Incredibly tiny explosion packs a big punch

Japanese researchers have captured the birth of a nanoplasma – a mixture of highly charged ions and electrons – in exquisite detail, as a high-powered X-ray laser roasted a microscopic cluster of atoms, tearing off electrons.

While it’s cool to witness an explosion lasting just half a trillionth of a second and occupying one-hundredth the diameter of a human hair, caused by an X-ray beam 12,000 times brighter than the sun, it’s also important for studies of tiny structures such as proteins and crystals.

To study small things you need light of a comparably small wavelength. The wavelength of the X-rays used by Yoshiaki Kumagai and his colleagues in this experiment at the Spring-8 Angstrom Compact free electron Laser (SACLA) in Japan is one ten billionth of a meter: you could fit a million wavelengths into the thickness of a sheet of paper.

This is the perfect wavelength for probing the structure of crystals and proteins, and the brightness of a laser gives a good strong signal. The problem, however, is that the laser itself damages the structure, says Kumagai, a physicist from Tohoku University in the city of Sendai.

“Some proteins are very sensitive to irradiation,” he explains. “It is hard to know if we are actually detecting the pure protein structure, or whether there is already radiation damage.”

The tell-tale sign of radiation damage is the formation of a nanoplasma, as the X-rays break bonds and punch out electrons from deep inside atoms to form ions. This happens in tens of femtoseconds (that is, quadrillionths of a second) and sets off complex cascades of collisions, recombinations and internal rearrangements of atoms. SACLA’s ultra short pulses, only 10 femtoseconds long, are the perfect tool to map out the progress of the tiny explosion moment by moment.

To untangle the complicated web of processes going on the team chose a very simple structure to study, a cluster of about 5000 xenon atoms injected into a vacuum, which they then hit with an X-ray laser pulse.

A second laser pulse followed, this time from an infrared laser, which was absorbed by the fragments and ions. The patterns of the absorption told the scientists what the nanoplasma contained. By repeating the experiment, each time delaying the infrared laser a little more, they built a set of snapshots of the nanoplasma’s birth.

Previous experiments had shown that on average at least six electrons eventually get blasted off each xenon atom, but the team’s set of new snapshots, published in the journal Physical Review X, show that it doesn’t all happen immediately.

Instead, within 10 femtoseconds many of the xenon atoms have absorbed a lot of energy but not lost any electrons. Some atoms do lose electrons, and the attraction between the positive ions and the free electrons holds the plasma together. This leads to many collisions, which share the energy among the neutral atoms. The number of these atoms then declines over the next several hundred femtoseconds, as more ions form.

Kumagai says the large initial population of highly-excited neutral xenon atoms were gateway states to the nanoplasma formation.

“The excited atoms play an important role in the charge transfer and energy migration. It’s the first time we’ve caught this very fast step in nanoplasma formation,” he says.