Speaker
Description
To resolve ultrafast events, we typically rely on controlled probe pulses that are shorter than the delay between these events. Likewise, to resolve closely-spaced spectral lines, we need to achieve a spectrometer resolution better than their spacing. These statements appear correct, at first. Here, we shed light on this common wisdom, statistically structured light to be more precise. Experiments at the extreme-ultraviolet (XUV) FEL FLASH at DESY taught us that correlations in statistically structured SASE FEL pulses can increase temporal resolution. Molecular wavepacket dynamics on time scales shorter than the FEL pulse duration are resolved in the D$_2$ molecular ion, observed by ion coincidence spectroscopy with a reaction microscope (ReMi) in an XUV-pump–XUV-probe experiment. Regarding enhanced spectral resolution, we learn from a recent "single-pulse" transmission spectroscopy experiment at EuXFEL. Here, correlations within the physical interaction process itself (X-ray stimulated Raman) can be harnessed (in a manner akin to super-resolution microscopy) to resolve spectral structures (100-meV fine-structure splitting in Ne), beyond the (0.2 eV) x-ray spectrometer resolution. Finally, we turn to our recent attosecond-pump–attosecond-probe experiment on electron dynamics in molecules. Making use of a recently established attosecond double-pulse operating regime at EuXFEL we measure spectral interference structures, encoding the time delay of the stochastically varying SASE pulses. Using post sorting based on shot-to-shot spectral diagnostics, we uncover transient changes of characteristic x-ray absorption below the oxygen K edge, encoding the motion of electrons in the vicinity of the two chemically distinct oxygen atoms in the chemically relevant COOH carboxy group found in organic acids.
