C. Kerner

Technische Universität Kaiserslautern, Kaiserslautern, Rhineland-Palatinate, Germany

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    ABSTRACT: We have carried out experimental and theoretical studies of Penning ionization processes occurring in thermal energy collisions of state-selected metastable He*(23S) and He*(21S) atoms with ground state alkaline earth atoms X(X=Mg, Ca, Sr, Ba). Penning ionization electron energy spectra for these eight systems, measured with a crossed-beam set-up perpendicular to the collision velocity at energy resolutions 40-70&#114meV, are reported; relative populations of the different ionic X+ (m&#146) states are presented and well depths D*e for the He*+X entrance channel potentials with uncertainties around 25&#114meV are derived from the electron spectra as follows: He*(23S)+Mg/Ca/Sr/Ba: 130/250/240/260&#114meV; He*(21S) +Mg/Ca/Sr/Ba: 300/570/550/670&#114meV. The spectra show substantial differences for the three ionic states X+(2S), X+(2P) and X+(2D) and reveal that transitions to a repulsive potential - attributed to He+X+(2P) 2‡ formation - are mainly involved for the X+(2P) channel. Ab initio calculations of potential curves, autoionization widths, electron energy spectra and ionization cross sections are reported for the systems He*(23S)+Ca and He*(21S)+Ca. The respective well depths D*e are calculated to be 243(15)&#114meV and 544(15)&#114meV; the ionization cross sections at the experimental mean energy of 72&#114meV amount to 101&#114ư and 201&#114&#432, respectively. Very good overall agreement with the experimental electron spectra is observed.
    Zeitschrift für Physik D 01/1996; 37(3):219-230.