Nonequilibrium lattice dynamics measurements with ultrafast x-ray pulses

There is growing interest in the use of ultrafast light pulses to drive nonequilibrium states of materials with novel properties. Our understanding of how to generate and control these states requires time-resolved atomic-scale probes of structure and dynamics. In this talk, we report results of femtosecond x-ray free-electron laser-based scattering experiments which help elucidate the excited-state dynamics of photo-excited materials. In the case of SnSe, we find that above-gap photoexcitation drives the material towards a higher-symmetry structure that does not exist thermal equilibrium. First principle calculations help us identify how photoexcitation from localized valence bands drives changes in the orbital hybridization leading to this instability. From the measured excited-state phonon dispersion we further identify changes in the interlayer bonding that are responsible for driving this novel instability, consistent with the changes in the orbital character of the bands. Our work suggest the importance of pump-wavelength for control of structural distortions through orbitally-selective above-gap excitation.