Ultrashort Laser Pulses Can Control X-Ray Switch

In the molecular world of chemical reactions and conformational changes in biological and other molecules, events often happen on time scales from hundreds to thousands of femtoseconds—the typical period of molecular vibrations. Femtochemistry (described in Physics Today, Physics Today 0031-9228 52 

 1999 19  December 1999, page 19 ) uses ultrashort laser pulses, only a few tens of femtoseconds in duration, to probe electronic changes on such time scales. From these experiments, dynamics information can be extracted. But to obtain direct structural information, one needs a probe whose wavelength matches the angstrom scale of interatomic spacing: x rays are a prime candidate (see the article by Eric Galburt and Barry Stoddard in Physics Today, July 2001, page 33).

Obtaining probes of appropriate wavelength, duration, and intensity is no easy matter, however. Laser-produced plasmas can generate sufficiently short bursts of x rays, but the bursts aren’t highly collimated...