Extreme timescale core-level spectroscopy with tailored XUV pulses

A new approach for few-femtosecond time-resolved photoelectron spectroscopy in condensed matter that balances the combined needs for both temporal and energy resolution is demonstrated. Here, the method is designed to investigate a prototypical Mott insulator, tantalum disulphide (1T-TaS2), which transforms from its charge-density-wave ordered Mott insulating state to a conducting state in a matter of femtoseconds. The signature to be observed through the phase transition is a charge-density-wave induced splitting of the Ta 4f core-levels, which can be resolved with sub-eV spectral resolution. Combining this spectral resolution with few-femtosecond time resolution enables the collapse of the charge ordered Mott state to be clocked. Precise knowledge of the sub-20-femtosecond dynamics will provide new insight into the physical mechanism behind the collapse and may reveal Mott physics on the timescale of electronic hopping.