To observe the ultrafast electron motion in the inner shells of atoms with short light pulses, the pulses must be ultrashort, very bright, and the photons that are delivered must have sufficiently high energy. This combination of properties has been sought in laboratories around the world for the past 15 years. Physicists at the Laboratory for Attosecond Physics (LAP), a joint venture between Ludwig-Maximilians-Universität (LMU) and the Max Planck Institute of Quantum Optics (MPQ), have met the conditions necessary to achieve this goal. The team observed the interaction of multiple photons in a single attosecond pulse, with electrons in one of the inner orbital shells around the atomic nucleus. The advance was made possible through the development of a novel source of attosecond pulses. After the interaction of a xenon atom with two photons from an attosecond pulse (purple), the atom is ionized and multiple electrons (green balls) are ejected. This two-photon interaction is made possible by the latest achievements in attosecond technology. Courtesy of Christian Hackenberger. The team used an attosecond extreme ultraviolet (XUV) source to investigate the absorption of two 100-eV photons within the giant resonance of xenon. The researchers observed distinct deviations with respect to free-electron laser results, which originated from the shorter XUV pulse duration provided by their system. Researchers concluded that these differences were indicative of dynamics on a few-femtosecond, or even shorter, timescale. The team’s experiment demonstrates the capability of few-cycle-driven high-harmonic sources to combine extreme temporal confinement with peak powers required for observing nonlinear interactions. Such energy scaling could stimulate efforts toward the development of next-generation high-intensity attosecond sources, and could pave the way for the real-time exploration into the unresolved area of inner-shell electron dynamics. The research was published in Optica, a publication of OSA, The Optical Society (doi:10.1364/OPTICA.5.000237).