Molecules in space
Like atoms, molecules absorb and emit radiation only at specific wavelengths, as they make transitions from one energy state to another. A molecule is a much more complicated object than an atom, however, and consequently has many more transitions available to it, especially at infrared and microwave wavelengths. This richness not only results in molecular spectroscopy being much more complex than atomic spectroscopy, but it potentially makes an enormous amount of astrophysical information available to astronomers. Molecular spectroscopy is of rapidly increasing importance in astronomy, not only because of the light it sheds on interstellar chemistry, but also because it is allowing us to study dark regions of space from which little or no light emerges.

The transitions of a simple molecule such as carbon monoxide (CO) can be divided into three main types: electronic, vibrational and rotational, as depicted. ELECTRONIC TRANSITIONS involve changes in the shape of .the cloud of electrons surrounding the constituent atoms. They are very analogous to the transitions seen in single atoms, and occur at similar wavelengths, namely visible and ultraviolet. Electronic transitions in inter¬stellar molecules can be studied only by looking at the absorption in front of a bright star. Only rather few, relatively transparent regions of space can be studied in this way, so although ultraviolet and visible observations of molecules can be useful in forming a general picture of the diffuse interstellar medium, they provide no clue to what happens in the denser, more opaque regions.

VIBRATIONAL TRANSITIONS occur as a result of elasticity within the molecule allowing its atoms to oscillate to and fro with respect to each other. Changes in the amplitude of these vibrations’ generally lead to transitions at infrared wavelengths. Although infrared spectroscopy is potentially one of the most promising methods for studying molecular clouds in our Galaxy, the technique is currently not very sensitive, and is confined to observations of cool stars or infrared sources such as IRC + 10216.

ROTATIONAL TRANSITIONS, resulting from changes in the rate a molecule is spinning, give spectral lines which are usually at microwave or millimeter wavelengths. Large numbers of molecules have been observed by their rotational transitions, although the commonest molecule, hydrogen, like other symmetric molecules including oxygen, nitrogen and carbon dioxide, is unfortunately unobservable by this technique. Nevertheless, as described in the next section, the study of molecules by their microwave transitions has led to a spectacular increase in our understanding of molecular clouds in the past five or ten years. Most of our know¬ledge about interstellar chemistry is based on these very recent observations, many of which are of regions whose existence was unsuspected a decade ago.

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