We have been developing a small-scale EUV laser facility at MIT with which we hope to observe gain in nickel-like molybdenum at 191 angstroms. The physics involved in the formation of laser gain is examined through the use of simple models, and optimum operating temperatures and electron densities are determined. The upper state population is determined primarily through a balance between direct collisional excitation to and from the upper laser state; the lower state population follows primarily from a balance between indirect excitation and radiative decay. Models describing these balances are used to estimate optimum conditions and the resulting population inversions.
Peter L. Hagelstein,
"Development of the MIT tabletop soft x-ray laser (Invited Paper)", Proc. SPIE 1551, Ultrashort Wavelength Lasers, (5 February 1992); doi: 10.1117/12.134825; https://doi.org/10.1117/12.134825