Impulsive Light-driven CDW wavevector modulation in 1T-TiSe₂

1-T titanium diseleinde (1T-TiSe₂) has been the subject of much study due to its prototypical second-order commensurate charge density wave (CDW) transition. The CDW phase competes with superconductivity, which is also typical of a large family of transition metal dichalcogenides. Below the CDW transition temperature, a soft TA phonon at the L point (q = [ ½ ½ ½]) condenses into long-range order. This phase transition was recently shown to have strong excitonic character with characteristics of an excitonic insulator, leaving open questions about the role of electron-phonon coupling in the CDW transition. To investigate this electron-phonon coupling, we performed optical pump, hard x-ray probe experiments on TiSe₂ at LCLS to perturb the electronic states and observe the impulsive structural response of the material far from equilibrium. We observe coherent oscillations in the scattered x-ray intensity as a function of time near the CDW wavevector of 1T-TiSe₂ upon optical excitation, which we interpret as a time-dependent modulation of the CDW wavevector based on the wavevector-dependent amplitude and phase of the coherences.