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 150 fs pulses centered at 400 nm. While we observe a comparably fast initial diffraction intensity decay at room temperature, we do not observe any bi-exponential dynamics that was previously reported in the dynamics of <110> peaks at fluences above 1 mJ/cm² with 800 nm centered excitation. On the other hand, at temperatures below the CDW transition temperature bi-exponential dynamics is observed above the threshold fluence of 0.5 mJ/cm². This threshold fluence is more than two times larger than the threshold fluence for CDW melting with an 800 nm pump. Bi-exponential behaviour is linked to a strong electron-phonon coupling process followed by a slower electron-lattice equilibration. This shows the importance of the Jahn-Teller mechanism for charge density wave formation in TiSe₂. We will also present ultrafast electron diffraction results on Cu_xTiSe₂ single crystals.