Ionization of small molecules by state-selected neon (3P0, 3P2) metastable atoms in the 0.06 <E <6 eV energy range

The velocity dependence and absolute values of the total ionisation cross section for the molecules H2, N2, O2, NO, CO, N2O, CO2, and CH4 by metastable Ne* (3P0) and Ne* (3P2) atoms at collision energies ranging from 0.06 to 6.0 eV have been measured in a crossed beam experiment. State selection of the two metastable states of Ne* was obtained by optical pumping with a cw dye laser. We observe a strongly different velocity dependence at collision energies below about 1 eV for the ionisation cross section of the systems Ne*---H2, N2, CO, and CH4, and the systems Ne*---O2, NO, CO2, and N2O, respectively. The first group shows an increasing cross section in this energy range, similar to the Ne*---Ar system, while the second group shows a very flat behaviour. This behaviour correlates with the difference in character (p or sb) of the orbital of the electron that is removed from the target molecule. For the molecules H2, N2, CO, and CH4 an electron from a sb orbital is removed from the molecule, whereas for O2, NO, N2O, and CO2 an outer p-ortibal electron is involved. For the systems Ne* (3P0, 3P2)---H2 we have derived the imaginary part of the optical potential by assuming a real potential similar to the theoretically calculated ground state Na---H2 potential of Botschwina et al. The resonance width G(r) as a function of the internuclear distance r shows a saturation at small r (r <2.8 Å) for both the Ne*(3P0)---H2 and the Ne*(3P2)---H2 interaction. This supports previous conclusions of Verheijen et al. and Kroon et al. Reliable values for the absolute value of the total ionisation cross section have been obtained by performing a careful calibration of the density—length product of the supersonic secondary beam. The results are in good agreement with the values of West et al. for experiments without state selection. The total ionisation cross sections for molecules with p-type ionisation orbitals, with their larger spatial extent, in general are larger than those for molecules with sb-type ionisation orbitals.