Tuning and Tracking of Coherent Shear Waves in Molecular Films.

We have determined the time-dependent displacement fields in molecular sub-micrometer thin films as response to femtosecond and picosecond laser pulse heating by time-resolved X-ray diffraction. This method allows a direct absolute determination of the molecular displacements induced by electron-phonon interactions, which are crucial for, for example, charge transport in organic electronic devices. We demonstrate that two different modes of coherent shear motion can be photoexcited in a thin film of organic molecules by careful tuning of the laser penetration depth relative to the thickness of the film.

Ultrafast Formation of a Charge Density Wave State in 1T-TaS_{2}: Observation at Nanometer Scales Using Time-Resolved X-Ray Diffraction.

Femtosecond time-resolved x-ray diffraction is used to study a photoinduced phase transition between two charge density wave (CDW) states in 1T-TaS_{2}, namely the nearly commensurate (NC) and the incommensurate (I) CDW states. Structural modulations associated with the NC-CDW order are found to disappear within 400 fs. The photoinduced I-CDW phase then develops through a nucleation and growth process which ends 100 ps after laser excitation.

Ultrafast structural dynamics of the orthorhombic distortion in the Fe-pnictide parent compound BaFe2As2.

Using femtosecond time-resolved hard x-ray diffraction, we investigate the structural dynamics of the orthorhombic distortion in the Fe-pnictide parent compound BaFe2As2. The orthorhombic distortion analyzed by the transient splitting of the (1 0 3) Bragg reflection is suppressed on an initial timescale of 35 ps, which is much slower than the suppression of magnetic and nematic order. This observation demonstrates a transient state with persistent structural distortion and suppressed magnetic/nematic order which are strongly linked in thermal equilibrium.

Femtosecond X-ray absorption study of electron localization in photoexcited anatase TiO2.

Transition metal oxides are among the most promising solar materials, whose properties rely on the generation, transport and trapping of charge carriers (electrons and holes). Identifying the latter's dynamics at room temperature requires tools that combine elemental and structural sensitivity, with the atomic scale resolution of time (femtoseconds, fs). Here, we use fs Ti K-edge X-ray absorption spectroscopy (XAS) upon 3.