engleski

The interaction of metastable helium atoms with alkaline earth atoms: He*(23 S,21 S)+Mg, Ca, Sr and Ba

We have carried out experimental and theoretical studies of Penning ionization processes occuring in thermal energy collisions of state-selected metastable He(2 3S) and He(2 1S) 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 De for the He +X entrance channel potentials with uncertainties around 25 meV are derived from the electron spectra as follows: He(2 3S) + Mg/Ca/Sr/Ba : 130/250/240/260 meV ; : He(2 1S) + Mg/Ca/Sr/Ba : 300/570/550/670 meV. The spectra show substantial differences for the free ionic states X(+)(2S), X(+)(2P) and X(+)(2D) and reveal that transitions to a repulsive potential - attributed to He + X(+)(2P) 2Sigma 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(2 3S) + Ca and He(2 1S) + 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 x10(-16) cm(2) and 201x10(-16) cm(2), respectively. Very good overall agreement with the experimental electron spectra is observed.

penning ionization ; cross section ; thermal energy collisions

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