In addition to that advantage that it more easily penetrates clouds of dust and gas, infrared radiation has an additional powerful attribute. It just so happens that the frequencies associated with the middle portion of the infrared spectrum (4000-400 cm^-1; 2.5-25 µm) span the same range as the vibrational frequencies of the adjacent atoms in most molecules.

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Thus, it is in this spectral region that most molecules will absorb infrared radiation at characteristic wavelengths determined by their atomic composition and bond structure. As a result, most molecules will absorb infrared radiation in such a way as to produce a unique infrared absorption 'fingerprint.' For example, the figures below show the infrared spectra of ices made up of CH₃OH (methanol) and NH₃ (ammonia). The absorption bands seen in the spectra correspond to a specific vibrational mode of the molecules. Since the molecules contain different types of atoms that share different bonds, they produce different, characteristic infrared "fingerprints."

The infrared spectrum of a sample can be used not only to identify the molecular components present, but can also provide a lot of information about the state of the material. For example, the figure below shows the infrared spectra of several different materials including a mixture of H₂O:CO₂ in both (a) the gas phase and in (b) the ice phase,(c) a CH₃OH (methanol) ice mix, (d) an apple, and (e) an orange. The differences between the spectra of the gaseous and solid H₂O:CO₂ samples are dramatic. Similarly, the infrared absorption bands produced by the H₂O:CO₂ ice are distinctly different from that of the CH₃OH ice.

This handy property of the interaction of infrared radiation and the interatomic bonds of molecules provides a powerful probe of the materials present in the interstellar medium and on the surface of planets. Infrared radiation emitted by a star and collected with a telescope will suffer differential absorption whose frequency dependence will be determined by the nature of the materials along the path of the infrared light. The result is that astronomer on Earth will see absorption features superimposed on the spectrum of the star (for a schematic, click here). These absorption features can compared to the infrared 'fingerprints' of molecules studied in the laboratory and matches can be used to identify the compounds present in the interstellar cloud. Similar comparisons can be used to help identify the materials present in the diffuse interstellar medium (DISM), in interstellar infrared emission objects, and on the surface of solar system objects.

While one of the advantages of working in the middle-infrared is that this region overlaps the vibrational frequencies of most molecules, it should be noted that this can also be a problem. The infrared spectra of complex samples can be quite confusing since they will contain the overlaping absorption bands of many different molecular species. This can make the interpretation of the spectra very difficult. In some cases, different samples having distinctly different, but related molecules can produce spectra that are nearly identical. An excellent example of this is provided by the infrared spectra of DNA. The DNA molecules of different living species (or even members of the same species for that matter) differ from each other in the order in which their nucleotides are arrange in large numbers of ways. Despite all of these differences, however, the infrared spectra of all DNA looks essentially the same, independant of whether it is from a bacterium, a fish, or a person (a humbling thought!). This is because all DNA is made up of four main molecular structures with their characteristic interatomic bonds and it is the bonds, not the overall molecules that are being probed by the infrared spectra. As an analogy, think of the molecule as being a house and the infrared spectrum as being a good device for measuring the kinds of bricks used to make the house. The infrared spectrum may be a great way to separate houses made with clay bricks from houses made with cinder blocks, but it can't help you too much in determining exactly how they are stacked!

An amusing example of this problem is provided by the comparison of the infrared spectra of an apple and an orange in the figure above. We can easily use our eyes to distinguish an apple from an orange, but they look very similar in the infrared because they contain very similar mixtures of molecular components. Who says you can't compare apples and oranges! [This comparison gained a certain notariety when Scott Sandford published it in the Annals of Improbably Research (AIR).