The fact that certain rotational “constants” change with the vibrational state of the molecule is well known. It is also true that the two substates of a II state have slightly different values of rotational constants associated with them. The difference of the two B values in a II state is designated by q (usually of the order of 10^-3 cm^-1), while the difference of the two D’ values is called µ(~10^-7 cm^-1). We have measured q and µ for the 11¹1-000 band of HCN with the aid of a Fabry-Perot etalon which had dielectric films for the reflecting surfaces. The method employed made it possible to ignore the effect of phase changes due to reflections in the etalon, and a crude optical calibration of the spacer was all that was necessary. In addition, B_d′-B″ and D_d′-D″ for the Q branch of the 01¹1-000 band of HCN and B_d′-B″ for the Q branch of the 11¹1-000 band of N₂O have been measured. The results are 11¹1-000 of HCN; q=7.76×10^-3cm^-1, µ=20×10^-8 cm^-1 01¹1-000 of HCN; B_d′-B″=-0.002 876 cm^-1, D_d′-D″=9.4×10^-8 cm^-111¹1-000 of N₂O; B_d′-B″=-0.004 067 cm^-l, D_d′-D″ <10^-8 cm^-1.

Resolution of 175 000 was obtained with the Q branch of the 01¹1 band of HCN. Taking account of the finite slit width and the inevitable Doppler line broadening, the resolution attained is shown to be 95% of that theoretically to be expected for the double passed grating spectrograph employed in this investigation.

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