A primer on observations and spectroscopy

More than 80 interstellar and/or circumstellar molecules have been detected in interstellar molecular clouds from their spectral lines. The detected wavelengths of the lines - predominantly located in the millimeter wave region - identify unequivocally the molecules, their state of excitation, the velocities with which they move and their chemical composition. In addition the line intensities and the shape of the line contain information about e.g. the densities, temperatures and dynamics prevailing in molecular clouds. Densities range from about 102 to 109 molecules/cm3 in the densest clumped material which are potential birthplaces of young stars. However derivation from the observed lines of physical quantities and chemical behavior requires in depth knowledge of atomic and molecular parameters, such as line positions, transition matrix elements etc. which has been and still has to be provided by laboratory work