Multi-photon excitation of a beam of SF6 molecules pumped and probed by cw CO2 lasers

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

“…In the present work, the laser was tuned to the lines X = 10-40, corresponding to wavenumbers of 1/k = (953-925) cm À1 . The infrared activity of the m 3 band is very high, and overtone pumping is possible for rather moderate IR intensities (0.1-10 MW/cm 2 ) [36,40]. In the present work, we apply transverse single pass excitation of the SF 6 molecules with a continuous wave, line-tunable single mode CO 2 laser (Edinburgh Instruments, model PL 5, line bandwidth 100 MHz) at powers up to 50 W and intensities up to about 400 W/cm 2 .…”

“…the shape of the cross section) as long as e/F is large compared to 1 and the energy dependence of r 0 (E) is well known. In order to obtain an estimate of the fraction F we take advantage of results for this quantity obtained by Liedenbaum et al [36,41] with bolometric measurements. In agreement with earlier work by Boschetti et al [42], Liedenbaum et al found a substantial redshift for optimal laser excitation and concluded that 4-5 photons are typically absorbed for laser fluences of about 0.05 J/cm 2 .…”

“…Additional results were obtained for T 0 = 450 and 550 K. The average number of absorbed photons depends strongly on the rovibrational energy of the molecules (which rises with increasing T 0 ), but for a fixed nozzle temperature the value of hN c i is rather similar for the pure and the seeded supersonic beam. Our supersonic beam (12.5% SF 6 in He) is intermediate between the pure beam and the 1% mixture in He used by Liedenbaum et al [36]. In our data evaluation, we adopted averages of the values measured for the two different supersonic beams and determined a fit function to these values for the range T 0 = 300-600 K which is given by (nozzle temperature T 0 in K):…”

“…Liedenbaum et al further demonstrated that the upper state vibrational population is free of IVR processes on the time scale of about 1 ms. More specifically, SF 6 molecules in both a pure as well as in a seeded supersonic beam (1% SF 6 in He) were excited by 10PX lines of a CO 2 laser at an intensity of 60 kW/cm 2 and for nozzle temperatures T 0 between 300 and 550 K (see Fig. 2 in [36]). From the bolometer signals with and without laser irradiation of the molecular beam, they estimated the average number hN c i of absorbed laser photons per molecule, as measured at the line for optimal excitation (X = [26][27][28][29][30].…”

“…Using the laser photoelectron attachment (LPA) method, electron energy resolutions down to 1 meV are realized; thus it is possible for the first time to demonstrate the dependence of the IR-enhanced SF À 5 yield on electron energy for several nozzle temperatures and laser lines. Using results of previous IR-excitation experiments by Liedenbaum et al [36], we estimate the fraction F of vibrationally excited SF 6 molecules and -in combination with the measured IRenhanced anion formation -we provide energy dependent DEA cross sections for IR-excited SF 6 molecules. 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.…”