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Rotational–vibrational spectroscopy
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<p>Rotational–vibrational spectroscopy is a branch of molecular <a href="/facts/Spectroscopy/mj71FuNK">spectroscopy</a> that is concerned with <a href="/facts/Infrared_spectroscopy/u1Hzx617">infrared</a> and <a href="/facts/Raman_spectroscopy/XY063k29">Raman spectra</a> of <a href="/facts/Molecule/N9ljSfFq">molecules</a> in the <a href="/facts/Gas/hVVAoodV">gas phase</a>. Transitions involving changes in both <a href="/facts/Molecular_vibration/Y0ovRukI">vibrational</a> and <a href="/facts/Rotational_transition/GhwCVVLL">rotational</a> <a href="/facts/Quantum_state/7jbxd7Dx">states</a> can be abbreviated as rovibrational (or ro-vibrational) transitions. When such transitions emit or absorb <a href="/facts/Photon/mFNhAEzA">photons</a> (<a href="/facts/Electromagnetic_radiation/Ynx7ujCW">electromagnetic radiation</a>), the <a href="/facts/Frequency/QUSbPaW8">frequency</a> is proportional to the difference in energy levels and can be detected by certain kinds of <a href="/facts/Spectroscopy/mj71FuNK">spectroscopy</a>. Since changes in rotational <a href="/facts/Energy_level/66x51Khc">energy levels</a> are typically much smaller than changes in vibrational energy levels, changes in rotational state are said to give fine structure to the vibrational spectrum. For a given vibrational transition, the same theoretical treatment as for pure <a href="/facts/Rotational_spectroscopy/QakXReKL">rotational spectroscopy</a> gives the rotational <a href="/facts/Quantum_number/AByP6F0y">quantum numbers</a>, energy levels, and <a href="/facts/Selection_rule/q0l2TSEq">selection rules</a>. 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 <a href="/facts/Molecular_dipole_moment/G1vRqruD">dipole moment</a> 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 <a href="/facts/Water_(molecule)/FFsl65l1">water</a> is important because of the presence of water vapor in the atmosphere. </p>