Vibrational Raman Spectra of Diatomic Molecules


If we approximate the potential energy curve of a vibrating bond as a parabola (i.e. assume that the vibration is harmonic) then the specific selection rule for vibrational Raman transitions is Δν = ±1.

Transitions for which Δν = -1 lie to high frequency of the incident radiation, and are called the Stokes lines.

Transitions for which Δν = +1 lie to low frequency of the incident radiation, and are called the anti-Stokes lines. These are usually more intense than the Stokes lines, as the Stokes lines require the molecules to initially be in excited vibrational states, which is a relatively rare occurrence.

In gas-phase spectra, the lines of the vibrational spectrum have a branch structure similar to that of the lines in an infra-red vibrational spectrum. They again arise from rotational transitions simultaneous with the vibrational transitions.

In this case, the selection rules are ΔJ = 0, ±2 (as in pure rotational Raman spectroscopy), and the three branches are termed the O branch (ΔJ = -2), the Q branch (ΔJ = 0), and the S branch (ΔJ = +2).

Note that in Raman spectroscopy, the Q branch is always observed.


This diagram is a schematic representation of the rotational structure within a vibrational line in a high resolution Raman spectrum.Note that in this diagram, unlike that for the infra-red rotation-vibration spectrum, the frequency (and thus the wavenumber) increases from right to left.