The preceding discussion has revealed that the energy possessed by electrons is quantized and that an atom or combinations of atoms called molecules can absorb light. This light is used by the atom or molecule to promote electrons to a higher energy level. The frequency and intensity of light picked up by a sample is measured by a device called a spectrometer. The spectrum shows how much light has been absorbed at each frequency. Figure 21 shows an ultraviolet spectrum.
Molecules also possess other types of energy that are quantized.
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.