Effect of Debye plasma environments on positronium formation in positron and alkali-metal atoms (Li, Na, K, Rb, & Cs) collisions
M. K. Pandey1*, Y. –C. Lin1, Y. K. Ho1
1Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
* presenting author:MUKESH KUMAR PANDEY, email:pandey@pub.iams.sinica.edu.tw
Recently, positron and alkali-metal atoms collisions have become a subject of great theoretical as well as experimental interest because of positronium (Ps) formation [1-3, and references therein] would appear in such processes. The effects of Debye plasmas on the plasma embedded atomic systems have also received considerable attention by many atomic physics research groups. However, Ps formation in positron-alkali-metal atoms collisions above 1 keV collision energy has not been examined till date as there are no reliable calculations and experiments reported on these processes in the given energy range. Aside from being a fundamental aspect, a study on collision processes in plasma environments will find profound importance in astrophysics and plasma physics. In this present work we have studied the effects of plasma screening on positronium formation cross sections using the Debye-Hückel model, which is known to be very reliable to describe the interaction potential of multielectron atoms [3]. Here, we consider the model potential suggested by Schweizer et al. [4], as

Vmodel=-1/r*(Z'+(Z-Z')exp(-a1r+a2r*exp(-a3r)).

The electron–ion interaction is described by the Debye Hückel potential (e is the unit charge)

V(r)=(Vmodel(r))exp(-r/rd).
.
In the present work we have found that in collisions of positron-alkali-atoms (Li, Na, K, Rb, & Cs) the variation behaviors of Ps formation cross sections for all the systems are almost the same. In these systems, the Ps formation cross sections ascend gradually until reaching a maximum around ~10 keV, and then start to decrease. Plasma screening effect reduces the Ps formation cross sections above ~20 keV energies with decreasing rd. Also, the Ps formation cross sections in all the systems below 20 keV, at all rd except for rd = 4a0, are larger than the unscreened cases. Eventually, we conclude that the plasma screening parameter plays a crucial role on the Ps formation cross sections for all Debye lengths in the entire collision energy range. Detail results will be presented at the meeting.
This work was supported by the Ministry of Science and Technology (MOST) of Taiwan.


Keywords: positronium (Ps) , Debye plasmas, positron-alkali-metal atoms collisions, Ps formation cross sections