Mapping the dispersion of the occupied and unoccupied band structure in photoexcited 1T-TiSe₂

Charge density waves (CDW) are states of broken symmetry with a periodic modulation of charge and lattice typically leading to the opening of a gap in the band structure. In the model CDW system 1T-TiSe₂ such a gap opens up between its Se_4p valence and Ti_3d conduction band, accompanied by a change of dispersion. These changes are crucial in understanding the CDW phase, as they provide a measure of the Se_4p-Ti_3d hybridization strength and characteristic mechanistic features. Using time- and angle-resolved photoelectron spectroscopy (trARPES), the unoccupied band structure is populated with near-infrared (NIR) pump pulses which allows to directly visualize the parabolically-shaped Ti_3d conduction band. Furthermore, we observe a transient change of effective mass in the Se_4p valence band following photoexcitation. This occurs alongside an overall reduction due to weakening of the CDW phase and is accompanied by an oscillating component with the frequency of the characteristic A_1g phonon. These observations, enabled by trAPRES, highlight the importance of the lattice contributions in establishing the CDW order in 1T-TiSe₂.