Atomic 'movies' made thanks to X-ray bursts

Illustrating the promise of bright ultrashort X-ray pulses, scientists have observed the wiggling of atoms within a piece of solid bismuth after exciting the element's electrons (Science 2007, 315, 633). The experiment marks several technological achievements in developing the Sub-Picosecond Photon Source, a precursor to the X-ray free-electron laser at the Stanford Linear Accelerator Center, Menlo Park, Calif. With their femtosecond-scale pulses and atomic-scale resolving power, such lasers are expected to allow monitoring of atomic motions during chemical reactions. David Mark Fritz led a team that bombarded a bismuth target with an infrared laser to excite electrons, then followed that with 100-femtosecond X-ray pulses. The researchers measured the resulting bismuth lattice vibrations by studying the diffraction efficiency of the pulses. They found that the bonds in the lattice "softened," or became weaker, in response to the electronic excitation.