The interaction of metastable helium atoms with alkaline earth atoms: He * (2 3 S ,2 1 S )+Mg, Ca, Sr and Ba

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 meV, are reported; relative populations of the different ionic X+ (ml) states are presented and well depths D*e for the He*+X entrance channel potentials with uncertainties around 25 meV are derived from the electron spectra as follows: He*(23S)+Mg/Ca/Sr/Ba: 130/250/240/260 meV; He*(21S) +Mg/Ca/Sr/Ba: 300/570/550/670 meV. 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 Σ 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 De* are calculated to be 243(15) meV and 544(15) meV; the ionization cross sections at the experimental mean energy of 72 meV amount to 101 A2 and 201 A2, respectively. Very good overall agreement with the experimental electron spectra is observed.