Molecular spectra are similarly useful in elaborating the structure of molecules, which has even greater interest for chemists than for physicists. Most molecular spectra are characteristically band spectra; that is, the spectrum consists of a series of bright bands, each of which appears similar to a piece of the continuous spectrum, separated by dark spaces. These bands are not continuous but consist of many closed spaced lines that can be resolved with high-resolution spectroscopes. The spacings of the lines in any series of molecular bands depend on whether the spectrum is rotational or vibrational. Because the rotational energy levels can be excited by small amounts of energy, and are thus close to one another, the lines in a rotational band are tightly packed with hardly any spacings. The vibrational levels, however, are much further apart, and the lines in a vibrational band are therefore much more widely spaced. The electronic energy levels of a molecule can also be excited, and the transitions of electrons between such levels give rise to the widely separated electronic lines in the molecular spectrum.
In addition to atomic absorption spectra, molecular absorption spectra also exist, which are obtained by passing continuous radiation through a molecular liquid or gas. This type of spectrum, consisting of dark bands separated by bright spaces, is the one that is most often used to study molecular structure. Other bands in molecular spectra are not resolvable into lines even by the most powerful instruments and are apparently continuous regions of absorption or emission of energy.

Applications of Spectrum Analysis
The two main uses of spectrum analysis are in chemistry and astrophysics.

Chemical Analysis
The spectrum of a particular element is absolutely characteristic of that element. Different elements, however, sometimes give rise to lines that are quite close together, leading to the possibility of error or misinterpretation. The Fraunhofer C line at 430.8 mµ, for example, is caused by two different lines, one formed by calcium with a wavelength of 430.7749 mµ and the other formed by iron with a wavelength of 430.7914 mµ. With an ordinary spectroscope, distinguishing between these two would be difficult. The other lines of calcium, however, are very different from those of the other lines of iron. Thus, the comparison of the entire spectrum of an element with a known spectrum simplifies its identification.