ULTRAFAST X-RAY PROCESSES WITH HOLLOW ATOMS

An ultrafast inner-shell ionization process, together with x-ray emiss ion stimulated by high-intensity short-pulse x rays, is studied. In th is process it is found that the multi-inner-shell ionization predomina tes, leading to the formation of hollow atoms. These hollow atoms play an instrumental role in high brightness x-ray measurements of the ult rafast dynamics in chemistry, biochemistry, biology, and materials. As the pumping x-ray power and thus the density of hollow atoms are incr eased, the emission of x rays is not only of significance for the meas urement but is good for x-ray lasing. This lasing mechanism has a clea r advantage over the conventional inner-shell ionization method, yield ing a gain value of more than 10(4) cm(-1) over a relatively long time . Classes of experiments of pump x-ray probe and x-ray laser are sugge sted.