For example, the atoms in molecules oscillate back and forth in a variety of directions. These vibratory motions, such as the symmetric stretching vibration shown for water in Figure 22, have discrete quantized energies. The spacing between the energy levels is such that light in the infrared region of the electromagnetic spectrum has the correct energy to cause vibratory excitation. Figure 23 shows an infrared spectrum. Each peak in the spectrum corresponds to the vibration of a group of atoms.
Infrared spectroscopy is therefore an important tool for the identification of a compound.
The recorder and sample compartment of an infrared spectrophotometer are shown in Figure 24.