The binding of positronium to alkali atoms.

Introduction.

The Positronium (Ps) atom consists of an electron and a positron orbiting their mutual center of mass. To a first approximation it can be regarded as a sort of light hydrogen atom.

The question of whether Ps could be bound to an atom was answered relatively early. It was over 50 years ago that Ore showed that Hydrogen Positride, i.e. HPs was electronically stable. Although the similarities of Ps with H suggest that Ps should be able to bind with a lot of atoms (just like H can combine with many atoms to form molecules), there is very little hard evidence stating explicitly whch atoms can form stable APs complexes and giving any detail about the structure of these exotic systems.

Part of the research effort has been the investigation of positronium binding to alkali atoms. So far, it has been shown that LiPs, NaPs, KPs and CuPs are electronically stable and binding energies and annihilation rates have been obtained.

Calculation methods.

Current list of atoms binding positronium.

The current list of atoms binding positronium is listed here. Atomic parameters are shown for all the different model calculations.

While the SVM model in general involves the minimum number of assumptions, it has been difficult to generate fully converged estimates of the Binding Energy for any of these systems.

Properties of positron-atom ground states.

System	Method	Atomic IP	Binding Energy	Gamma	Mean Positron Distance
One valence electron
PsH	SVM	0.500000	0.0391944	2.452	3.661
	There have been a number of calculations of this object. The results in this table were calculated by us. The energy and annihilation rate are in good agreement with other state of the art calculations.
LiPs	FCSVMpol	5.39115	0.33293	2.156	6.393
NaPs	FCSVMpol	5.12652	0.19519	2.080	7.676
KPs	FCSVMpol	4.32593	0.12	1.99	8.253
	These are rough estimates as a current calculation on this atom has not been completed.
CuPs	CIpol	7.71076	0.12	1.94	7.253
	These are rough estimates as a current calculation on this atom has not been completed.
FPs	QMC
	This results was not obtained by us.
ClPs	QMC
	This results was not obtained by us.
BrPs	QMC
	This results was not obtained by us.

Explanation of terms in above table.

Parent Atom	Self-explanatory.
Method	The method used for the calculation.
Atomic IP	The Ionization Potential (in eV) of the parent atom. This is not the experimental IP, it is the IP derived from calculations. (1.0 Hartree = 27.2114 eV)
Binding Energy	The calculated binding energy (in eV).
Gamma	The annihilation rate (in 109 sec-1).
Mean Positron Distance	The mean distance of the positron from the nucleus (in a0). (1.0 a0 = 0.52918x10-10m)

Heuristic description of the structure

One picture of the these atoms would be to imagine a positronium cluster aborting the atom.

Positronium Binding to other atoms

The list of elements that can bind positronium will almost certainly not terminate with the present list. There are many promising candidates for binding.