Multiphoton Ionization of Metastable Hydrogen Atom by Electron and Positron Impact

Charge asymmetry is studied for laser assisted multiphoton ionization (n γ e, e) of the hydrogen atom from its metastable(2s) state using coplaner geometry. The laser field is chosen to be linearly polarized, homogeneous, monochromatic with polarisations parallel and perpendicular to the incident momentum. The binary peak for the positron impact always lies above that for the electron impact while the reverse is true for the recoil peak. The famous Kroll Watson sum rule is found to be valid for multiphoton exchange while for single photon case, the laser assisted cross sections are highly suppressed with respect to the field free ones.