We developed a high-repetition rate extreme ultraviolet (XUV) laser delivering 500,000 pulses of photons with 22 eV energy and a duration of approximately 20 femtoseconds. These light pulses are used to photoemit electrons from a crystal in order to obtain a map of the material’s electronic structure. As we use a second, visible laser pulse to excite a small fraction of the material’s electrons into excited states shortly before the XUV pulses arrive, we are now able to do excited state mapping and to take movies of how electrons scatter in the band structure.

This achievement required years of development of new laser sources (see Puppin et al., Opt. Exp. 23 1491, 2015), high-harmonic generation, and a sophisticated surface science apparatus. This novel technique is developed in close collaboration with the Dynamics of Correlated Materials group.