The interaction of metastable helium atoms with alkaline earth atoms: He*(23 S,21 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*(2S) and He*(2S) 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* C for the He*#X entrance channel potentials with uncertainties around 25 meV are derived from the electron spectra as follows: He*(2S)#Mg/Ca/Sr/Ba: 130/250/240/260 meV; He*(2S) #Mg/Ca/Sr/Ba: 300/570/550/670 meV. The spectra show substantial differences for the three ionic states X>(S), X>(P) and X>(D) and reveal that transitions to a repulsive potential -attributed to He#X>(P) formation -are mainly involved for the X>(P) channel. Ab initio calculations of potential curves, autoionization widths, electron energy spectra and ionization cross sections are reported for the systems He*(2S)#Ca and He*(2S)#Ca. The respective well depths D* C 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 A s and 201 A s , respectively. Very good overall agreement with the experimental electron spectra is observed.

Controlled formation of weakly bound atomic negative ions by electron transfer from state-selected Rydberg atoms

“…f ci = (3(2 + 1)) 1/2 for m = 0, f ci = (6 ( + 1)(2 + 1)) 1/2 /(2R c (2EA) 1/2 ) for m = 1, and f ci = 0 for m > 1 [13], [19]. Realistic Ca − wave functions were calculated on the basis of a model potential with appropriate long-range behaviour 1/r 2 − α/2r 4 (α = Ca polarizability = 159 au [20]) and was chosen to yield the proper Ca electron affinity; somewhat dependent on the choice of the potential we found N i = 0.117 for Ca − ( 2 P 3/2 ) and N i = 0.132 for Ca − ( 2 P 1/2 ) (within 5%). We assume that the Ne + ( 2 P 3/2 ) core acts as spectator and can be considered as fixed in the electron transfer process.…”

Long-range interactions of excited He atoms with the alkaline earth atoms Mg, Ca, and Sr

“…We also list in Table I the values of C 6 and C 8 which have been used by Ruf et al to study Penning ionization collisions of state-selected metastable He(2 1, 3 S) atoms with the ground state alkaline earth atoms Mg, Ca, Sr, and Ba. 39 Ruf et al 39 have calculated the coefficients C 6 for the systems He(2 1, 3 S)-Ca(4s 2 1 S) with the so-called configuration interaction plus core self consistent field method (CISCF), which is similar to the CICP. Our C 6 values for these two systems are in agreement with those obtained by Ruf et al 39 within 0.5%.…”

“…39 Ruf et al 39 have calculated the coefficients C 6 for the systems He(2 1, 3 S)-Ca(4s 2 1 S) with the so-called configuration interaction plus core self consistent field method (CISCF), which is similar to the CICP. Our C 6 values for these two systems are in agreement with those obtained by Ruf et al 39 within 0.5%. Their estimated uncertainty (2%) is also consistent with our calculations.…”

“…Their estimated uncertainty (2%) is also consistent with our calculations. Ruf et al 39 have estimated the coefficients C 8 for the above two systems by fitting their experimental parameters. Their C 8 for the He(2 1 S)-Ca(4s 2 1 S) interaction is about 2% smaller than our value, while their C 8 for the He(2 3 S)-Ca(4s 2 1 S) interaction is about 46% larger than our result.…”

“…The causes for this large difference are unknown. When studying the Penning ionization involving the systems He(2 1, 3 S)-X (X=Mg, Sr, and Ba), Ruf et al 39 have used the coefficients C 6 determined from the atomic static dipole polarizabilities [7] in the Slater-Kirkwood approximation (SKA). 40 From Table I, it is noted that the SKA results are 10%-37% larger than our values, which should be more accurate.…”