Spectroscopy of TaS2 through the metal-insulator transition

Soft X-ray spectroscopies allow the analysis of the electronic structure of matter. They are specific to different elements, to their chemical surroundings and to individual orbitals, resolving their polarization. In combination with the newly available pulsed soft X-ray sources, they constitute an ideal tool to locally probe ultra-fast electron dynamics of phase transitions. The layered material 1T-TaS2 for example is at room temperature in a Mott insulating phase and exhibits a transition to a metallic state at liquid nitrogen temperatures, connected with the evolution of a charge density wave and a slight shift in atomic positions. This is well established around the Ta-atoms but should show effects on the electronic structure at S-centers as well. At the synchrotron sources MAX-lab and BESSY, we have studied the occupied and unoccupied electronic states. We observe large differences at both atomic centers depending on sample temperature and on polarization. In a next step, we will drive the phase transition through excitation with a short-pulsed optical laser and study the electron dynamics on a femtosecond timescale at the free electron laser in Hamburg (FLASH)