Listening to the Ultrafast Chat of Excited Electrons and Asking Them Some Quick Physics Questions

Electrons interact via the Coulomb force, repel each other and feel attracted by a nucleus that normally keeps them trapped inside an atom. When both electrons of a Helium atom are in excited states, they keep communicating repulsively until one of them drops back to the ground state, giving its energy to the other one to escape the atomic binding potential (autoionization). Here, I will show how this very fast communication and the corresponding fundamental dynamical processes are recorded and translated into a time-domain physics language. The key methods in our experimental research are the combination of ultrafast laser/light fields (including High-Harmonic Generation (HHG) and Free-Electron Lasers (FELs)) and multi-dimensional measurement techniques accessing time scales of 1 femtosecond (10-15 s) and shorter. Moreover, we ask some correspondingly short questions encoded into visible frequencies of time- and intensity-tunable laser pulses. Listening to the electrons' optical response by extreme-ultraviolet (XUV) spectroscopy we learned to interpret a fundamental quantum interference process—the Fano resonance—in the time domain. This understanding currently fuels several emerging science and technology applications ranging from laser-like hard-x-ray gain (without inversion) to frequency combs locked to nuclear resonances for precision spectroscopy.