Molecular dynamics calculations of the photodissociation of HCl and C1 2 in a Xe crystal have been performed using diatomics-in-molecules (DIM) Hamiltonians to describe the electronic interaction. Nonadiabatic transitions between the adiabatic states are explicitly taken into account in the equations of motion. It is shown that the transitions occur on a subpicosecond-time scale. They induce fast recombination of the fragments, thus leading to a reduction of the quantum yield of permanent dissociation. In addition to the octahedral and tetrahedral interstitial sites the DIM model predicts two further energy minima for Cl in a Xe crystal. One of them, a D2 site between two neighboring xenon atoms, is populated after dissociation of Cl 2 . In this case, our calculations led to recombination of the molecule when the system is annealed at temperatures below the onset of thermal diffusion of Cl atoms.

Nonadiabatic dynamics accompanying the photodissociation of HCI via excitation X II + -+A I II have been investigated. The energy-dependent branching ratio' 2Pll2/2P3/2 of the CI fragment resulting from the "exact" numerical solution of the time-dependent nuclear Schrodinger equation is compared to the branching ratio gained from a classical-path ansatz. Different sets. of potential energy functions taken from previous investigations are used. It is shown that discrepancies between the results of two recent papers are due to substantial differences concerning the nonadiabatic transition moments between the corresponding potential functions.

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Fluorine mobility at different structural positions in monocrystalline with the tysonite structure is analysed using NMR line-shape analysis. The method is sensitive to ionic exchange with correlation times in the range . For the temperature range between 240 K and 400 K the motion is restricted mainly to the ions in the fluorine layers perpendicular to the main symmetry axis (the sublattice), while ions in the La plane remain immobile. No significant anisotropy of the -ionic diffusion within the layers and along the c-axis is found ( at 400 K). From NMR spectra it is clear that mobility is strongly heterogeneous. The motional disorder can be described well by a broad distribution of correlation times, , which has a shape close to a log - Gaussian function and reflects the potential energy landscape in the superionic state. The variation of the centre position and width of with temperature differs from an Arrhenius law behaviour. Ionic mobility on the microscopical scale, therefore, cannot be considered a process which is activated only thermally. Applying MD techniques shows that the presence of vacancies may lead to pronounced changes of the potential energies, and supports the idea that there is a distribution of activation energies.

We investigate the solution properties of a generalized discrete nonlinear Schrödinger equation describing a nonlinear lattice chain. The generalized equation interpolates between the integrable discrete Ablowitz-Ladik equation and the nonintegrable discrete Schrödinger equation. Special interest is paid to the creation of stationary localized solutions called breathers. To tackle this problem we apply a map approach and illuminate the linkage of homoclinic and heteroclinic map orbits with localized lattice solutions. The homoclinic and heteroclinic orbits correspond to exact nonlinear solitonlike eigenstates of the lattice. Normal forms and the Melnikov method are used for analytical determinations of homoclinic orbits. Nonintegrability of the map leads to soliton pinning on the lattice. The soliton pinning energy is calculated and it is shown that it can be tuned by varying the ratio of the nonintegrability parameter versus the integrability parameter. The heteroclinic map orbit is derived on the basis of a variational principle. Finally, we use homoclinic and heteroclinic orbits as initial conditions to excite designed stationary localized solutions of desired width in the dynamics of the discrete nonlinear Schrödinger equation. In this way we are able to construct coherent solitonlike structures of profile determined by the map parameters. ͓S1063-651X͑96͒10211-7͔