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, 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 1111-000 of HCN; q=7.76×10-3cm-1, µ=20×10-8 cm-1 0111-000 of HCN; Bd′-B″=-0.002 876 cm-1, Dd′-D″=9.4×10-8 cm-11111-000 of N2O; Bd′-B″=-0.004 067 cm-l, Dd′-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.
D. H. RANK, T. A. WIGGINS, A. H. GUENTHER, and J. N. SHEARER, "Changes of Rotational Constants in Some II —Σ Bands of HCN and N2O," J. Opt. Soc. Am. 46, 953-954 (1956)