Selection Rule for Rotational and Vibrational Spectra

Selection Rule for Rotational and Vibrational Spectra

The restriction applied on the transition are governed by the certain rules known as selection rules. If these rules are followed the transition can take place and is called allowed transition. If these are not followed transition can not take place and is called forbidden transition. The selection rules are generally expressed in terms of change in quantum numbers for allowed transitions.
For pure rotational transition, the selection rule is ΔJ = ± 1, where, J is rotational quantum number (ΔJ = +1 corresponds to absorption and ΔJ = -1 corresponds to emission).
Similarly for pure vibrational transition, the selection rule is Δv = ± 1, where v is vibrational quantum number.

NOTE:   Selection rule for rotational-vibrational Raman spectra, rotational quantum number(J) can be ± 2, ± 3 and so on...
In a nomenclature, spectral lines are designated O, P, Q, R and S branches corresponding to ∆J values of -2, -1, 0, +1 and +2, respectively.