Tapas Mukherjee

Electron affinity of positronium embedded in Debye plasma

Abstract The effect of environment like that of a plasma on the ground state energy of positronium minus ion (Ps − ) has been estimated variationally using multi-term correlated basis sets. Debye model has been applied to include the plasma screening effect in the interaction energy between two charge particles. The electron affinity of positronium is estimated for different Debye screening parameters and is found to decrease monotonically with increasing screening.

A critical assessment of the bubble model for positronium annihilation in liquids

The bubble model conventionally used to fit the observed characteristics of the pick-off component of ortho-positronium decay in liquids is subjected, in the present study, to a critical assessment. It is demonstrated that in its usual form (namely that of a bubble with a sharp boundary) the model is untenable, when confronted conjointly with experimental data on the lifetime and angular correlation of the decay gammas. A modified version of the model that is relatively free from such shortcomings is presented.

A modified bubble model for positronium atoms in liquids

Positronium self-trapping in liquids is frequently described through the bubble model in order to account for the observed decay characteristics, namely, the enhanced pick-off lifetime and the details of the angular correlation of the decay gammas. This model is improved upon in the present paper by modifying the idealised picture of a sharp bubble radius and thereby a formalism is developed which is not only more realistic but also provides elegant analytic solutions.

Variational calculation for the doubly excited state (2p2)3Pe of BeIII

Nonrelativistic energy of the (2p(2))(3)P(e) state of Be(2+) has been calculated using Ritz-variational method. The trial wave function is of Hylleraas type. The upper-bound energy E=-3.382 712 420 77 a.u. calculated by us is the lowest yet obtained.

Modification of the bubble model for positronium atoms in high surface tension liquids

The bubble model is widely used for the analysis of the decay characteristics of the positronium atom in liquids. However, according to some authors this description is inappropriate in the case of polar liquids with high surface tension. It has been advocated that for such media, rather than employing the value of the bulk surface tension (which appears as a parameter in the model), one should incorporate the notion of a transition layer between the liquid phase and the cavity that encloses the positronium. Accordingly, the usual bubble model with sharp boundaries is modified in the present work through the introduction of a diffusivity in the bubble surface, thus developing the proper setting (without involving any extra free parameters) such that liquids with high surface tension may also be meaningfully discussed in this context and confronted with experimental data.

Exotic affinities under Debye plasma

Muonic affinities of the exotic system π+μ− have been calculated variationally using a general three-body formalism. The system is found to be stable in the ground state under Coulomb coupling. The stability of this system under an external plasma environment has been analyzed using multiterm correlated basis sets of Hylleraas type. The effect of external plasma has been incorporated using the standard Debye screening model. The system tends toward gradual instability under the increased strength of the plasma, and the affinities have been found to decrease gradually and systematically. The effect of correlation on the exotic affinities has been analyzed in detail. The effect of angular correlation on exotic affinities is found to be around 40–80%.

The role of vibrational coupling in low-energy positron scattering from molecular targets

The effect of dynamically coupling the motion of the impinging positron with that of the molecular nuclei during the scattering process is examined. At the collision energies below the threshold of positronium formation one finds that the effects are increasing the size of the total integral cross sections but are different at different energies. Comparison with experiments is carried out for the target and shows rather good agreement between calculations and the available measurements. The presence of a threshold peak in the inelastic channel is also discussed in analogy with the case of electron scattering from the same molecule.

Ground-state Ps formation in e+-H2 scattering using the FBA

Ground-state capture cross sections in e+-H2 scattering are calculated in the energy region 10-1000 eV using the first Born approximation. The approximation employed by Sural and Mukherjee (1970) in predicting the first Born capture cross section is found to be unjustified. Present capture cross sections for incident positron energies E>or=30 eV are in good agreement with the measured data.

A molecular basis of the bubble model of positronium annihilation in liquids

The bubble model of positronium annihilation in liquids universally used for the description of positron annihilation in liquids involves macroscopic notions, based on a continuum description of the liquid (such as surface tension extended to nano-particle dimensions), which is somewhat unsatisfactory. An elementary molecular level description is presented. The consequent clarification of underlying concepts (such as the location of the surface of tension) is injected into the bubble model to obtain a modified version. It is shown that there is a considerable difference between the effective surface tension in such microbubbles and the corresponding bulk values. The concept of a work function for the positronium in the liquid is introduced on the basis of this model from which some conclusions are drawn regarding the quasi-positronium, the precursor of the positronium in the bubble, as being a rather extended delocalized entity.

Positron annihilation in simple molecular gases: A study of vibrational effects

Abstract The dynamics of positron annihilation in molecular gases containing simple diatomics is studied within a quantum formulation and is implemented by treating the positron–molecule interaction via an ab initio, nonempirical approach for all the forces at play. The process is analysed for room temperature conditions of the ambient gas and the behaviour of the corresponding annihilation rate coefficients, Z eff , is studied in detail by considering the role of the dynamical coupling between the positron motion and the vibrational degree of freedom of the molecular partner.