Computational Study of the Structure and Thermodynamic Properties of Ammonium Chloride Clusters Using a Parallel J-Walking Approach

The thermodynamic and structural properties of (NH$_4$Cl)$_n$ clusters, n=3-10 are studied. Using the method of simulated annealing, the geometries of several isomers for each cluster size are examined. Jump-walking Monte Carlo simulations are then used to compute the constant-volume heat capacity for each cluster size over a wide temperature range. To carry out these simulations a new parallel algorithm is developed using the Parallel Virtual Machine (PVM) software package. Features of the cluster potential energy surfaces, such as energy differences among isomers and rotational barriers of the ammonium ions, are found to play important roles in determining the shape of the heat capacity curves.Comment: Journal of Chemical Physics, accepted for publicatio

A Semi-Empirical Potential for Simulations of Transition Metal Clusters: Minima and Isomers of Ni\u3csub\u3en\u3c/sub\u3e (n=2-13) and their Hydrides

A potential energy surface (PES)for bare, mono and di-hydrogenated nickel clusters is constructed using the extended-Hϋckel approximation. The parameters are optimized and good agreement with theoretical and experimental results is obtained without including a posteriori coordination dependent terms. The global minimum and the first few low-lying isomers of several nickel clusters are investigated using a variety of minimization techniques. The difference in energy between isomers is much smaller than the Ni-Ni dissociation energy. Both geometric and optical isomers are found for many cluster sizes. In some cases symmetric nuclear configurations give rise to orbital degeneracies in the adiabatic surface which lead to distortions. The hydrogen atom is most frequently found on the surface. All isomers of NinH2 contain a dissociated hydrogen molecule. The results are in good agreement with quantitative and qualitative experimental findings on this system

Time-Resolved Fluorescence Anisotropy in Chromophore Pairs

Electronic excitation transfer has been the subject of numerous experimental and theoretical studies. Recent experimental measurements of time-resolved fluorescence and pump-probe anisotropy have revealed sub-picosecond excitation transfer processes, seen as decays in the anisotropy. The experiments of Xie et al.[1] on Allophycocyanin (APC) and C-Phycocyanin (C-PC) isolated from photosynthetic antenna systems in cyanobacteria, measuring time-resolved fluorescence anisotropy with ~ 100 fs time resolution, found that the ultrafast decay component in the anisotropy showed a strong dependence on excitation/detection wavelength. The sub-picosecond decay anisotropy decay was seen in trimers of APC and C-PC and not in monomers, and, therefore, this fast decay has been attributed to pairwise excitation transfer among chromophores on different monomers of APC and C-PC.</jats:p