Ultrafast Electron Diffraction in Charge Density Wave State of TiSe₂

We probe the ultrafast laser response in thin TiSe₂ single crystals below and above the CDW transition temperature using ultrafast electron diffraction. The electron-phonon dynamics is initiated by a 150-fs photon pulse centered at 400 nm that induces a strain pulse and inter-layer
coherent acoustic breathing mode phonons. While we observe a fast (~1.5 ps) initial diffraction intensity decay at room temperature indicative of strong electron-phonon coupling, we do not observe any bi-exponential dynamics. In the CDW phase there exists a threshold fluence that causes change in the relaxation dynamics. We associate this threshold with electronic melting of CDW order. The ultrafast electron diffraction dynamics results on Cu_xTiSe₂ single crystals will be presented as well.