Infrared and Raman Spectroscopy for Thin-Film Analysis

Abstract

A molecule that is a coupled oscillator of atoms is vibrated by irradiation of light. When the vibration is delayed in terms of phase after that of light, the oscillator absorbs light; otherwise, the light passes through the matter although the oscillation frequency matches the frequency of the light. This phenomenon can be monitored by infrared (IR) spectroscopy. Since the light absorption is correlated with the polarity of the matter, IR spectroscopy reveals anisotropic permittivity, which depicts molecular orientation. On the other hand, the molecular vibration is also excited by irradiating a high-energy photon to generate a complex vibration that is a product of normal-mode functions, which emits Raman scattering comprising of two steps: the dipole excitation by irradiating light and the emission from the dipole. The first step is done via the Raman tensor, which reflects the symmetry of polarizability defined at the molecular coordinate. By analyzing these physical processes, details of the chemical structure and molecular interactions in a thin film are revealed.