Contours are calculated for the pairs of H35Cl and H37Cl rotation lines for a range of spectral slitwidths between 0.10 cm−1 and 3.0 cm−1. The peak positions are compared with the frequencies of the H35Cl lines with a view to predicting the displacement of each peak due to overlap of the H37Cl lines. Experimental measurements of the J = 12 and 13 lines show that the predictions are correct. It is suggested that the calculated peaks be used as calibration data for instruments, although it is pointed out that if the wavenumber error of the instrument exceeds one-twentieth of the spectral slitwidth, the displacements can be ignored.
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