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This article analyses the results for ground state anti-hydrogen scattering off meta-stable helium (He(21/3S)) targets at thermal energies using atomic orbital expansion technique. The variation of phase shifts indicates that the effective leptonic potential for each system has neither any barrier nor any hump. The zero energy cross sections for both the targets are much greater than the ground state target: σHe(21S)-H¯(1s)=150σHe(11S)-H¯(1s) and σHe(23S)-H¯(1s)=31σHe(11S)-H¯(1s). The maxima of the p-wave cross sections occur in the vicinity of the minima of s-wave result.

Collisions involving atoms and antiatoms have been investigated at low incident energies using close-coupling approximation (CCA). We investigate ortho -positronium– ortho -positronium ( o -Ps– o -Ps) scattering up to incident energy 3.4 eV. Antihydrogen–hydrogen elastic scattering has been studied in the energy range 10 −10 to 10 −2 a.u., whereas antihydrogen–helium system has been investigated in the range 10 −16 to 10 −2 a.u. The s-wave elastic scattering parameters, viz., scattering length, effective range, phase shift, cross section etc., have been reported. The present predictions have been compared with the other corresponding theoretical predictions wherever available. Usefulness of CCA model to investigate these systems at thermal energies has been emphasized.

We investigate the scattering of orthopositronium (o-Ps) atom off different atomic alkali targets (Na to Cs) at low and medium energies (up to 120 eV). Projectile-elastic and target-elastic close-coupling models have been employed to investigate the systems in addition to the static-exchange model. Elastic, excitation and total cross sections have been reported for all four systems. The magnitude of the alkali excitation cross section increases with increasing atomic number of the target atom while the position of the peak value shifts towards lower incident energies. The magnitudes of the Ps excitation and ionization cross sections increase steadily with atomic number with no change in the peak position. The reported results show regular behaviour with increasing atomic number of the target atom. Scattering parameters for the Ps–Rb and Ps–Cs systems are being reported for the first time.

The low-energy (below 10-3 au) capture processes in hydrogen-anti-hydrogen scattering have been investigated using the plane wave approximation. The capture cross sections for the different excited states (up to the principal quantum number N = 24) of the protonium (Pn) atom, keeping the positronium (Ps) atom in its ground state, have been explicitly calculated. The present cross sections for the excited states of the Pn atom (N = 24, L = 0) are found to be in fair agreement with those of Jonsell et al (Jonsell S, Saenz A, Froelich P, Zygelman B and Dalgarno A 2001 Phys. Rev. A 64 052712). The s-state capture cross sections for the N = 17-23 excited states are found to be quite significant. The p-state cross sections are also not negligible.

Positronium-hydrogen elastic scattering is investigated using the six-state full close coupling approximation employing the basis set Ps(1s,2s,2p) + H(1s,2s,2p). S-wave singlet and triplet scattering and also the corresponding scattering length and effective range are compared with the corresponding predictions of Drachman and Houston and the Belfast group. We also report phase shifts up to l = 2 and the integrated elastic cross sections. Present results indicate that the effect of target excitation as well as the effect of excitations of both the atoms are significant in the energy region considered. We also report the angle integrated Ps(1s) + H(1s) → Ps(2p) + H(1s) and Ps(1s) + H(1s) → Ps(1s) + H(2p) excitation cross sections.

This survey reports theoretical studies involving positronium-atom scattering. Investigations carried out on the interaction of positronium (Ps) atoms in gases during the last three decades have been briefly reviewed in this article. The recent studies using the Ps close coupling method form a part of the present resume. The partial wave theory using the close coupling method in which both the atoms are allowed to be excited to non-zero orbital angular momentum states has been presented. This article advocates the need to include the effect of exchange and to retain the excited states of both the atoms to predict reliable results.

Quantal calculations for scattering of ground-state antihydrogen by metastable (n=2S) helium atoms have been performed using the nonadiabatic, atomic orbital expansion technique at thermal energies. The zero-energy elastic cross sections of the present systems are much greater than the corresponding value for the ground-state helium target. The low-energy elastic cross section for the singlet metastable helium [He(2 {sup 1}S)] target is higher than the corresponding value when the target is in the metastable triplet state [He(2 {sup 3}S)].

We have investigated the $s$-wave elastic scattering of antihydrogen atoms off atomic alkali-metal targets (Li, Na, K, and Rb) at thermal energies $({10}^{\ensuremath{-}16}--{10}^{\ensuremath{-}4}\phantom{\rule{0.3em}{0ex}}\mathrm{a.u.})$ using an atomic orbital expansion technique. The elastic cross sections of these systems at thermal energies are found to be very high compared to $\overline{\mathrm{H}}\text{\ensuremath{-}}\mathrm{H}$ and $\overline{\mathrm{H}}\text{\ensuremath{-}}\mathrm{He}$ systems. The theoretical models employed in this study are so chosen to consider long-range forces dynamically in the calculation. The mechanism of cooling suggests that Li may be considered to be a good candidate as a buffer gas for enhanced cooling of antihydrogen atoms to ultracold temperature.

In this dissertation, scattering of positronium (Ps) atoms by atomic hydrogen (H) and helium (He) targets has been reviewed at low and intermediate energies. This article mentions the measurements for He target and details the theoretical works employing eigenstate expansion method or the close-coupling approximation (CCA). Different versions of the CCA models have been discussed along with their merits and demerits. The other theoretical models employed for studying Ps impact scattering have also been touched. The dramatic difference in the measured data and the theoretical predictions for the zero energy results of He have been emphasized. Reasons for the difference amongst the theoretical predictions as well as the measured data at intermediate energies have been justified. Suggestions have been made for the future works to be carried out both theoretically as well as experimentally.

The s-wave elastic phase shifts and cross sections for the H-Li system are predicted using an ab initio and nonadiabatic quantal method at very low energies. The smooth and monotonic change of the phase shifts with added eigenstates indicates that the leptonic potential for the present system has no barrier as noticed in the case of the H-He system, or even a hump which is present in H-H interaction. The very high value of the s-wave elastic cross section at zero or near zero energy implies that Li will be a more efficient buffer gas in cooling the antiatom to ultralow temperatures. This study may stimulate new experiments in thermalizing H atom.

The present study reports the nonzero partial wave elastic cross sections together with s-wave results for the scattering of an antihydrogen atom off a gaseous helium target at thermal energies (up to 10{sup -2} a.u.). We have used a nonadiabatic atomic orbital method having different basis sets to investigate the system. The consideration of all the significant partial waves (up to J=24) reduces the oscillatory nature present in the individual partial wave cross section. The added elastic cross section is almost constant up to 10{sup -7} a.u. and then decreases steadily and very slowly with increasing energy.

Projectile-elastic, target-elastic, and full close-coupling approximation models have been employed to investigate scattering of ortho-positronium off Li and Na atoms at low energies. Nine different basis sets for each system have been used to find the relative importance of atomic (target and projectile) states on the elastic scattering parameters and the convergence of the predicted results with added eigen- and pseudostates. The s-wave elastic phase shifts, scattering lengths and cross sections of both the systems are reported upto the incident energy k=0.5 a.u. and compared with the corresponding existing theoretical predictions. It has been found that the van der Waals and higher order long-range interactions play a crucial role in predicting s-wave elastic parameters for Ps-alkali-metal-atom systems at low energies.

This paper reports the elastic scattering cross sections for a few low-lying partial waves and also the converged elastic cross sections with added partial waves in the energy range 10{sup -10}-10{sup -2} a.u. for the H-bar-H system using atomic orbital techniques. The present s-wave predictions are in good agreement with the other existing theoretical estimates. Nonzero low-order partial-wave elastic cross sections show dips like for s-wave scattering. The converged elastic cross section shows structurelike behavior in the energy range 4.2x10{sup -4}-10{sup -2} a.u.

In view of the current interest in trapping antihydrogen H(bar sign) atoms below 1 K, the s-wave elastic-scattering parameters for the antihydrogen scattering off atomic helium target are calculated in the energy range 1x10{sup -16}-1x10{sup -2} a.u. using close-coupling models. The predicted cross section will help to simulate the evolution of a realistic mixture of He and H{sub 2} through the process involving ejection of H(bar sign) from the trap. The trend of the present results with different basis sets conclusively indicates the reliability of the predicted results.

In view of the recent interest in the trapping of antihydrogen atom $\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{},$ at very low temperatures, $\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{-}\mathrm{H}$ scattering has been investigated at low incident energies using a close-coupling model with the basis set $\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{}(1s,2s,2\overline{p})+\mathrm{H}(1s,2s,2\overline{p}).$ The predicted s-wave elastic phase shifts, scattering length, and effective range are in a good agreement with the other recent predictions of Jonsell et al. and of Armour and Chamberlain. The results indicate that the atomic orbital expansion model is suitable to study the $\mathrm{H}\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{-}\mathrm{H}$ scattering at ultracold temperatures.

The scattering of orthopositronium off atomic lithium (Li) target has been investigated using the close-coupling approximation (CCA) with different basis sets. The predictions of different projectile-elastic and target-elastic CCA models have been presented along with static-exchange results. We report the scattering length, the effective range, and the zero-energy cross section for singlet and triplet scattering. The spin averaged zero-energy cross section is significantly improved over the static-exchange prediction by the inclusion of the virtual Ps excitation channels in the basis set. All the present models indicate a binding of lithium-positride (LiPs) in the attractive singlet channel. The low-energy (below first Li excitation threshold) and low order (up to l=2) phase shifts are also reported and are found to be variationally consistent. Except the s-wave singlet scattering, other phase shifts are influenced more by the target distortion effect than the projectile inelastic effect. Estimates of the different single-atom excitation cross sections have also been reported along with an estimate of the total cross section. The peak in the total cross-section curve for the present system is not prominent at energies around the Ps inelastic threshold, unlike the Ps-Na case. For incident energy 20 eV or more the Ps ionization is the dominant contributor to the total cross section.

We investigate the scattering of orthopositronium by atomic sodium target at low and medium energies (up to 50 eV) using target-elastic and projectile-elastic close-coupling approximations (CCA) having different basis sets together with the static-exchange model. The low-energy elastic-scattering parameters are found to be consistent and the estimated binding energy for NaPs (in spin-singlet scattering) runs from 0.0042-0.0052 a.u. compared to value 0.005 892 a.u. obtained by using stochastic variational method [G. Ryzhikh and J. Mitroy, J. Phys. B 31. L401 (1998)]. The elastic cross sections as obtained by different CCA models, except the static-exchange one, differ very marginally among themselves in the energy range 0.017-25 eV and 15 eV, onwards all the partial cross sections are very close to the corresponding exchange plane-wave estimates. At higher energies (above 15 eV), it is found that the major contribution to the total cross section comes from the positronium inelastic channels.

Scattering of orthopositronium off helium target has been investigated using close-coupling method in the energy range 0--110 eV. Two basis sets, (a) $\mathrm{Ps}(1s)+\mathrm{He}{(1s}^{2},{1s2}^{1}s,{1s2}^{1}p)$ and (b) $\mathrm{Ps}(1s,2p)+\mathrm{He}{(1s}^{2},{1s2}^{1}s,{1s2}^{1}p),$ have been employed to find the scattering parameters. Low-order phase shifts, scattering length, and elastic and excitation cross section up to $n=2$ are reported using close-coupling approximation. Total cross section is also given by adding other partial cross section and compared with available measured data and existing theoretical predictions. Our total cross section at zero energy is very close to the theoretical prediction of Drachman and Houston [J. Phys B 3, 1657 (1970)] and measured data of Canter et al. [Phys. Rev. A 12, 375 (1975)]. Present total cross section is in qualitative agreement with measured data of the University College London group in the energy range considered. In particular, present predictions are in good agreement with the UCL group in the energy range 20--30 eV. It has been found that elastic cross section is dramatically reduced, at zero or near zero energies, with the inclusion of target excitation in the expansion scheme. Moreover, ionization cross section of the Ps atom is found to be a major contributor to the total cross section above the ionization threshold.

Scattering of a Ps atom off a He target has been investigated in the framework of the close-coupling approximation using two basis sets: (a) $\mathrm{Ps}(1s)+\mathrm{He}{(1s}^{2}{,1s2}^{1}{s,1s2}^{1}p)$ and (b) $\mathrm{Ps}(1s,2p)+\mathrm{He}{(1s}^{2}{,1s2}^{1}{s,1s2}^{1}p).$ Target inelastic channels reduce the elastic cross sections appreciably near zero energy. The present results are in good agreement with the theoretical prediction of Drachman and Houston and the measured data of Canter, McNutt, and Roellig and Coleman et al. and are also in good agreement with the measured data of the UCL group from 15--20 eV.