ABSTRACT
This chapter explores a more detailed discussion of the molecular spectral lines that are commonly observed at radio frequencies. Spectral lines result from the interaction of light with atoms, ions and molecules. The most important broadening mechanism for radio wavelength spectral lines is Doppler broadening. The optical depth in a spectral line varies with frequency and depends on the frequency offset relative to the line center frequency according to the line profile function. The temperature of the gas is often larger than the radiation temperature, and therefore, in these cases, the excitation temperature of radio spectral lines is bounded by the gas temperature and by the radiation temperature. The first detection of a radio-wavelength spectral line from a molecule was made in 1963. The most important broadening mechanism for radio wavelength spectral lines is Doppler broadening. In fact, at radio frequencies, most of the spectral lines detected are due to molecules not atoms.
