Electrons move on the time scale of attoseconds, each of which is a vanishingly short billionth of a billionth of a second. With ultrafast pulses of light, researchers have measured the photoemission of electrons from their valence shells, a process that was once considered instantaneous. For developing the tabletop laser systems necessary to glimpse such electron dynamics, three researchers were awarded the 2023 Nobel Prize in Physics (see Physics Today, December 2023, page 13).
Now Stanford University’s Taran Driver, Agostino Marinelli, and James Cryan and their colleagues have harnessed recent advances in x-ray free-electron lasers (XFELs) to witness the photoelectric effect in the electrons closest to an atom’s nucleus. Complex, multielectron interactions can be studied with measurements of core electrons, but the energies of such electrons had been out of reach for attosecond pulses of light produced with low-energy tabletop lasers.
The new results were obtained with the kilometer-scale Linac Coherent Light Source at SLAC. It’s an XFEL source that sends a relativistic bunch of electrons through a series of alternating magnets to generate femtosecond x-ray pulses. Compared with tabletop lasers, XFEL sources generate much more intense pulses of light with a broad range of tunable wavelengths. But to study core-electron emission, researchers needed attosecond pulses of x-ray light. To do so, Marinelli, Cryan, and colleagues used a technique called enhanced self-amplified spontaneous emission. It creates an ultrashort current spike in the electron beam, which results in x-ray pulses lasting just a few hundred attoseconds.
Equipped with the necessary attosecond x-ray pulses, Driver and colleagues measured the photoionization timing of core electrons in gaseous nitric oxide with the angular streaking method. For the measurement, an ionizing attosecond x-ray pulse is overlapped with a circularly polarized IR laser field. The rotating electric field of the IR laser deflects the photoionized electrons radially, and the time at which the electron is released from the molecule can be calculated from the deflection angle. Compared with the emission of low-energy electrons in the molecule, the core-level electrons from the oxygen atom were emitted as much as 700 attoseconds later, which is sluggish relative to theoretical predictions.
The researchers’ numerical simulations show that some of the delay may be caused by a core electron’s unique interactions with other electrons. Unlike a valence electron, a core electron usually is quickly replaced once it’s emitted from its shell. That process, known as Auger–Meitner decay, causes the molecule to emit a high-energy secondary electron. Its interaction with the outgoing core electron may, according to the simulations, be partially responsible for the duration of the delay. (T. Driver et al., Nature 632, 762, 2024.)
