Molecular Structure of Water-Like Positronium Complexes

It is well known that positrons can live long enough to form complexes with atoms. The theoretical study of these complexes is important to interpret experimental data from both positron scattering by atoms and molecules and positron annihilation spectroscopy. There are typically two different kinds of positron complexes. In one of them a positron attaches to a neutral or ionic atom or molecule A with a positive positron affinity, yielding the formation of the complex [e;A]. In the other kind, a pair (e−,e+) (the positronium (Ps) atom) acts as a light isotope of hydrogen and binds to matter in almost the same way as the H atom does. Examples considered here are the two different species of positronic water, [PsOH] and [Ps2O]. Most theoretical studies of positron complexes consider the three different types of particles: heavy nuclei, light electrons and positrons in the same footing and aim to treat all the correlations between any pair of them. As a consequence, these calculations are computationally quite hard and limited to very small systems. We have found, on the other hand, that treating a positron as a light nucleus and performing an adiabatic separation of the electronic and nuclear motions, a lot of valuable information can be obtained from the potential energy surface (PES)