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Associative ionization in collisions of state-selected metastable Ar*, Kr* and Xe* atoms with hydrogen and deuterium atoms

Recent detailed experimental and theoretical studies of Penning and associative ionization in the basic collision system He*(2 3S) + H(1 2S) -> (He + He+) +e-(E) (1), have demonstrated that these processes are well described by local complex potential theory within the Born-Oppenheimer approximation. An analogous situation is found for the Ne*(3s 3P2) + H(1 2S) collision complex. In low energy collisions of the heavier metastable rare gas atoms Rg* = Ar*, Kr* and Xe* with hydrogen atoms, on the contrary, ionization is only possible through RgH+ formation (associative ionization AI) owing to the high proton affinity of Ar, Kr and Xe (>= 3.8 eV): Rg* + H(1 2S) -> RgH+(v+, J+) +e-(E) (2). These systems are subject to electron emission at short range following non-adiabatic transfer through the (RgH)** Rydberg manifold. In this contribution we present systematic investigations of associative ionization (AI) in thermal energy collisions (Erel ~ 50 meV) of state-selected metastable rare gas atoms Ar*(4s 3P2,0), Kr*(5s 3P2,0) and Xe* (6s 3P2,0) with hydrogen and deuterium atoms. Mass- and electron-spectrometric experiments were carried out in a crossed beam setup. Efficient state selection of the two metastable levels 3P2 and 3P0 was achieved with an optical pumping method involving scanning diode lasers. By mass spectrometry, we have determined the absolute cross sections of process (2) for both H and D atoms. The ratio sig2/sig0 of the cross sections for Rg*(3P2) and Rg*(3P0) decreases dramatically from Ar*(0.80) to Xe*(< 1.7x10^(-5)) while the absolute cross sections sig0 rise monotonously from Ar* (sig0(ArH+) ~ 0.50 A^2) to Xe* (sig0(XeH+) ~ 8.0 A^2). A clear isotope effect in favour of D atoms is observed which scales approximately with the ratio of the reduced masses. Electron spectrometry reveals that at electron energies E >= 0.2 eV the shape of the AI spectra from the reactions (2) is similar to the high energy, associative ionization part of the electron energy distributions for the collision systems He*(2 3S) + H and Ne*(3S 3P2) +H. Quantum mechanical model calculations of the electron spectra for (2) indicate that the AI processes associated with E >= 0.2 eV can be well described by electronic decay from a diabatic local complex potential Vc(R) = V*(R) - (i/2)G(R) with suitably chosen real and imaginary parts. The most critical parameters are the crossing point Rc of V*(R) with the ionic potential V+(r) for RgH+ and the slope of V*(R) around Rc, as will be discussed in detail for relatively simple system Xe*(3P0) + H. As a side product of the precise comparison between the measured and the calculated AI electron spectra we determine dissociation energies for the RgH+ and RgD+ molecular ions which are accurate to better than 50 meV. This work has been supported by the Deutsche Forschungsgemeinschaft, by the Stiftung Rheinland-Pfalz fuer Innovation, By the Graduiertenkolleg "Laser und Teilchenspektroskopie" and by the European Union through the HCM network "Lasers, atoms and molecules: dynamical interactions". We gratefully acknowledge M. Reicherts for experimental support and A. Merz and W. Meyer for helpful discussions.

associative ionization; state-selective collisions

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