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Transition Densities (transition + density)

Distribution by Scientific Domains

Business, Economics, Finance and Accounting	50%

Selected Abstracts

Exciton Migration in Conjugated Dendrimers: A Joint Experimental and Theoretical Study

CHEMPHYSCHEM, Issue 18 2009
Muhammet E. Köse Prof.
Abstract We report a joint experimental and theoretical investigation of exciton diffusion in phenyl-cored thiophene dendrimers. Experimental exciton diffusion lengths of the dendrimers vary between 8 and 17 nm, increasing with the size of the dendrimer. A theoretical methodology is developed to estimate exciton diffusion lengths for conjugated small molecules in a simulated amorphous film. The theoretical approach exploits Fermi's Golden Rule to estimate the energy transfer rates for a large ensemble of bimolecular complexes in random relative orientations. Utilization of Poisson's equation in the evaluation of the Coulomb integral leads to very efficient calculation of excitonic couplings between the donor and the acceptor chromophores. Electronic coupling calculations with delocalized transition densities revealed efficient coupling pathways in the bulk of the material, but do not result in strong couplings between the chromophores which are calculated for more localized transition densities. The molecular structures of dendrimers seem to be playing a significant role in the magnitude of electronic coupling between chromophores. Simulated diffusion lengths correlate well with the experimental data. The chemical structure of the chromophore, the shape of the transition densities and the exciton lifetime are found to be the most important factors in determining the size of the exciton diffusion length in amorphous films of conjugated materials. [source]

Bootstrap Methods for Markov Processes

ECONOMETRICA, Issue 4 2003
Joel L. Horowitz
The block bootstrap is the best known bootstrap method for time-series data when the analyst does not have a parametric model that reduces the data generation process to simple random sampling. However, the errors made by the block bootstrap converge to zero only slightly faster than those made by first-order asymptotic approximations. This paper describes a bootstrap procedure for data that are generated by a Markov process or a process that can be approximated by a Markov process with sufficient accuracy. The procedure is based on estimating the Markov transition density nonparametrically. Bootstrap samples are obtained by sampling the process implied by the estimated transition density. Conditions are given under which the errors made by the Markov bootstrap converge to zero more rapidly than those made by the block bootstrap. [source]

Specification Analysis of Diffusion Models for the Italian Short Rate

ECONOMIC NOTES, Issue 1 2005
Monica Gentile
In recent years, diffusion models for interest rates became very popular. In this paper, we perform a selection of a suitable diffusion model for the Italian short rate. Our data set is given by the yields on 3-month BOT (Buoni Ordinari del Tesoro), from 1981 to 2001, for a total of 470 observations. We investigate among stochastic volatility models, paying more attention to affine models. Estimating diffusion models via maximum likelihood, which would lead to efficiency, is usually unfeasible because the transition density is not available. Recently, Gallant and Tauchen (1996) proposed a method of moments which gains full efficiency, hence its name of Efficient Method of Moments (EMM); it selects the moments as the scores of an auxiliary model, to be computed via simulation; thus, EMM is suitable to diffusions whose transition density is unknown, but which are convenient to simulate. The auxiliary model is selected among a family of densities which spans the density space. As a by-product, EMM provides diagnostics that are easy to compute and interpret. We find evidence that one-factor models and multi-factor affine models are rejected, while a logarithmic specification of the volatility provides the best fit to the data. [source]