Motional narrowing in the v 1 /2 v 2 Fermi resonance diad of CO 2

Studies of the v 1 and 2v 2 Q branches in the Raman spectrum of carbon dioxide have been carried out at a resolution of ∼30 MHz using the methods of Raman Amplification (Raman Gain) Spectroscopy. The v 1 band at 1286 cm-1 is not resolved at atmospheric pressure. The 2v 2 band, although similarly unresolved over the pressure range 120–760 Torr, is extremely narrow with a full width at half maximum (0·0076 cm-1 at 760 Torr) which is comparable with the Doppler width of a single transition and very markedly narrower than the width that would be expected of a single rotational S or O branch line at this pressure. This result is attributed to the effects of motional narrowing, which are thought to be very pronounced for 2v 2 because of the near coincidence of the individual J components within the Q branch.