Researchers from the Max Born Institute, University College London, and ELI-ALPS in Szeged, Hungary, demonstrated attosecond-pump attosecond-probe spectroscopy. Demonstration of the technique allowed the collaborators to study nonlinear multiphoton ionization of atoms. Femtosecond pump-probe spectroscopy has been instrumental to understanding extremely fast processes. For example, the dissociation of a molecule can be initiated by a femtosecond laser pump pulse and then be observed in real time using a time-delayed femtosecond probe pulse. The probe pulse interrogates the evolving state of the molecule at various time delays, making it possible to record a movie of the molecular dissociation. Ar2+ and Ar3+ ion yields as a function of the time delay between two attosecond pulse trains. The Ar2+ ion yield (red curve) is only weakly modulated as a function of the XUV-XUV time delay, whereas clear oscillations with a period of 1.3 fs are observed in the delay-dependent Ar3+ ion yield (blue curve). These results indicate that Ar2+ is generated via the sequential absorption of two photons. Subsequently, two additional photons are simultaneously absorbed to form Ar3+. Courtesy of Max Born Institute. Some processes are even faster — taking place on attosecond timescales. To date, attosecond-pump attosecond-probe spectroscopy has been demonstrated for relatively simple processes involving the absorption of two photons. However, since all-attosecond pump-probe spectroscopy is quite challenging to achieve, most experiments in attosecond science use only one attosecond (pump or probe) pulse in combination with one femtosecond pulse. In the team's recent results, it performed a pump-probe experiment in which complex multiphoton ionization processes were studied using two attosecond pulse trains. The experiment required the generation of intense attosecond pulses, for which a large laser system was used. Consequently, the team performed the experiment in the largest laboratory available at the Max Born Institute. At the same time, the two attosecond pulses had to be overlapped with attosecond temporal and nanometer spatial stability, demonstrating the challenging nature of such experiments. The experiment saw attosecond pulse trains interacting with an argon atom. Following absorption of four photons from the attosecond pulses, three electrons were removed from the atom. The researchers varied the time delay between the two attosecond pulses and observed how many ions were generated to find out in detail how the electrons were removed from the atom. The yield of the doubly charged argon ions was almost independent of the time delay. In contrast, the yield of the triply charged argon ions showed pronounced oscillations when varying the time delay between the two attosecond pulses. The researchers concluded that the multiphoton absorption occurred in three steps. In each of the first two steps, a single photon was absorbed, whereas in the third step, two photons were absorbed at the same time. These results were confirmed by computer simulations that were carried out at University College London and at ELI-ALPS. The experimental technique could be used to study complex processes not only in atoms but also in molecules, solids, and nanostructures, the researchers said. Further, they hope to be able to determine how several electrons interact with one another. This determination could help the researchers understand some of the most fundamental processes on the shortest timescales. The research was published in Optica (www.doi.org/10.1364/OPTICA.456596).