HomePhysical → Vibrational Spectroscopy

Vibrational Spectroscopy

Read in this section

Combination Differences

The rotational constant of a vibrationally excited state of a diatomic molecule will be slightly smaller than that of the vibrational ground state, as the excited state will have a slightly longer bond than the ground state (due to the anharmonicity of the vibration).

Vibrational Raman Spectra of Diatomic Molecules

The gross selection rule for the observation of vibrational Raman transitions is that the polarisability of the molecule should change as the molecule vibrates. Both homonuclear and heteronuclear diatomics fulfill this requirement, so both molecules are vibrationally Raman active.

Vibration – Rotation Spectra

If the vibrational spectrum of a gas-phase heteronuclear diatomic molecule is obtained at high enough resolution, it is found that each line of the spectrum actually consists of a large number of closely spaced components.

Anharmonic Oscillation

The approximation of the potential energy to a parabola cannot be correct at all extensions, as it does not permit dissociation of the bond. At high vibrational excitations (i.e. in states with high values of the quantum number ν), the parabolic approximation is particularly poor.

Vibrational Selection Rules

The gross selection rule for vibrational transitions is that the electric dipole moment of the molecule must change in the course of the vibrational motion.

Molecular Vibrations

A typical potential energy curve for a diatomic molecule has the following form.