The Photo­electric Effect in Stereo

Depending on whether the electron is close to the oxygen or to the carbon atom, the laser pulse will eject it more or less quickly. That difference can now be precisely measured. (Source: ETHZ)

When a photon hits a material, it can eject an electron from it provided it has enough energy. Albert Einstein found the theo­retical expla­nation of this photo­electric effect in Bern during his “year of wonders” 1905. That explanation was a crucial contribution to the develop­ment of quantum mechanics, which was under way at the time, and it earned him the Nobel Prize in Physics in 1921. An inter­national team of physicists led by Ursula Keller at the Institute for Quantum Elec­tronics of the ETH Zurich has now added a new dimension to the experi­mental inves­tigation of this important effect. Using atto­second laser pulses they were able to measure a tiny time difference in the ejection of the electron from a molecule depending on the position of the electron inside the molecule.

“For quite some time, people have studied the time evolution of the photo­electric effect in atoms”, says PhD student Jannie Vos, “but very little has so far been published on molecules.” That is mainly due to the fact that molecules are consi­derably more complex than single atoms. In an atom, the outermost electron moving around the atomic nucleus is essen­tially cata­pulted out of its orbit. In a molecule, by contrast, two or more nuclei share the same electron. Where it is located depends on the interplay between the different attrac­tive potentials. Exactly how the photo­electric effect happens under such conditions could only now be studied in detail.

To do so, Keller and her co-workers used carbon monoxide molecules, which consist of one carbon and one oxygen atom. Those molecules were exposed to an extreme ultra­violet laser pulse that only lasted for a few atto­seconds. The energy of the ultra­violet photons ripped an electron out of the molecules, which subse­quently broke up into their consti­tuent atoms. One of those atoms turned into a posi­tively charged ion in the process. Using a special instrument, the researchers then measured the directions in which the electrons and ions flew away. A second laser pulse, which acted as a kind of measuring stick, also allowed them to deter­mine the precise instant at which the electron left the molecule.

“In this way we were able, for the first time, to measure the Stereo Wigner time delay,” explains Laura Cattaneo, who works as a post­doctoral researcher in Keller’s group. The stereo Wigner time delay measures how much earlier or later an electron leaves the molecule if it is located close to the oxygen atom or to the carbon atom when photo­ionization occurs. The extremely short laser pulses make it possible to measure that instant to within a few atto­seconds. From that infor­mation, in turn, it is possible to deter­mine the location of the ionization event inside the molecule to within a tenth of a nano­meter. The experi­mental results agree well with theo­retical predic­tions that describe the most likely position of an electron at the time of photo­ionization.

Next, the ETH researchers want to take a closer look at larger molecules, starting with the laughing gas. The extra atom in that molecule already makes the theo­retical description quite a bit more difficult, but at the same time the physicists hope to obtain new insights, for example into the charge migra­tion inside molecules, which plays an important role in chemical process. In principle it should even be possible to use atto­second laser pulses not just to study those processes, but also to deli­berately steer them and thus to control chemical reactions in detail. Right now, however, such atto-chemistry is still a long way off, as Jannie Vos points out: “In theory that’s all very exciting, but a lot remains to be done before we get there.” (Source: ETHZ)

Reference: J. Vos et al.: Orientation-dependent stereo Wigner time delay and electron localization in a small molecule, Science 360, 1326 (2018); DOI: 10.1126/science.aao4731

Link: Institute for Quantum Electronics, Dept. of Physics, ETH Zurich, Zurich, Switzerland • Max Planck Institute for the Physics of Complex Systems, Dresden, Germany • Max Born Institute, Berlin, Germany

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