Ultrafast melting of a charge-ordered state in 1T-TaS2 investigated with photoelectron spectroscopy at FLASH

The Free-Electron-Laser in Hamburg (FLASH) generates highly brilliant, ultrashort, and coherent pulses in the XUV regime enabling many fascinating experiments that are not possible at any other source. After various challenges concerning space-charge effects as well as timing and synchronization issues have recently been solved optical pump-XUV probe photoelectron spectroscopy on solid surfaces is now possible over a wide probing photon energy range (∝24-570 eV) with time and energy resolutions of ∝700 fs and ∝300 meV, respectively. Our most recent experiments, performed on the correlated layer compound 1T-TaS2 deep in the charge-density-wave (CDW) state, demonstrate that core-level dynamics on the femto-, pico- and nanosecond time scale can be investigated at FLASH. We find that long-range charge order in 1T-TaS2 collapses promptly and that a domain-like CDW state is reached within about 1 ps. The results imply that the CDW and the accompanying periodic lattice distortion, which are strongly coupled in equilibrium, are decoupled after photoexcitation on the time scale for electron-phonon thermalization.