Ultrafast Electron Crystallography. 3. Theoretical Modeling of Structural Dynamics
A systematic theoretical study of structural dynamics for a substrate and an adsorbate is presented and compared with experimental results of ultrafast electron crystallography. Using the harmonic lattice model of a chain of atoms, including anharmonic forces, we investigate influences of the impulsive force on the temporal behavior of Bragg diffraction; the roles of damping and anharmonicity are also discussed. Extensive simulations are made in order to gain an understanding of the controlling factors of the atomic motions in the substrate, with or without the adsorbate. The results elucidate the importance of coherent wave propagation in the nonequilibrium regime and provide the atomic-scale description of surface and bulk dynamics. The influence of the attenuation length of the impulsive force and the thickness of the substrate (adsorbate) are also considered. For adsorbates, the mismatch of force constants and the coupling with the substrate are examined, thus establishing the conditions for wave-type propagation.