The dependence of low-energy electron attachment to CF3Br on electron and vibrational energy

Abstract

In a joint experimental and theoretical effort, we have studied dissociative electron attachment (DEA) to the CF3Br molecule at electron energies below 2 eV. Using two variants of the laser photoelectron attachment method with a thermal gas target (T(G) = 300 K), we measured the energy dependent yield for Br- formation over the range E = 3-1200 meV with resolutions of about 3 meV (E < 200 meV) and 35 meV. At the onsets for excitation of one and two quanta for the C-Br stretching mode nu3, downward cusps are detected. With reference to the recommended thermal (300 K) attachment rate coefficient k(A)(CF3Br) = 1.4 x 10(-8) cm3 s(-1), absolute cross sections have been determined for Br- formation. In addition, we studied Br- and (CF3Br)Br- formations with a seeded supersonic target beam (10% CF3Br in helium carrier gas, with a stagnation pressure of 1-4 bars and nozzle temperatures of 300 and 600 K) and found prominent structure in the anion yields due to cluster formation. Using the microwave pulse radiolysis swarm technique, allowing for controlled variation of the electron temperature by microwave heating, we studied the dependence of the absolute DEA rate coefficient on the mean electron energy E over the range of 0.04-2 eV at gas temperatures T(G) ranging from 173 to 600 K. For comparison with the experimental results, semiempirical resonance R-matrix calculations have been carried out. The input for the theory includes the known energetic and structural parameters of the neutral molecule and its anion; the parameters of the resonant anion curves are chosen with reference to the known thermal rate coefficient for the DEA process. For the gas temperature T(G) = 300 K, good overall agreement of the theoretical DEA cross section with the experimental results is observed; moreover, rate coefficients for Br- formation due to Rydberg electron transfer, calculated with both the experimental and the theoretical DEA cross sections, are found to agree with the previously reported absolute experimental values. At T(G) = 300 K, satisfactory agreement is also found between the calculated and experimental attachment rate coefficients for mean electron energies E = 0.04-2 eV. The strong increase of the measured rate coefficients with rising gas temperature, however, could be only partially recovered by the R-matrix results. The differences may result from the influence of thermal excitations of other vibrational modes not included in the theory.

On the validity of the Arrhenius equation for electron attachment rate coefficients

“…Examples for case ͑b͒ include the molecules CH 3 Br, 16,19 CH 3 Cl, 16,28 and CF 3 Y ͑Y=Cl,Br͒. 18,29,30 As a special example, we present in Fig. 2 an Arrhenius plot of the DEA rate coefficient for the CF 3 Br molecule.…”

“…2 an Arrhenius plot of the DEA rate coefficient for the CF 3 Br molecule. 18 Ignoring details of the experimental observations ͑symbols͒ and of the theoretical results ͑full curve͒, one observes that the validity of the Arrhenius equation is certainly limited toward low temperatures and probably toward high temperatures as well. Because of these limitations, the question arises as to what is the relation between the activation energy and the reaction barrier.…”

“…In the present model calculations, all parameters determining the surface amplitude ␥͑͒ were chosen the same as in the CF 3 Br calculations. 18 To calculate DEA cross sections, we incorporate nuclear dynamics and solve basic equations of the resonance R-matrix theory in the quasiclassical approximation. 33 As part of this procedure, we calculate electron scattering wave functions outside the R-matrix sphere that include dipolar and polarization interactions.…”

Highly resolved absolute cross-sections for dissociative electron attachment to SF5CF3

“…2b) which is attributed to the interaction of the primary attachment process with the channel for vibrational excitation of the 4 (a 1 ) mode. As is known from the previous high-resolution work on molecules such as SF 6 [20,22], CCl 4 [38], CF 3 Br [39], and CF 3 I [40], these Wigner cusps are characteristic for the vibrational modes which are active in the attachment process. Note that for SF 5 − formation, the vibrational modes which may be considered to promote dissociation of the primary anion complex (SF 5 CF 3 ) − towards the fragments SF 5 − + CF 3 are not identical with the channel interaction mode The observation of the cusp structure at the 4 (a 1 ) = 1 vibrational onset was incorporated into our theoretical model, discussed in Section 3.2, whose results are also included in Fig.…”

“…This deviation from the experimental observations [8] is attributed to the one-dimensional approximation employed in the calculations which does not incorporate IVR and dissociation. This issue was recently discussed in some detail for DEA to CF 3 Br [39]. Over the energy range 0.2-0.9 eV (see Fig.…”