Mapping the interatomic potential of photoexcited bismuth: Ultrafast optical and x -ray studies.

Abstract

Material properties can change dramatically under intense femtosecond photoexcitation. These properties are directly determined by the transient shape of the excited state interatomic potential. Details of the potential energy surface are required for predictive models of the non-equilibrium behavior. Until now, no experiment has succeeded in measuring the potential of a solid with the atomic scale spatial and femtosecond temporal resolution required to understand the underlying dynamics. The first detailed mapping of the carrier density-dependent interatomic potential of a highly excited material approaching a solid-solid phase transition is presented in this dissertation. Optical and X-ray scattering techniques were used to characterize the <italic> A</italic>₁<italic>_g</italic> optical phonon mode of crystalline bismuth that is excited in response to an impulsive excitation of charge carriers. The properties of this vibrational mode, in particular the oscillation frequency and position the atoms are oscillating about (quasi-equilibrium), provides insight into the shape of the interatomic potential energy surface. Optical experiments were preformed using two pump pulses to coherently control the optical phonon amplitude at a fixed carrier density. This method allowed separation of the effects of carrier dynamics from lattice anharmonicity. Our results show that the time dependent frequency of the phonon is dominated by electronic softening of the interatomic potential. The X-ray experiments are the first to combine stroboscopic techniques using a high flux linear accelerator based X-ray source with pulse-by-pulse timing reconstruction for femtosecond resolution. These advances enabled us to follow the curvature and minima position of the double well interatomic potential of bismuth over several picoseconds, for a variety of carrier densities. These two key parameters determine the interplay between electronic softening and the Peierls distorted structure. Our measurements validate previous density functional theory calculations of the free energy surface and gives credence to this approach for predicting the excited state properties.