He(2¹S, 2¹P) excitation and positronium formation in positron-helium collisions at intermediate energies

Abstract

The close-coupling approximation has been used to investigate positronium formation into the n=1 and n=2 levels and helium excitation (11S-21S, 21P) in positron-helium collisions at energies between 31.3 eV and 200 eV. The authors have used a target basis set of the form He(1s, 2s, ks, 2p, kp) and investigated the effect upon excitation due to the addition of the positronium basis set Ps(1s, 2s, 2p). An independent electron model has been combined with a model potential to represent the helium atom. Positronium formation reduces the 21S excitation below 50 eV but the 21P excitation is significantly reduced for all energies up to 150 eV. The present excitation results are in closer agreement with previous close-coupled calculations than with other theoretical methods. The He(n=2) excitation total is in agreement with experimental data below 36 eV. The calculated total positronium formation cross section is in agreement with experiment below 100 eV.

The Scattering of Positrons and Positronium by Atoms in the Close-Coupling Approximation

“…We have also performed calculations, for positron scattering by helium atoms, within the single-active electron approximation [9]. Our results were in agreement with the experimental total cross-sections for the formation of positronium.…”

“…We begin by briefly describing our treatment of the positron-argon scattering system and refer to our previous papers for further details [7][8][9]. We treat the argon atom within the framework of the single-active electron approximation, i.e.…”

Excited-State Positronium Formation from Helium, Argon, and Xenon

“…The results of the close-coupling approximation calculations of Refs. [17,20] are, respectively, a factor of 2 and 3 higher in magnitude at their peak than the experimental Q Ps ð2PÞ.…”

“…A decade later, using a similar method, Ps à was also observed in low density gases (namely, Ne, Ar, and H 2 ) formed by positron projectiles of a few tens of eV [14]. While formation cross sections for Ps in the 2S and 2P states have been theoretically calculated for H (e.g., [15]), He [16][17][18][19][20], and the noble gases [21], experimental determinations have been lacking until now. It was recently considered that structure observed in the total positronium formation cross section (Q Ps ) for the heavier noble gases might be due to Ps à [22][23][24], and estimates were made which inferred increasing contributions from Ps à to Q Ps as the target atomic number was increased from He to Xe [22].…”

“…[19], the close-coupling calculations of Refs. [17,20], the distorted-wave approach of Ref. [16], and from 100 eV, the second Born approximation of Ref.…”