Modeling and Simulation in Spectroscopic Study

Abstract

Dear Readers:Modeling and simulation has been increasingly applied to spectroscopic study over the last decade. Modeling is the process of producing a model; a model is a representation of the construction and working of some system of interest. A model is similar to but simpler than the system it represents. One purpose of a model is to enable the analyst to predict the effect of changes. A simulation of a system is the operation of a model of the system. The model can be reconfigured and experimented with, usually, this is impossible, too expensive or impracticable to do in the system it represents. The operation of the model can be studied, and hence, properties concerning the behavior of the actual system or its subsystem can be inferred. In its broadest sense, simulation is a tool to evaluate the performance of a system, existing or being proposed, under different configuration of interest and over long periods of real time.Through the following representative example of Raman and infrared spectroscopic study, let us look at how modeling and simulation plays an indispensable role in predicting and interpreting spectra of matters.Spinel phases of lithium manganese oxides are important positive electrode and lithium-selective ion-sorption materials. Vibrational spectroscopy is a powerful tool to examine structural changes in these oxides [1], being sensitive to the short range environment of oxygen coordination around both manganese and lithium ions. In order to establish the relationship between the lattice structures and the vibrational spectra of lithium manganese oxides, there is a need of a fundamental understanding of the lattice vibrations, particularly those that are Raman and infrared active. Ammundsen et al.[2] used atomistic simulation methods based on an interatomic potential model to determine the normal modes of the zone-center (k = 0) lattice phonons in spinel LiMn 2 O 4 . They compared the results of these calculations with Raman and infrared experimental data of the LiMn 2 O 4 bulk phase. Their simulation allowed the Raman and infrared spectra of spinel LiMn 2 O 4 to be assignable.By atomistic simulation [3]-[5], the calculations were carried out using energy minimization procedures embodied in the GULP (General Utility Lattice Program) code. The interatomic potentials are based on the Born model of the solid, which includes a long-range Coulombic interaction and a short-range term to model overlap repulsions and van der Waals forces. The electronic polarizability of the ions is described by the shell model, which is effective in simulating the dielectric and lattice dynamical properties of metal oxides.