This chapter concerns interactions of light with matter that can be used to identify specific structures at the level of the chemical bonds making up the sample. This is in contrast to the scattering phenomena described in Chapter 4 where information is obtained on length scales comparable to the wavelength of the light itself. The ability to discriminate between different chemical constituents is accomplished by taking advantage of couplings between incident light and various modes of energy contained within the sample. For this reason, the interactions presented here do not conserve energy, as is the case with Rayleigh light scattering.

There are many spectroscopic interactions of light and matter, all of which rely on particular molecular structural changes that affect the electric and magnetic fields of light. In this chapter, dichroism resulting from either electronic or vibrational transitions is discussed, with the most emphasis being given to vibrational absorption spectroscopy in the infrared. That discussion is followed by a second vibrational spectroscopy: Raman scattering. Polarized fluorescence is also presented. In every case, the focus of the discussion is on the spectroscopic measurement of orientation.