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

The influence of IR photon excitation of the ν3 vibrational mode in SF6 on dissociative electron attachment (SF5-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 T0 = 300-600 K, corresponding to vibrational temperatures TV ≈ T0 − 100 K) and transversely excited by the 10PX lines (X = 10-40) in the 10.6 μm band of a continuous CO2 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 SF5- yield is found to be optimal on the 10P28 line (936.8 cm−1) at all nozzle temperatures (with the maximum reached for T0 ≈ 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 SF6 beams by CO2 laser light, the fraction of laser-excited SF6(ν3 ⩾ 1) molecules is estimated, and the absolute cross sections σL(E) for SF5- formation involving the IR-excited molecules are determined; they exceed the cross sections σ0(E) for thermal molecules in a way which strongly depends on electron energy and initial vibrational energy. In contrast, the cross section for SF6- 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 SF6-andSF5- formation and the responsible potential energy surfaces are discussed in the light of the available experimental data.