IR photon enhanced dissociative electron attachment to SF 6: Dependence on photon, vibrational, and electron energy

Abstract

The influence of IR photon excitation of the ν 3 vibrational mode in SF 6 on dissociative electron attachment ( SF 5 - formation ) is investigated at high electron energy resolution (down to 1 meV) over the energy range E = 0–0.5 eV with the laser photoelectron attachment method. The molecules are contained in a collimated seeded supersonic beam (nozzle temperatures T 0 = 300–600 K, corresponding to vibrational temperatures T V ≈ T 0 − 100 K) and transversely excited by the 10PX lines (X = 10–40) in the 10.6 μm band of a continuous CO 2 laser at intensities up to about 400 W cm −2. The IR excitation and the attachment regions are separated by 5 cm. The IR photon induced enhancement of the SF 5 - yield is found to be optimal on the 10P28 line (936.8 cm −1) at all nozzle temperatures (with the maximum reached for T 0 ≈ 390 K) and monotonically decreasing with rising electron energy from 0 eV over a range of about 0.3 eV. For a fixed spatial profile of the exciting IR beam, the enhancement at E ≈ 0 eV follows a near-square-root dependence on laser power. With reference to previous work on the excitation of supersonic SF 6 beams by CO 2 laser light, the fraction of laser-excited SF 6( ν 3 ⩾ 1) molecules is estimated, and the absolute cross sections σ L( E) for SF 5 - formation involving the IR-excited molecules are determined; they exceed the cross sections σ 0( E) for thermal molecules in a way which strongly depends on electron energy and initial vibrational energy. In contrast, the cross section for SF 6 - formation is found to be almost independent of laser-excitation and temperature. The experimental findings are discussed with regard to the multiphoton character of the IR excitation, and comparisons are made with the effects of thermal excitation. The mechanisms for SF 6 - and SF 5 - formation and the responsible potential energy surfaces are discussed in the light of the available experimental data.