Debarshi Gangopadhyay

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.

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.

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.

Enigmatic features of positronium acceptor reactions in associative solvents

Abstract Positronium (Ps) reactions with the diamagnetic acceptor molecule, namely, nitrobenzene, have been critically studied by us at various temperatures in water and water/methanol mixture (1:1 v/v), with the special emphasis on the Arrhenius plot of the overall reaction rate constant. While the Ps–Ac reaction rate constant in water takes over an anomalous course at higher temperatures (departing from normal Arrhenius paradigm), the reaction in water/methanol mixture, however, shows an entirely different systematics. The temperature dependences of surface tensions and viscosities of these solvents when taken in conjunction with the other physical parameters, enable us to explain and compare the plausible mechanism in the respective solvents.

Temperature, pressure and solvent dependence of positronium acceptor reactions

Abstract Positronium (Ps) reaction rates ( κ ) with weak Acceptors (Ac) leading to the formation of Ps–Ac complexes show several intriguing features: non-monotonic temperature dependence of κ (departing from the usual Arrhenius paradigm), considerable variability of κ with respect to different solvents, and anomalies in response to external pressure at ambient temperature (large changes of κ in some media and hardly any in others). We explain all these phenomena, introducing the novel concept of a critical surface tension, which unifies observations in diverse non-polar solvents at different temperatures and pressures.

Mechanism of positronium–nitrobenzene complex formation in water

Rate constants for complex formation between the positronium (Ps)-atom and the weak acceptor (Ac) nitrobenzene in water have been measured by us at various temperatures with particular emphasis on the slopes of the Arrhenius plot on either side of the observed turn-over point. Taken together with the temperature dependence of the surface tension and the viscosity of water, our data enable us to discuss the implications of our experimental findings as to the tenability of a proposed underlying reaction mechanism.

Solvent effects in positronium complex formation and the bubble model

The formation of positronium–acceptor complexes (Ps–Ac) with nitro aromatics and quinone has attracted much attention. The dependence of such reactions on temperature and on the nature of the solvent is sometimes discussed within the ambit of the bubble model of Ps in liquids. However, the description generally adopted is that of a sharp bubble boundary which is not only unrealistic as a liquid interface but has also been shown recently, through a series of papers by the present authors, to be in contradiction with experimental observations. An explanation is sought for the observed occurrence of both Arrhenius and anti-Arrhenius temperature dependence in the reaction rate for Ps–Ac complex formation seen over different temperature regimes in terms of the Kramers’ turnover in the context of the improved version of the bubble model.