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

The population inversion between n = 2 and 1 in hydrogen-like helium is cal culated with a non-Maxwellian electron energy distribution in a plasma prod uced by optical field-induced ionization. Collisional relaxation of the ele ctron energy distribution after passage of the laser pulse is followed by u sing a non-linear Fokker-Planck equation. In the low density regime the rel axation process plays an important role in limiting the population inversio n, because the cold component of the electron distribution is reduced. A si gnificant gain is obtained with a pulse length of less than 30 fs.