https://doi.org/10.1351/goldbook.08611
Line-broadening by mechanisms that affect every molecule of the same species in the sample in the same way.
Note:
Such mechanisms do not require an ensemble of spectroscopically non-equivalent molecules of the same species. Such mechanisms are the natural line width, which is usually negligible in vibrational spectroscopy, and intra- or inter- molecular interactions, including anharmonic vibrational interactions (see anharmonicity), that reduce the lifetime of the excited state.
Such mechanisms do not require an ensemble of spectroscopically non-equivalent molecules of the same species. Such mechanisms are the natural line width, which is usually negligible in vibrational spectroscopy, and intra- or inter- molecular interactions, including anharmonic vibrational interactions (see anharmonicity), that reduce the lifetime of the excited state.