A molecule is a tightly bound group of atoms held together by interactions among their orbiting electrons—interactions that we call chemical bonds. Much like atoms, molecules can exist only in certain well-defined energy states, and again like atoms, molecules produce emission or absorption spectral lines when they make a transition from one state to another. Because molecules are more complex than individual atoms, the rules of molecular physics are also much more complex. Nevertheless, as with atomic spectral lines, painstaking experimental work over many decades has determined the precise frequencies (or wavelengths) at which millions of molecules emit and absorb radiation.

In addition to the lines resulting from electron transitions, molecular lines result from two other kinds of change not possible in atoms: molecules can rotate, and they can vibrate. Figure 4.14 illustrates these basic molecular motions. Molecules rotate and vibrate in specific ways. Just as with atomic states, only certain spins and vibrations are allowed by the rules of molecular physics. When a molecule changes its rotational state or its vibrational state, a photon is emitted or absorbed. Spectral lines characteristic of the specific kind of molecule result. These lines are molecular fingerprints, just like their atomic counterparts, enabling researchers to identify and study one kind of molecule to the exclusion of all others.

Figure 4.14 Molecules can change in three ways while emitting or absorbing electromagnetic radiation. Sketched here is the molecule carbon monoxide (CO) experiencing (a) a change in electron arrangement in which an electron in the outermost orbital of the oxygen atom drops to a lower-energy state, (b) a change in rotational state, and (c) a change in vibrational state.

As a rule of thumb, we can say that

1. Electron transitions within molecules produce visible and ultraviolet spectral-line features.
2. Changes in molecular vibration produce infrared spectral features.
3. Changes in molecular rotation produce spectral lines in the radio part of the electromagnetic spectrum.

Molecular lines usually bear little resemblance to the spectral lines associated with their component atoms. For example, Figure 4.15(a) shows the emission spectrum of the simplest molecule known—molecular hydrogen. Notice how different it is from the spectrum of atomic hydrogen shown in part (b) of the figure.

Figure 4.15 (a) The spectrum of molecular hydrogen. Notice how it differs from the spectrum of the simpler atomic hydrogen (b).