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 Π state have slightly different values of rotational constants associated with them. The difference of the two B values in a Π 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, Bd′−B″ and Dd′−D″ for the Q branch of the 0111–000 band of HCN and Bd′−B″ for the Q branch of the 1111–000 band of N2O have been measured. The results are
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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