INTRODUCTION

Raman is a vibrational spectroscopic technique and is complementary to infrared (IR) and near-infrared (NIR) spectroscopy. The Raman effect itself arises as a result of a change in the polarizability of molecular bonds during a given vibrational mode and is measured as inelastically scattered radiation. The appearance of a Raman spectrum is much like that of an IR absorbance spectrum. The intensities, or the number of Raman photons counted, are plotted against the shifted energies. The Raman shift is usually expressed in wavenumber and represents the energy difference between the incident photon energy and the inelastically scattered photon energy. Unlike IR or NIR spectra that arise when vibrational modes in a molecule have a dipole change, Raman spectra have frequencies and intensities that arise when these same modes cause a change in polarizability. The spectrum is interpreted in the same manner as the corresponding mid-infrared spectrum. The positions of the (Raman-shifted) wavenumbers for a given vibrational mode are theoretically identical to the wavenumbers of the corresponding bands in an IR absorption spectrum. However, the stronger peaks in a Raman spectrum are often weak, or not present in an IR spectrum, and vice versa. Similarly, wavenumbers above 1500 cm⁻¹ are group frequencies, and strong bands that absorb below 1500 cm⁻¹ can be either group frequencies or fingerprint bands for the molecule. Thus, the two spectroscopic techniques are often said to be complementary. For discussion of the theory and principles of measurements, see Raman Spectroscopy—Theory and Practice 〈1858〉, which may be a helpful, but not a mandatory, resource. For further discussion of the theory and applications of chemometrics, see Chemometrics 〈1039〉.