THE INTERACTION OF METASTABLE HELIUM-ATOMS WITH ALKALINE-EARTH ATOMS - HE-ASTERISK(2(3)S,2(1)S)+MG, CA, SR AND BA

We have carried out experimental and theoretical studies of Penning io nization processes occurring in thermal energy collisions of state-sel ected metastable He(2(3)S) and He*(2(1)S) atoms with ground state alk aline earth atoms X(X = Mg, Ca, Sr, Ba). Penning ionization electron e nergy spectra for these eight systems, measured with a crossed-beam se t-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 entran ce channel potentials with uncertainties around 25 meV are derived fro m the electron spectra as follows: He(2(3)S) + Mg/Ca/Sr/Ba: 130/250/2 40/260 meV; He(2(1)S) + Mg/Ca/Sr/Ba: 300/570/550/670 meV. The spectra show substantial differences for the three ionic states X(+)(S-2), X( +)(P-2) and X(+)(D-2) and reveal that transitions to a repulsive poten tial - attributed to He + X(+)(P-2) (2) Sigma formation - are mainly i nvolved for the X(+)(P-2) channel. Ab initio calculations of potential curves, autoionization widths, electron energy spectra and ionization cross sections are reported for the systems He(2(3)S) + Ca and He*(2 (1)S) + Ca. The respective well depths D-e are calculated to be 243(1 5)meV and 544(15)meV; the ionization cross sections at the experimenta l mean energy of 72 meV amount to 101 Angstrom(2) and 201 Angstrom(2), respectively. Very good overall agreement with the experimental elect ron spectra is observed.