IN AND OUT Passing through a thin germanium crystal (top), an X-ray beam is split into two equal-intensity beams. But when a short, intense laser pulse is directed at the crystal, the lattice spacings in a thin surface region (yellow) are altered--leading to picosecond switching between the diffracted beams.

By shining intense laser light on a thin germanium crystal while the crystal is transmitting X-rays, University of Michigan physicists Matthew F. DeCamp, David A. Reis, and Philip H. Bucksbaum and colleagues can control X-ray transmission [Nature, 413, 825 (2001)]. In the absence of the laser pulse, an X-ray beam passes through the crystal and emerges as two diffracted beams with roughly equal intensity. But when the laser light irradiates the crystal, the lattice spacings are altered rapidly, allowing the team to switch the X-ray beams on and off on a subpicosecond timescale.

A number of other advances in ultrafast methods for probing reaction dynamics using light have been reported in the past several years, but they have all had shortcomings. For example, scientists developed techniques in which femtosecond pulses of visible light are used to take snapshots of reactions as they occur. But the long wavelength of the fast laser limits the technique to fairly simple molecules.

High-energy (hard) X-rays have also been used as the basis of fast techniques in dynamics, but while not limited to simple systems, they do have shortcomings. For example, one technique uses commercial lasers to generate hard X-rays indirectly. The pulse duration is less than 1 picosecond, but the intensity is low. A high-intensity synchrotron method was recently reported, but it requires much costly equipment.

The Michigan group may have just found a way around those problems with their fast X-ray on-off switch.

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