For the development of femtosecond X-ray laser using inner-shell L23M1 transition in Ca

A new physical scheme for femtosecond X-ray lasers, where the upper lasing is pumped by X-ray photons and the lower lasing level is depopulated via a Coster-Kronig radiationless transition, is analyzed for Ca. The transition wavelength is 4.1 run, which is inside the water window. The peak spectral brightness of the X-ray laser output at 4.1 rim. is predicted to be as larg e as 5 x 10(25) photons/sec./(mm(2) mrad(2) 0.1% bandwidth), which is 4 to 5 orders of magnitude brighter than a typical undulator radiation in the si milar spectral region. In addition to the high flux, the expected duration of X-ray lasing of similar to3 fs. will be useful for the study of fast dyn amics in physical and biological sciences.