Rotation spectrum of hydrogen chloride

To determine the moment of inertia and the rotational characteristic temperature of HCl from its rotation spectrum in the far infra-red. 

The absorption spectrum of HCl in the far infra-red was observed by Hausler and Oetjen (J. Chem. Phys. 1953, 21, 1340). Their measurements are given in table 1. 

The rotational level T expressed as a wave-number is related to the rotational quantum number / by the formula, neglecting centrifugal stretching, 

The selection rule for absorption is / - / -f- 1 and the wave-number of the line so specified is consequently 

It is obvious by inspection that the observed wave-numbers are at least approximately in the ratio 4 5 . .. 10 llso that these are the values of / -J- 1. 

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