Attoseconds Break Into Atomic Interior

In order to observe the ultra­fast electron motion in the inner shells of atoms with short light pulses, the pulses must not only be ultra­short, but very bright, and the photons delivered must have suffi­ciently high energy. This combination of proper­ties has been sought in labora­tories around the world for the past 15 years. Physicists at the Labora­tory for Atto­second Physics LAP, a joint venture between the Ludwig-Maximilians-Univer­sity Munich LMU and the Max Planck Insti­tute of Quantum Optics MPQ, have now succeeded in meeting the condi­tions necessary to achieve this goal.

After the interaction of a xenon atom with two photons from an attosecond pulse, the atom is ionized and multiple electrons are ejected. (Source: C. Hackenberger)

In their latest experi­ments, they have been able to observe the non-linear inter­action of an atto­second pulse with electrons in one of the inner orbital shells around the atomic nucleus. The inter­action involves more than one photon – in this parti­cular case two are involved. This break­through was made possible by the develop­ment of a novel source of atto­second pulses. The experi­mental procedure used to film electrons in motion makes use of the pump-probe approach. Electrons within a target atom are first excited by a photon contained within the pump pulse, which is then followed after a short delay by a second photon in a probe pulse. The latter essen­tially reveals the effect of the pump photon. In order to implement this proce­dure, the photons must be so tightly packed that a single atom within the target can be hit by two photons in succes­sion. Moreover, if these photons are to have a chance of reaching the inner electron shells, they must have energies in the upper end of the extreme ultra­violet (XUV) spectrum. No research group has previously succeeded in gene­rating atto­second pulses with the required photon density in this spectral region.

The techno­logy that has now made this feat possible is based on the upscaling of conven­tional sources of atto­second pulses. A team led by Laszlo Veisz has developed a novel high-power laser capable of emitting bursts of infrared light – each consisting of only a few oscil­lation cycles – which contain 100 times as many photons per pulse as in conven­tional systems. These pulses, in turn, allow the gene­ration of isolated atto­second pulses of XUV light containing 100 times more photons as in conven­tional atto­second sources.

In a first series of experi­ments, the high-energy atto­second pulses were focused on a stream of xenon gas. Photons that happen to interact with an inner shell of a xenon atom eject electrons from that shell and ionize the atom. By using an ion micro­scope to detect these ions, the scien­tists were able to observe the inter­action of two photons confined in an attosecond pulse with electrons in the inner orbital shells of an atom. In previous atto­second experi­ments, it has only been possible to observe the inter­action of inner shell electrons with a single XUV photon.

“Experi­ments in which it is possible to have inner shell electrons inter­acting with two XUV attosecond pulses are often referred to as the Holy Grail of atto­second physics. With two XUV pulses, we would be able to film the electron motion in the inner atomic shells without per­turbing their dynamics,” says Boris Bergues, the leader of the new study. This represents a significant advance on atto­second experi­ments involving exci­tation with a single atto­second XUV photon. In those experi­ments, the resulting state was photo­graphed with a longer infrared pulse, which itself had a signi­ficant influence on the ensuing electron motion. “The electron dynamics in the inner shells of atoms are of parti­cular interest, because they result from a complex inter­play between many electrons that interact with each other,” as Bergues explains. “The detailed dynamics resulting from these inter­actions raise many questions, which we can now address experi­mentally using our new atto­second source.”

In the next step, the physicists plan an experi­ment in which they will time resolve the inter­action by splitting the high-inten­sity atto­second pulse into separate pump and probe pulses. The success­ful appli­cation of non-linear optics in the atto­second domain to probe the behaviour of electrons in the inner orbital shells of atoms opens the door to a new under­standing of the complex multi­body dynamics of subatomic particles. The ability to film the motion of electrons deep in the interior of atoms promises to reveal much about a mys­terious realm that has remained hidden from our gaze. (Source: MPQ)

Reference: B. Bergues et al.: Tabletop nonlinear optics in the 100-eV spectral region, Optica 5, 237 (2018); DOI: 10.1364/OPTICA.5.000237

Link: Laboratory for Attosecond Physics, Max Planck Institute of Quantum Optics, Garching, Germany

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