An inner-shell vacancy in an atom can decay very efficiently via the emission of an Auger electron. Thus far, Auger decay has been observed in atoms, molecules and clusters following the ionization or excitation by light with high photon energies in the extreme-ultraviolet and X-ray regime. Surprisingly, we have discovered Auger decay after the interaction of methane clusters with intense near-infrared laser pulses, even though the photon energy is not sufficient to directly generate an inner-shell vacancy. However, due to very efficient ionization avalanching in clusters, electrons from outer as well as inner shells are removed from their atoms during the laser pulse, and a nanoscale plasma is formed. Subsequent recombination of electrons to outer shells of ions and to high-lying Rydberg orbitals then results in a population inversion of the cluster atoms. By observing a clear peak in the electron spectrum, evidence was provided for the first time that Auger decay is one of the relaxation channels of the highly excited system. In the future, the observed population inversion could be exploited for the development of a table-top X-ray laser.
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Unlike natural light sources such as the sun, lasers have special coherence properties, resulting e.g. in the intriguing observation of interference, where the overlap of two lasers can lead to darkness. The coherence of lasers can also be used to push electrons in a quantum swing inside an atom, meaning that electrons oscillate between two quantum states that lie on different energy levels. Such quantum swings are known as Rabi oscillations and have been observed with low-frequency lasers up to the ultraviolet spectral range. In this paper, we show that Rabi oscillations can be driven in an argon atom by an intense laser pulse in the extreme-ultraviolet spectral range, which is invisible for the human eye. By using a second laser pulse, the ultrafast oscillation of electrons between the ground state (the lowest energy level) and an excited state (at a higher energy level) is traced directly in the time domain, showing that it takes place within tens of femtoseconds (corresponding to 10^-14 seconds). Our results pave the way towards multi-photon coherent control techniques in the extreme-ultraviolet range, allowing extension to situations where interactions between electrons play a role, or more complex systems including molecules.
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