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Attosecond quantum beats of inner-shell excited atoms

Article by Marcel Mudrich, IFA et al. in Nature Communications 14 February 2020

An international collaboration with the participation of Marcel Mudrich from IFA has succeeded in observing ultrafast quantum interferences induced by the coherent excitation of an electron out of an inner shell of a rare gas atom. The measured quantum beat reflects the time evolution of a coherent superposition of two electronic states spaced by no less than 28 eV of energy; they have a period of just 150 attoseconds (one attosecond is a billionth of a billionth of a second). This was achieved by exciting the atoms with specially shaped laser pulses up-converted into the extreme ultraviolet spectral range by the first and only seeded free-electron laser FERMI in Trieste, Italy. To track the atom’s response, a new measurement technique was used, which allows researchers to follow the quantum dynamics of atoms and molecules with extremely high time resolution. These results are presented in the latest edition of Nature Communications.

Numerous chemical reactions, such as the breaking of bonds in molecules, are triggered by the absorption of light. In the first instant, it’s the distribution of the electrons in the atomic shell that changes, thereby inducing the reaction. Conventional spectroscopic techniques which use visible laser pulses are not fast enough to track the motion of the electrons. Therefore, researchers around the world are currently developing new laser sources and spectroscopic techniques using pulses of extreme ultraviolet or even X-ray radiation. The group headed by Prof. Stienkemeier in Freiburg, Germany, has extended a technique known from the visible spectral range -- coherent pump-probe spectroscopy -- to the extreme ultraviolet spectral range. In essence, a sequence of two ultrashort extreme ultraviolet laser pulses was created where the time interval between the pulses as well as their phase relation are precisely controlled. The first pulse excited the electron in the atom (pump step). The second pulse then probed the state of the electron shell after a well-defined delay (probe step). By precisely varying the delay and phases of the pulses, the researchers were able to trace the coherent evolution of the electron’s quantum state, which turned out to be limited by the spontaneous ionization of the atom in the super-excited state. “The greatest challenge was to achieve precise control over the pulse properties and to isolate the weak signals”, explains Andreas Wituschek, who was the leading scientist of this experiment and is the first author of the publication. “This study paves the way for probing coherent quantum dynamics of highly excited atoms and molecules using radiation in the extreme ultraviolet and x-ray ranges”.

Original publication:

Wituschek A., Bruder L., Allaria E., Bangert U., Binz M., Borghes R., Callegari C., Cerullo G., Cinquegrana P., Giannessi L., Danailov M., Demidovich A., Di Fraia M., Drabbels M., Feifel R., Laarmann T., Michiels R., Mirian N.S., Mudrich M., Nikolov I., O’Shea FH., Penco G., Piseri P., Plekan O., Prince K.C., Przystawik A., Ribi? P.R., Sansone G., Sigalotti P., Spampinati S., Spezzani C., Squibb R.J., Stranges S., Uhl D. & Stienkemeier F. (2020): Tracking attosecond electronic coherences using phase-manipulated extreme ultraviolet pulses. In: Nature Communications, Volume 11, Article number 883 (2020). DOI: 10.1038/s41467-020-14721-2

Figure caption:

Laser pulses generate and track electronic quantum interferences in super-excited atoms.