MOLECULE-LIKE METASTABLE STATES OF ANTIPROTONIC AND MESIC HELIUM

The exotic helium atom, consisting of a helium nucleus He2+, an electron e-, and a massive, negatively charged particle X-, such as an antiproton (pBAR) or a kaon (K-), is studied theoretically. This exotic atom is more like a polar molecule with two nuclei He2+ and X-, if e- is in a low-lying orbital and X- is in a highly excited orbital. Such states are generally formed in the capture of X- by helium. The Born-Oppenheimer (BO) separation of the e- motion and the He2+-X- motion is a good approximation for these states, and affords a transparent, unified perspective of exotic helium with different X- and different isotopes of He2+ . A propensity rule that favors small transition energies (except for the smallest one) is found for radiative emission by low vibrational, high rotational levels. This rule resembles the selection rule for infrared emission by the usual diatomic polar molecules, and contradicts the conventional expectation that radiative transitions in exotic atoms favor larger transition energies. The radiative lifetimes tau of highly excited states formed immediately after the capture of X- are of the order of musec for He-4(2+)-p-e-BAR and He-3(2+)-p-e-BAR, and those for He-4(2+)-K--e- are about half the values for the antiprotonic helium. The effect of the electronic charge polarization by X- on tau is explained quantitatively in a simple picture in terms of the molecular dipole moment. Low vibrational, high rotational states are described as circular or near-circular states in the atomic picture. They are stable against Auger decay, which the nonadiabatic perturbation on the BO states induces. An infinite series of bound SIGMA electronic states of exotic helium negative ions (He2+-X--e-)-e- exists for any He2+-X- distances R beyond 0.819 a.u. in the BO approximation. An infinite PI series exists for R > 3.811 a.u., and an infinite DELTA series for R > 9.532 a.u.