High Quality Content by WIKIPEDIA articles! Rotational-vibrational spectroscopy is a branch of molecular spectroscopy concerned with the infrared and Raman spectra of molecules in the gas phase. Changes in rotational state give fine structure to the vibrational spectrum. For a given vibrational transition, the same theoretical treatment as for pure rotational spectroscopy gives the rotational quantum numbers, energy levels and selection rules. In linear and spherical top molecules rotational lines are found as simple progressions at both higher and lower frequencies relative to the pure vibration frequency. In symmetric top molecules the transitions are classified as parallel when the dipole moment change is parallel to the principal axis of rotation, and perpendicular when the change is perpendicular to that axis. The ro-vibrational spectrum of the asymmetric rotor water is important because of the presence of water vapor in the atmosphere.