SLAC researchers have developed a laser-timing system that could lead to X-ray snapshots fast enough to reveal the triggers of chemical and material reactions.
"Previously, we could see a chemical bond before it's broken and after it's broken," said Ryan Coffee, an LCLS scientist whose team developed this system. "With this tool, we can watch the bond while it is breaking and 'freeze-frame' it."
The success of most LCLS experiments relies on precise timing of the X-ray laser with another laser, a technique known as "pump-probe." Typically, light from an optical laser "pumps" or triggers a specific effect in a sample, and researchers vary the arrival of the X-ray laser pulses, which serve as the "probe" to capture images and other data that allow them to study the effects at different points in time.
Timing tools now in place at most LCLS experimental stations can measure the arrival time of the optical and X-ray laser pulses to an accuracy within 10 femtoseconds, or quadrillionths of a second. The new pulse-measuring system, which is highlighted in the July 27 edition of Nature Photonics, builds upon the existing tools and pushes timing to attoseconds, which are quintillionths (billion-billionths) of a second.
Nick Hartmann, an LCLS research associate and doctoral student at the University of Bern in Switzerland who is the lead author of the study detailing the system, said, "An X-ray laser with attosecond timing resolution would open up a new class of experiments on the natural time scale of electron motion."
The new system uses a high-resolution spectrograph, a type of camera that records the timing and wavelength of the probe laser pulses. The colorful patterns it displays represent the different wavelengths of light that passed, at slightly different times, through a thin sample of silicon nitride.
This material experiences a cascading reaction in its electrons when it is struck by an X-ray pulse. This effect leaves a brief imprint in the way light passes through the sample, sort of like a temporary interruption of vision following a camera's flash.
This X-ray-caused effect shows up in the way the light from the other laser pulse passes through the silicon nitride – it is seen as a brief dip in the amount of light recorded by the spectrograph, like the after-image of a camera flash. An image-analysis algorithm then precisely calculates, based on the recorded patterns, the relative arrival time of the X-ray pulses.