Austrian and German researchers have demonstrated that the speed of x-ray-generated photoelectron wave packets can be controlled by a strong laser field. They produced soft x-ray bursts several hundred attoseconds long by pumping neon gas with 7-fs, 750-nm laser pulses. The burst ionized a neon gas jet, creating electron wave packets. Laser light with field strengths in excess of 108 V/cm was used to up- and downshift the packet's energy spectrum.The effectiveness of this method relies upon the attosecond-scale duration of the x-ray burst and on its timing relative to the phase of the laser light. The research, done by a team from the Technische Universität Wien in Austria and from German institutions Universität Würzburg, Fachhochschule Koblenz in Remagen and Universität Bielefeld, was published in the Aug. 16 issue of Science.