Molecular astronomy

There are in principle an infinite number of series beginning at higher quantum numbers with i = 6, 7, 8, 9... but they become increasingly difficult to observe. For the higher n series to be seen in the spectrum the levels have to be populated, so some hydrogen atoms must be in the n = 5 level to see the Pfund series. We shall see that the presence of the Balmer series in the spectrum of a star is indicative of the stellar temperature, which is a direct consequence of the population of the energy levels. More of this in Chapter 4. 

The problem with molecular astronomy is knowing where to start and, more importantly, where to stop. The laboratory-based big brother of molecular astronomy is the field of high-resolution spectroscopy. The synergy between these two subjects 

The next most useful is vibrational spectroscopy but identification of large molecules is still uncertain. In the laboratory, vibrational spectroscopy in the infrared (IR) is used routinely to identify the functional groups in organic molecules but although this is important information it is not sufficient to identify the molecule. Even in the fingerprint region where the low wavenumber floppy vibrational modes of big molecules are observed, this is hardly diagnostic of structure. On occasion, however, when the vibrational transition can be resolved rotationally then the analysis of the spectrum becomes more certain. 

Consider the simple diatomic molecule CO with a carbon atom at one end of the bond and an oxygen atom at the other. The moment of inertia is a measure of how heavy each atom is and the length of the bond between them. The moment of inertia carries important information regarding the structure of the molecule and, more importantly, is very useful in identifying a molecule. The energy separation between the allowed end-over-end rotations of a diatomic, Ej, is given by  

Reduced mass comes from the definition of moment of inertia, which will be discussed shortly. 

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One of the oldest problems in molecular astronomy concerns identification of the molecules responsible for diffuse interstellar bands (DIBs). Since their first observation in 1922 some 127 bands have been detected all over the electromagnetic spectrum, shown schematically in Figure 3.19, but the origins of the transitions, the so-called carriers of DIBs, have not been determined. 

The first faltering steps of molecular astronomy were intimately related to the birth of modern spectroscopy. It was the discovery at the beginning of the nineteenth century that the Sun and stars are composed of the same elements as the Earth, which led astronomers to the idea that spectroscopic techniques can be used to observe cosmic chemical processes. 

One task awaiting molecular astronomy is to find the missing links that separate 11-atom molecules from PAHs and fullerenes with 20 to 60 atoms, whilst a whole world lies between the most evolved molecules and a simple dust grain which contains, at the lowest estimate, 1 billion atoms. 

Molecular astronomy of carbon molecules is very rich. Of about 120 known interstellar molecules more than three-quarters contain carbon atoms diatomic molecules include CO, CN, C2, CH, CH+, CN+, and CO+ polyatomic include CH2, CH4, C2H2j CH OH, CH3CH2OH, H2CO and HNC large complex unsaturated radicals and polycyclic aromatic hydrocarbons are also detected. These all play a role in the thermochemistry of interstellar clouds. The 2.6-millimeter line of CO diagnoses density and temperature in molecular clouds, as do other molecules. 

Absorption spectroscopy in the visible and near-ultraviolet region of the spectrum dominated molecular astronomy in the three decades from 1937 to 1968. The species CH, CH+, and CN, observed by using bright hot stars as light sources, were seen in the spectra of many stars. The spectra of these species, as shown in Figure 17.1, establish that the population in the quantum levels is strongly biased toward the lowest levels. The population distributions are fit by a temperature of 2.73 K (Bortolot et al., 1969). ... 

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