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. A spectrometer consists of a source of light, a sample holder, a detector that responds to light that has passed through the sample, and a recorder that produces a print-out or spectrum (Figure 20). The spectrum shows how much light has been absorbed at each frequency. Figure 21 shows an ultraviolet spectrum. The peaks around 250 nm tell the chemist that a benzene ring is present in this compound (the electrons in the benzene ring undergo excitation at this wavelength).

schematic of a spectrometer

Figure 20. A schematic of a spectrometer.
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ultraviolet spectrum

Figure 21. 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. Because molecules are complex systems of atoms, each molecule has its own pattern of infrared peaks; that is, the infrared spectrum of a compound serves as a fingerprint of the compound. 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.

Figure 22. The symmetric stretching vibration of water.

infrared spectrum

Figure 23. An infrared spectrum.

photo of recorder and sample compartment of an infrared spectrophotometer

Figure 24. A photograph of the recorder and sample compartment of an infrared spectrophotometer.