states n=2 and n=1 have slightly less energy than transitions between the ground state and first excited state. Such a transition gives rise to a hot band. To describe vibrational levels of an anharmonic oscillator, Dunham expansion is used. Intensities In an infrared spectrum the intensity of an absorption band is proportional to the derivative of the molecular dipole moment with respect to the normal coordinate. Likewise, the intensity of Raman bands depends on the derivative of polarizability with respect to the normal coordinate. There is also a dependence on the fourth-power of the wavelength of the laser used. See also Coherent anti-Stokes Raman spectroscopy (CARS) Eckart conditions Fermi resonance GF method Infrared spectroscopy of metal carbonyls Lennard–Jones potential Near infrared spectroscopy Nuclear resonance vibrational spectroscopy Resonance Raman spectroscopy Transition dipole moment References Further reading External links Free Molecular Vibration code developed by Zs. Szabó and R. Scipioni Molecular vibration and absorption small explanation of vibrational spectra and a table including force constants. Character tables for chemically important point groups Chemical physics Spectroscopy