In the experiment, the X-ray pulse first ejected an electron from an orbit close to the nucleus of the atom. The team measured the time it took this vacancy to refill by carefully controlling the time delay between the X-ray pump and subsequent probe laser pulses.

This has not been feasible before because the decay processes in the electron cloud take just a few femtoseconds – and the fastest laser sources have similar pulse durations. However, the attosecond source recently developed by Ferenc Krausz and colleagues in Vienna has opened up new possibilities for probing ultrafast atomic phenomena.

“It is essential that the exciting X-ray pulse is much shorter than the decay process being investigated to achieve good temporal resolution,” says Drescher. The pump pulse must be in the X-ray region because the pulse duration is fundamentally limited by its wavelength. “The limit for visible pulses, given by the light oscillation period, is about 2.5 femtoseconds,” he explains. “For our [X-ray source] the fundamental limit is pushed to 40 attoseconds." Drescher says that the team’s next aim is to measure sub-femtosecond phenomena, such as ionization processes involving two or more competing reaction pathways.

Commenting on the work in Nature, Louis DiMauro of the Brookhaven National Laboratory likened the attosecond source to the shutter speed of a camera – but one which is fast enough to capture electrons moving near the nucleus. “There are few papers that announce the beginning of a new era, but [this paper] falls into that category,” he says. “We are entering a new realm of hyperfast measurement – the age of attophysics has begun.”