Ultrafast x-ray processes with hollow atoms

An ultrafast inner-shell ionization process, together with x-ray emission stimulated by high-intensity short-pulse x rays, is studied. In this process it is found that the multi-inner-shell ionization predominates, leading to the formation of hollow atoms. These hollow atoms play an instrumental role in high brightness x-ray measurements of the ultrafast dynamics in chemistry, biochemistry, biology, and materials. As the pumping x-ray power and thus the density of hollow atoms are increased, the emission of x rays is not only of significance for the measurement but is good for x-ray lasing. This lasing mechanism has a clear advantage over the conventional inner-shell ionization method, yielding a gain value of more than 10⁴cm^-1 over a relatively long time. Classes of experiments of pump x-ray probe and x-ray laser are suggested.