State selected total penning ionisation cross sections for the systems Ne *( 3P 0, 3P 2) + Ar, Kr, Xe and N 2 in the energy range 0.06 < E0(eV) < 8.0

Abstract

The velocity dependence and absolute values of the total ionisation cross sections of Ar, Kr, Xe and N 2 by metastable Ne*( 3P 0) and Ne*( 3P 2) atoms have been measured in a crossed beam scattering experiment. State selection of the beam of metastable atoms has been performed by optical pumping with a cw dye laser. Our technique, which uses the UV photons released in the radiative decay following the laser excitation to measure the density of metastable atoms in the scattering centre, is very insensitive to details of the process of optical pumping. Systematic errors in detection efficiencies of the metastable atoms are largely eliminated in this approach. We have analysed our experimental data in terms of an optical potential, using a least-squares method to determine the potential parameters. For the real part we use an ion—atom Morse—Morse—spline—van der Waals potential ( V 0( r) as proposed by Siska. The well area for V 0( r) ⩽ 0.1 eV is left unmodified. The energy dependence of the cross sections for the Ne*-rare gas systems points unambiguously to a pronounced “kink” in the repulsive branch at 0.1–0.2 eV. For the imaginary part, with the usual exponential behaviour, we have to introduce a saturation to a constant value at small internuclear distances r < r Im with r Im in the range 2.1 < r Im(Å) < 2.6 which is only probed for energies 0.15 eV. These modifications result in a satisfactory description of the data for the Ne*( 3P 2)—Ar, Kr, Xe systems. For the Ne*( 3P 0 systems additional modifications of the well area are necessary. By calibrating the density—length product of the secondary beam and using the available detection efficiencies for ions and UV photons of the spiraltron detector we have also determined absolute values of the total ionisation cross section. In the thermal energy range (0.06–0.16 eV) they are in fair agreement with the rate constants for the quenching of metastable atoms as measured by Brom in flowing afterglows.