Excited states of the positronium molecule

We investigate the spectrum of the positronium (Ps) molecule consisting of two electrons and two positrons by the stochastic variational method (SVM). By calculating the energies of states with L-pi = 0(+/-), 1(+/-) and 2(+/- ), we confirm that only four states, 0(+)(A(1)), 0(+)(B-2), 0(+)(E) and 1(- )(B-2), are bound. We investigate the various properties of these bound sta tes as well as the decay probabilities for the 2 gamma emission by electron -positron annihilation leaving a particular Ps final state. The binding ene rgies of a biexciton, a Coulomb four-body system analogous to Ps(2), are al so examined as a function of the electron-hole mass ratio. (C) 2000 Elsevie r Science B.V. All rights reserved.