SLAC measures hard X-ray flashes under 400 attoseconds

At SLAC's Linac Coherent Light Source, a beam of high-energy X-rays is split in two. One half runs straight to a detector that records its energy spectrum. The other is focused onto a spot 150 nanometers wide on a sheet of copper foil, which gives off fluorescent light. Read together, the two measurements let the team calculate how long the pulse lasted: between 100 and 400 attoseconds, at energies around 9 keV.

Making such a pulse is hard, and measuring one is harder. Ichiro Inoue, a professor at the University of Tokyo and lead author, proposed the indirect technique based on amplified spontaneous emission, a light-emission process. Agostino Marinelli, a SLAC professor and study author, said the team had spent several years learning to create these high-energy attosecond pulses, and that measuring them is much harder still. The new diagnostic is the first to show the pulses really are only a few hundred attoseconds long. The method also extends attosecond science to about 9 keV, an order of magnitude beyond earlier soft X-ray experiments that operated below 5 keV.

The reason to go higher in energy is depth. Soft X-rays, the lower-energy kind, are well suited to chemical bonds in an atom's outer shell. Hard X-rays above 5 keV penetrate dense matter, which lets scientists pinpoint electrons with atomic precision. Sam Vinko, a physicist at the University of Oxford, describes the usual problem with plasma: the sample is excited, then relaxes and recombines before a measurement can be taken, so researchers often rely on models and assumptions. Scientists can now target extreme systems, such as those in inertial fusion energy experiments, and capture core-excited states before they blur or evolve.

X-rays often destroy the delicate samples they scan. These flashes finish before damage or decay can occur, so researchers can photograph a system in its untouched state. Tais Gorkhover of the University of Hamburg thinks this ability may improve nanoscale imaging and could eventually help control, or even reverse, light-matter interactions.

The people who gain first are those already using the flashes, scientists tracking electron motion in atoms and dense plasmas, and fusion-energy researchers whose experiments are named as targets. For them, the change is practical: a shutter faster than atomic decay, with the sample left intact. The study gives no date for wider access to such pulses, and it does not say when other facilities could reproduce the measurement.