Rotational-vibrational Spectroscopy - Rotational-Vibrational Spectroscopy

Rotational-Vibrational Spectroscopy

Generally vibrational transitions occur in conjunction with rotational transitions. Consequently, it is possible to observe both rotational and vibrational transitions in the vibrational spectrum. Although many methods are available for observing vibrational spectra, the two most common methods are infrared spectroscopy and Raman spectroscopy.

The energy of rotational transitions is on the order of J whereas vibrational transitions have energies on the order of J. Therefore, highly-resolved vibrational spectra will contain fine structure corresponding to the rotational transitions that occur at the same time as a vibrational transition. Although molecular vibrations and rotations do have some effect on one other, this interaction is usually small. Consequently, the rotational and vibrational contributions to the energy of the molecule can be considered independently to a first approximation:

where is the vibrational quantum number, is the rotational quantum number, h is Planck's constant, is the frequency of the vibration, is the speed of light, and is the rotational constant.