Region Spectroscopy Process Involved Radio waves Nuclear magnetic Resonance* Changing nuclear spin orientation Microwave Electron spin Resonance Changing electron Spin orientation (pure) Rotational Changing molecular vibrational states Infrared Vibrational** Changing molecular vibrational states Ultraviolet Electronic Changing atomic or molecular electronic states X ray Inner electronic Changing electronic states or ejecting electrons Gamma ray Mössbauer Changing nuclear energy levels * These types of spectroscopy require a magnetic field to differentiate between different spin states.

Ultimately, the word “spectrum” came to represent the permanent record, rather than the dispersed white light (or other region of the electromagnetic spectrum). In this chapter, we will treat a spectrum as the physical record of which wavelengths/frequencies/energies of light are absorbed or emitted by a sample. With that in mind, we will find that there are several popular ways of displaying a spectrum, although different types of spectroscopy typically display spectra in one or two common ways.

J. Gordon, R. A. Ford, The Chemist’s Companion, John Wiley & Sons, New York, 1972. 1 Introduction In this chapter, we will consider the spectrometer itself. The problem is that no single description of a spectrometer exists. Spectrometers are designed to take advantage of the unique way that different types of light interact with matter. By the same token, most spectrometers have certain common elements. We will consider those elements. First we will differentiate between the several general classes of spectrometers.