X-rays have long proved useful in investigating the microscopic structure of matter. Now scientists have taken an important step toward bringing such x-ray imaging to the next level. A report in the current issue of the journal Nature describes the development of an ultrafast switch that could allow researchers using x-ray beams to capture the dynamics of molecular reactions.
Switching x-rays on and off fast enough to freeze atomic movement is no easy task. But Matthew F. DeCamp of the University of Michigan and colleagues found that they could achieve this speed using a germanium crystal to control the timing of x-ray pulses. The resulting switch functions much like a camera's shutter. Given sufficient light, the faster the shutter speed, the better the camera can freeze an object in motion. But whereas a camera shutter might reach speeds amounting to a thousandth of a second or so, the new switch allows x-ray pulses in picosecond time (that is, a trillionth of a second)fast enough to document changes in molecular structure during chemical reactions.
Other techniques for controlling x-rays on a comparably fast timescale do exist, but they are variously invasive and applicable to only certain x-ray beam lines. The switch developed by DeCamp's team, in contrast, "is a versatile tool that could be added to nearly every beam line without having to touch the source," Ferenc Krausz and Christian Spielmann of the Vienna University of Technology write in an accompanying commentary. "Many challenges remain, such as shortening the switching time by using electronic excitations, and improving the switching efficiency to produce x-ray pulses with good contrast," they add. "But once these problems have been solved, such ultrafast switches could become a key component in the x-ray toolbox for probing the structural dynamics of matter."