Part of the spectrum showing the first overtone 2ν 20 composed of a parallel band and a perpendicular band very close in energy. While the shape of the two Q branches is visible, the perpendicular band has a poor signal-to-noise ratio and shows a very strong perturbation.

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... 2ν 20 band, centered around 595 cm −1 , is composed of a parallel sub-band (A 1 ) and a perpendicular sub-band (E) very close in energy as illustrated in Fig. 7. The E sub-band is less intense but Q branches are clearly visible for both sub-bands. Using parameters fitted for ν 20 as a starting point of a new fit gives a prediction that seems close in a first look. Nevertheless, while line assignment in the A 1 sub-band seems to be not too difficult thanks to the fair enough signal intensity, ...