Molecules in the vapor phase can have both vibrational and rotational transitions simultaneously. These spectra are quite rich in information since the moment of inertia determines the rotational line spacing. Anharmonic and centripedal corrections cause changes in the line space in the progression. Therefore, rovibrational data sets tell us a great deal about the properties of molecules in the gas phase. Both the vibrational and rotational selection rules must be obeyed in rovibrational transitions. This requirement defines the shape of the spectra. There are three branches called P, Q and R, which corresponds to changes in the vibrational quantum of J' = J - 1, J' = J and J' = J + 1, respectively. The Q branch is not observed because the selection rule for rotation does not allow J' = J. All of the rovibrational bands have a change in the vibrational quantum number, v' = v + 1. The nature of the transitions is shown using potential energy surfaces in the short presentation below. Rovibrational spectroscopy can be conducted on any molecules in the gas phase. Sometimes molecular samples are heated to increase their vapor pressure. Relatively large molecules have been studied using this technique. Quite detailed information can be obtained.
