Study of innershell photo-ionization-pumped L₂₃M₁ transition in Ca for the development of femtosecond X-ray laser

Abstract
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 nm, which is inside the water window. The peak spectral brightness of the X-ray laser output at 4.1 nm is predicted to be as large as 5 x l0²⁵ photons/sec./(mm² mrad² 0.1% bandwidth), which is 4 to 5 orders of magnitude brighter than a typical undulator radiation in the similar spectral region. In addition to the high flux, the expected duration of X-ray lasing of ~ 3 fs. will be useful for the study of fast dynamics in physical and biological sciences.