisions with the buffer gas and approximating the solution to the Master Equation using the many-shots approach. 13,46 As determined with the kinetic modeling, neither stabilization of C 2 F − * 5 through collisions with the buffer gas, process (9), nor autodetachment (8) occur quickly enough to compete with the dissociation, process (10). The failure of (9) to compete is consistent with no parent anion signal being observed and not surprising as dissociative electron attachment is exothermic by 0.5 eV, as calculated with GAUSSIAN-3 (G3) theory.…”

“…The temperature dependence is very similar to that of attachment to CF 3 . 13 Although not measured, it is expected that due to the rapid dissociation rate of C 2 F − * 5 that the electron attachment rate constant will not have any pressure dependence even up to atmospheric pressures, which is very different to CF 3 , where non-dissociative attachment occurs and there is a measurable positive pressure dependence at pressures on the order of 1 Torr. This difference stems primarily from the fact that dissociative attachment to CF 3 is slightly endothermic, while dissociative attachment to C 2 F 5 is exothermic.…”

“…13 In that work, formation of F − in dissociative electron attachment was observed along with formation of CF − 3 anions in associative attachment; the ratio of these two channels depended strongly on the temperature and pressure of the gas. The temperature and pressure dependence of the measurements was fitted well with a kinetic modeling approach.…”

“…The temperature and pressure dependence of the measurements was fitted well with a kinetic modeling approach. 13 …”

A novel technique for measurement of thermal rate constants and temperature dependences of dissociative recombination: CO2+, CF3+, N2O+, C7H8+, C7H7+, C6H6+, C6H5+, C5H6+, C4H4+, and C3H3+

“…The technique, dubbed variable electron and neutral density attachment mass spectrometry (VENDAMS), 11 has been applied to measure rate constants and product distributions of mutual neutralization [12][13][14][15][16] and electron attachment to short-lived species such as radicals. 11,17,18 Here, a variation of the technique is used to determine thermal DR rate constants. Unlike other VENDAMS applications, the method does not involve electron attachment; however, for consistency the acronym is retained.…”