Collisional effects on population inversion in optical-field-ionized plasmas

The population inversion between n=2 and 1 hydrogen-like helium is calculated with a non-Maxwellian electron energy distribution in a plasma produced by optical field-induced ionization. Collisional relaxation of the electron energy distribution after passage of the laser pulse is followed by using a non-linear Fokker-Planck equation. In the low density regime the relaxation process plays an important role in limiting the population inversion, because the cold component of the electron distribution is reduced. A significant gain is obtained with a pulse length of less than 30 fs. (author)