Laser-driven inner-shell excitation in high-Z atoms: a shell-selective impact ionization mechanism

Summary form only given. The efficient generation of inner-shell population inversion in high-Z elements (Z>30) is the most direct route to the development of an atom-based coherent multi-kilovolt X-ray source (i.e. a/spl sim/1 A X-ray laser). We describe a shell-selective, coherent impact ionization mechanism which is capable of providing a required pumping rate in excess of 1W/atom for the generation of inner-shell population inversion in laser-driven plasmas. The evidence for the existence of such a mechanism is contained in the Xe L-shell (2p/spl larr/3d) emission spectra obtained under laser excitation of 5-20 atom Xe clusters at irradiances of 10/sup 18/-10/sup 19/ W/cm/sup 2/. The spectra display three striking features: (i) the generation of "hollow" atoms, including the inverted ions Xe/sup 27+/(2p/sup 5/3d/sup 10/) and Xe/sup 28+/(2p/sup 5/3d/sup 9/), (ii), a/spl sim/1000/spl times/ reduction in the efficiency of Xe(L) emission as the pump-laser wavelength is increased by a factor of /spl sim/3 from 248 nm to 800 nm/sup 2/ and, most significantly, (iii) the observation of ionic species with double 2p vacancies only under 248 nm excitation.