Self-trapped states of positrons and positronium in dense gases in liquids

Abstract

In dense media positrons and positronium atoms are bound in self-trapped states. These states are density fluctuations stabilised by the light quantum particles. The positronium atoms are trapped in a bubble, while positrons are trapped in a cluster. Transitions to these states at changes of the density or temperature of the medium produce an essential effect on the annihilation rate. It can be considered as a local phase transition in the vicinity of the positron (or positronium). The present article is a review of the present state of the physics of positron and positronium self-trapped states in dense gases and liquids. Basic properties of the positronium bubbles and positron clusters are considered, as well as the conditions under which they exist and their effect on the annihilation rate. Experiments in which the self-trapping was detected are discussed. The positron and positronium self-trapping is considered to be closely related to other similar electronic phenomena, known for dense gases, liquids and other dense media. The main results from the study of positron and positronium interactions with isolated atoms are also presented. The density effects in moderately dense gases anticipating self-trapping are considered.

Theory of Positron Annihilation on Molecules

“…For small positron binding energies |ε 0 | < k B T , and "large" ω > k B T , only one lowest resonance contributes noticeably to the sum, and Z (29) are kept constant: ρ ep = ρ Ps and |ε 0 | k B T . Apart from the points for CF 4 and H 2 O, the rest of the data seem to correlate well with the slope of the curve. For molecules on the high-frequency side of the graph, the contribution of resonant annihilation is small.…”

“…As a result, most of the annihilation events involve electrons of the valence and near-valence subshells, making Z eff smaller. On the other hand, outside the target the positron motion is affected by an attractive long-range polarisation potential −α d /2r 4 , where α d is the dipole polarisability of the target. This leads to an increase of the positron density near the target and enhances Z eff .…”

“…For thermalised positrons the latter is Maxwellian, and the experimental value of Z eff corresponds to a Maxwellian average of Z eff (k). Equation (6) is also used to describe experiments at large densities where Z eff becomes density dependent [4].…”

“…The resonances have finite energy widths Γ =h/τ , where τ is their lifetimes. The latter are determined by the positron annihilation rate in the (quasi)bound state, as well as the rate of positron emission back into the continuum 4 .…”

“…However, at low energies the positron wavefunction is dominated by the s wave, and exchange of angular momentum between the positron and the molecule is suppressed, at least for non-polar molecules. 4 If the quasibound positron-molecule complex undergoes collisions with other molecules, it can be stabilised against positron emission. This effect will result in density dependent Z eff [26], and should not be considered in the binary-collision regime.…”