Abstract

Ionization is the dominant response of atoms and molecules to intense laser fields and is at the basis of several important techniques, such as the generation of attosecond pulses that allow to measure electron motion in real time. We present experiments where metastable Xenon atoms are ionized by intense 7-micrometer laser pulses from a free electron laser. Holographic structures are observed that record underlying electron dynamics on a sub-laser cycle time scale, enabling photoelectron spectroscopy with a time resolution almost two orders of magnitude higher than the duration of the ionizing pulse. After a strong laser field ionizes an atom or molecule, the liberated electron is accelerated by the oscillatory laser electric field and driven back towards the ion (1). Electron-ion re-collision leads to the emission of XUV radiation with