Fast, Faster, Zeptosecond

Probable position of the remaining electron after photoemission of an electron from a helium atom. (Source: M. Ossiander / TUM, M. Schultze / MPQ)

Probable position of the remaining electron after photoemission of an electron from a helium atom. (Source: M. Ossiander / TUM, M. Schultze / MPQ)

If light hits the two electrons of a helium atom, one must be very fast to observe what occurs. Besides the ultra-short periods in which changes take place, quantum mechanics also comes into play. Laser physicists at the Max Planck Institute of Quantum Optics, the Technical Uni­versity of Munich and the Ludwig Maxi­milians University Munich have now measured such an event for the first time with zepto­second precision.

Either the entire energy of a photon can be absorbed by one of the electrons or a division takes place, if a photon hits the two electrons of a helium atom. Regardless of the energy transfer, one electron leaves the atom. It takes between five and fifteen atto­seconds from the time a photon interacts with the electrons to the time one of the electrons leaves the atom, as physicists already discovered in recent years. With their improved measure­ment method, laser physicists can accu­rately measure events at a rate of up to 850 zepto­seconds. The researchers shone an atto­second-long, extremely ultraviolet (XUV) light pulse onto a helium atom to excite the electrons.

At the same time, they fired a second infrared laser pulse, lasting about four femto­seconds. The electron was detected by the infrared laser pulse as soon as it left the atom following excitation by XUV light. Depending on the exact electro­magnetic field of this pulse at the time of detection, the electron was accelerated or decelerated. Through this change in speed, the physicists were able to measure photo­emission with zepto­second precision.

The researchers were also able to determine for the first time how the energy of the incident photon is quantum-mechani­cally divided between the two electrons of the helium atom in a few atto­seconds before the emission of one of the particles. “With the measure­ment of the electronic corre­lation, our experiments solved a promise of atto­second physics, namely the temporal reso­lution of a process which is inacces­sible with other methods,” says Reinhard Kienberger, professor of the Chair of Laser- and X-Ray Physics at TUM. The physicists were also able to correlate the zepto­second precision of their experiments with the theo­retical predictions of their peers from the Institute of Theo­retical Physics at the Technical University of Vienna.

Helium is the only multi electron system that can be calculated completely quantum mechani­cally. This makes it possible to reconcile theory and experiment. “We can now derive the complete wave mechanic description of the inter­connected systems of electron and ionized helium mother atoms from our measure­ments,” says Martin Schultze, project leader at the Max Planck Institute of Quantum Optics in Garching. With their metrology experiments in zepto­second time dimensions, the laser physicists have maneuvered another important puzzle piece in the quantum mechanics of the helium atom into position, and thus advanced measuring accuracy in the microcosm to a whole new dimension. (Source: TUM)

Reference: M. Ossiander et al.: Attosecond correlation dynamics, Nat. Phys., online 07 November 2016; DOI: 10.1038/nphys3941

Links: Max-Planck-Institute for Quantum Optics, Garching, Germany • Physics Dept., Technical University Munich, Garching, Germany

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