X-ray Excitation of Energetic Metastable Levels in Atoms and Ions

A fundamental problem associated with the development of XUV lasers is that, at constant oscillator strength, the radiative lifetime of an excited level in an atomic system varies as the square of the transition wavelength. For a strong transition near 200 A, this lifetime is of the order of 10^-11 seconds.

by R.G. Caro, J.C. Wang, J.F. Young, and S.E. Harris