Population inversion in plasmas produced by picosecond laser pulses

Abstract The expansion of plasmas formed by picosecond laser pulses produces rapid cooling. If the cooling rate is faster than the rate of recombination frozen ionization results with the plasma in a much higher ionization state than predicted by the equilibrium relations. Since recombination is much more rapid into the higher bound states, inversions of population between the upper and lower states of the recombining ion may be expected to occur. The conditions necessary for this to take place in hydrogenic ions are examined.