Numerical simulation of IR-spectroscopic experiments

In this work we demonstrate the application of numerical simulation for the investigation of the light distribution in an optical system, such as an FT-IR microscope. Because of the complexity of simulating the optical arrangement of a complete IR-microscope we have applied a program based on a Monte Carlo type wavelength-dependent ray-tracing technique (SPRAY). The program is capable of computing spectral features in a complex three-dimensional system. Spectral Features of organic layers and bodies are calculated from the real and imaginary parts of the dielectric functions determining the interaction of light with the material considered. The dielectric functions can be derived from various models. Previous work has shown that for organic substances a harmonic oscillator model can be used with sufficient accuracy. As microscopy usually exploits plane sample carriers, which do not seriously affect the path of the IR-beam, we calculated and rested cylindrical and spherical carriers, resulting in a higher energy throughput to the detector. In the simulation virtual screens could be placed before, inside and after the carrier to visualize the path of the radiation and the focusing effects, corroborating the experimental results. This work demonstrates the importance of wavelength-dependent numerical simulation of IR-spectroscopic experiments. The software developed offers a valuable tool for the verification of experimentally obtained IR-spectroscopic results and for the development and optimization of optical systems.