Positron-Impact Excitation Of The Lowest Autoionizing State In Rubidium Atom Using Distorted Wave Method

ABSTRACT

Many calculations on atomic collisions and scattering processes have been performed on electron

impact excitation of the lowest autoionizing state of rubidium, but not much attempt has been

made with positron impact which is of equally fundamental importance and is receiving attention

nowadays with the availability of improved positron beam experiments. So, in this study, total

cross-sections, differential cross-sections, lambda parameter, R parameter and the alignment

parameter for positron impact excitation of the lowest autoionizing state of rubidium have been

calculated using Distorted Wave method. The wave functions used are the Roothan Hatree Fock

double zeta and multi zeta wave functions due to Clementi and Roetti. Variations in distortion

potential have been made such that the static potential of the initial state of rubidium atom is

used as the initial channel distortion potential and a linear combination of static potentials of the

initial and final states as the final channel distortion potential to check its effect on crosssections.

Numerical calculations have been done using a modified DWBA1 FORTRAN

computer program which was originally made for hydrogen atom. The results for positron impact

excitation of the lowest autoionizing state of rubidium have been analyzed and compared with

experimental and theoretical results for positron and electron impact excitation of the same state

available in literature. From the comparison of the results, it is seen that in general the electron

impact excitation cross section results are higher than the positron impact excitation cross section

especially near excitation threshold energy. This can be attributed to the exchange process which

takes place in the case of electron impact and not in the case of positron impact and also due to

larger interaction between the projectile and the target in case of electron impact than in case of

positron impact. It is also found from the alignment parameter results that the integral cross

section results for m=0 level are larger compared to m=1 level for impact energies up to about

500 eV beyond which integral cross-sections for the magnetic sublevel m=1 become greater. The

lambda parameter indicates that more particles are scattered towards m=0 for electron impact

compared to positron impact excitation near threshold energy. R parameter results have been

calculated to account for phases of scattered amplitudes in the collision process.