Interstellar molecules detected by their radiofrequency, microwave or millimetre wave spectra

6 Interstellar molecules detected by their radiofrequency, microwave or millimetre wave spectra 

Both emission and absorption processes rely on the background radiation, which is present throughout the universe and which has a wavelength distribution characteristic of a black body and a temperature of about 2.7 K. This radiation is a consequence of the big bang with which the universe supposedly started its life. 

Since 1963 spectra of many molecules have been detected, mainly in emission but some in absorption. Telescopes have been constructed with more accurately engineered paraboloids in order to extend observations into the microwave and millimetre wave regions. 

The regions of space where molecules have been detected are the nebulae which are found not only in our own galaxy but also in other galaxies. In our galaxy the nebulae are found in the Milky Way, which appears as a hazy band of light as a result of its containing millions of stars. Associated with the luminous clouds composing the nebulae are dark clouds of 

The first polyatomic molecule was detected in 1968 with use of a telescope having a dish 6.3 m in diameter at Hat Creek, California, USA, designed to operate in the millimetre wave region. Emission lines were found in the 1.25 cm wavelength region due to NH3. The transitions are not rotational but are between the very closely spaced 2 = 0 and 2 = 1 levels of the inversion vibration V2 (see Section 6.2.5.4). 

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