Dispersion analysis of asymmetric band shapes: Application to the IR absorption spectrum of silica glass

The IR absorption spectrum of silica glass in the 3800 to 8000 cm^–1 region is analyzed based on the assumption of asymmetric band shapes. For simulating the asymmetric band shapes, a generalized version of the convolution model for the complex dielectric constant is proposed that involves different magnitudes of the standard deviation for an oscillator distribution in the wavenumber regions below and above the distribution center. A computational program for the dispersion analysis of the absorption spectra using such a model is developed. Best fit to the IR spectrum of silica glass obtained with the asymmetric band shapes contains no systematic errors throughout the contours of the 4400–4600 and 7000– 7300 cm^–1 absorption maxima, thus having appreciably better quality than fits obtainable with the symmetric band shapes inherent in the usual convolution model. A high accuracy of simulating the 7000–7300 cm^–1 absorption maximum with the asymmetric band shapes is attained when using as few as two bands in this region, which is in contrast to available literature sources assuming four bands. The band frequencies and intensities calculated with the asymmetric and symmetric band shapes are compared.