Spectral and Temperature Dependence of Laser Induced Dissociative Attachment in SF6

Abstract

We consider the influence of resonant laser excitation upon the dissociative reaction between SF6 molecules and very low energy electrons. Experimentally, the use of valence electrons of atoms in Rydberg states ensures a well-defined energy resolution of the scattering electrons in the crossed beam apparatus. A spectroscopic model of the infrared absorption of CO2 laser radiation by SF6 gives a fair determination of the energy levels and vibrational transitions involved in the dissociative process. This reaction between highly excited atoms and a molecule is interpreted as a three step process: attachment of a quasi-free electron, followed by the interaction between the atomic ionic core and the negative molecular ion leading to a partial stabilization and, finally, the competition between the dissociating and the autodetachment channels.

IR photon enhanced dissociative electron attachment to SF6: Dependence on photon, vibrational, and electron energy

“…Stricklett and Burrow [35] detected changes in the total electron scattering cross section at energies E = 0-14 eV, as induced by CO 2 laser excitation of SF 6 molecules in a free jet. Barbé et al [32] investigated the spectral and temperature dependence of SF À 5 formation in electron transfer from Ar ** (nl) Rydberg atoms with a broad range of n (14-35) and l to laser-excited SF 6 molecules with initial temperatures of 300-1000 K; one of the problems in the interpretation of the results was the fact that at the given medium principal quantum numbers n energy exchanges between the Ar + core and the target molecule play a role. In a pioneering paper, Chen and Chantry [8] reported enhancement in the SF À 5 ion yield for free electron attachment to SF 6 at low electron energies, both as a function of gas temperature (over the range 300-880 K) and for IR excitation of SF 6 with a line-tunable CO 2 laser.…”

“…Several electron collision experiments involving SF 6 molecules, vibrationally excited by CO 2 laser lines, have been carried out earlier [8,17,[31][32][33][34][35]. Stricklett and Burrow [35] detected changes in the total electron scattering cross section at energies E = 0-14 eV, as induced by CO 2 laser excitation of SF 6 molecules in a free jet.…”

“…(c) To what extent does selective excitation of the m 3 mode differ from a thermal rise in the (average) vibrational energy with regard to SF À 5 formation? (d) How many IR photons on an average are absorbed by a SF 6 molecule and which fraction of the SF 6 molecules is excited under the conditions of the previous experiments [8,17,32]? (e) What is the electron energy dependence of the cross section for the formation of both SF À 5 and SF À 6 ions in electron attachment to IR-excited SF 6 (m 3 P 1) molecules?…”