Number 192 (Story #1), August 30, 1994 by Phillip F. Schewe and Ben Stein A SOURCE OF COHERENT X RAYS , an x-ray laser, is hard to achieve because of the enormous power needed to bring about a population inversion in which atoms in the laser medium reside in excited states long enough in time and high enough in energy to promote a worthwhile laser action. In general the power requirement is proportional to the inverse fifth power of the wavelength of the emitted light. Charles Townes of the University of Illinois at Chicago has partly gotten around this problem by using light from an ultraviolet laser to excite not atoms but clusters of atoms. The incoming pump light excites outer electrons in the cluster (xenon) atoms and these in turn can, by consolidating their energy, ionize a more tightly-bound, inner electron in one of the atoms, creating thereby a sort of "hollow" atom. The ejected electron is quickly replaced by an outer electron in a transition that gives rise to a hard x ray with a wavelength of 2-3 angstroms. This whole process seems to be aided by a self-focusing of the pump laser beam along the ionization trail through the laser medium. X-ray lasers in this wavelength range, when fully realized, will facilitate microscopy with a much better spatial resolution than is now possible. (A. McPherson et al., Nature, 25 August 1994.)