Controlling the Inner Electron Dance

January 9, 2012

When an electron is propelled out of an atom’s innermost orbital, the hole the departing electron leaves usually is filled within a few micro-billionths of a second by a different electron moving in from a higher orbital. But is there a way to force the original electron back where it came from, making the atom immune to such electron rearrangements?

That’s what researchers using SLAC’s Linac Coherent Light Source wanted to know. So, in only the fifth experiment conducted at the LCLS, they tuned the new X-ray free-electron laser to the exact resonance between the inner and outer orbitals of neon ions to see whether ousted inner electrons could be pushed home again.

They described the results from the experiments they conducted in October and November 2009 in the Dec. 2 Physical Review Letters. Why so long between experimentation and publication? Scientists previously haven’t been able to explore the frontier of inner-orbital electrons, so proper analysis of the data they collected two years ago required that lead author Elliot Kanter at Argonne National Laboratory, in Illinois, develop an entirely new set of tools to do so, said his colleague at Argonne and co-researcher Linda Young, who is also vice-chair of the American Physical Society’s Division of Atomic, Molecular and Optical Physics.

Using the first two quadrillionths of a second in the LCLS’s 10-femtosecond laser pulse, the team stripped away one electron from a neon atom’s outer orbit. That created a hole for an inner electron to fill. Then, using the remaining eight femtoseconds of the same pulse, the team managed to push that inner electron back where it started from.

But because the LCLS laser beam is “spiky” – because its beam energy ebbs and spikes during each pulse – the experimenters could only push the same electron back successfully in a handful of the tens of thousands of neon atoms subjected to each pulse.

“There was a small effect,” Young said. But a new process called seeding scheduled to begin this year at LCLS will smooth the beam considerably, she said. “Smoothing the time distribution and energy distribution would be a big boon for people who want to control how inner electrons dance around in an atom.”

She noted that future experiments need to take into account a key, albeit inadvertent, finding of their work: The LCLS’s high-fluence X-ray pulses can break open inner electron shells at far lower energy levels than expected, unleashing damaging electron decay cascades. “In other words: Beware!” she said. “These ionic resonances are well hidden and it’s not easy to calculate where they are going to be!”