Ultrafast soft X-ray absorption spectroscopy with sub-20-fs resolution

The most challenging application of time-resolved spectroscopy is to watch the motion of atoms directly. In the current experiment, we were able to follow the displacement of atoms in silicon after excitation with an intense near infrared femtosecond laser pulse with time-resolved x-ray absorption spectroscopy. The sub-20-fs x-ray probe pulses in an energy range up to several 100 eV were generated by high harmonic generation of intense femtosecond laser pulses at a repetition rate of 1 kHz. In a proof of principle experiment, we found an oscillatory motion of the atoms which can be explained by the excitation of coherent phonons. Our setup can be easily extended for studying structural dynamics in many important systems not only in solids but also in gases and liquids.