Spectroscopic Theory for Chemical Imaging

All images require some typeof contrast to differentiate regions of interest in a field of view. Themost common source of image contrast is variation in the intensity of reflected light. Contrast can, however, be based upon any measurable property of the sample that can be expressed as a function of location. Contrast is improved for measurements having a wider dynamic range and bymeasuring a larger number of variables for each pixel, as in color versus black-and-white photography. Contrast may also be enhanced with one or more of a wide range of techniques including digital image processing and structured illumination. This book will focus on chemical images generated using vibrational spectroscopic contrast. Such contrast is generated by quantifying one ormore attribute(s) of an infrared absorption, infrared emission, or Raman scattering spectrum for each pixel. By providing a window into the spatial distribution of properties such as molecular composition, structure, state, and concentration, images based on vibrational spectroscopies open up a new way of seeing the world. Imaging can be accomplished by measuring a property from the entire field of view simultaneously (global imaging) or by measuring a property from individual points in the field of view sequentially and combining the points to create the image (mapping). Since mapping requires a large number of measurements, each measurement must be relatively fast for mapping to be practical. For example, an image consisting of 640 480 pixels contains over 300,000 measurements and would take more than 3.5 days to acquire if each measurement required 1 s. Mapping speed can be increased by simultaneously measuring a property at multiple points in a subregion of the field of view and combining those subregions to create the image. The subregion may consist of a single column of measurement points (line imaging) or may contain multiple columns (mosaic imaging). In most cases, even global imaging requires multiple frames, each containing different spectroscopic information, to be collected sequentially and overlaid to form a single image. Sample changes during the course of sequential measurements can confound the interpretation of spectral images. This chapter provides an introduction and theoretical background for vibrational spectroscopies, as used to produce chemical images. Infrared, Raman, and related spectra result from the interactions of electromagnetic radiation with molecular vibrations, so this chapter begins with a description of relevant aspects of molecular vibration, followed by a section on electromagnetic radiation and its interactions with matter. Next are three sections on infrared spectroscopies, divided by spectral region. After that, several different types of Raman spectroscopy that are used for chemical imaging are described. The final section briefly presents the use of Raman and infrared spectroscopies for creating large chemical images by remote sensing. Remote sensing is probably responsible for the majority of chemical images created because of its use in mapping the atmosphere, planets including Earth, and moons, and in astronomy.