INFLUENCE OF VIBRATION–ROTATION INTERACTION ON THE INTENSITIES OF PURE ROTATION LINES OF DIATOMIC MOLECULES

The magnitude of the effect of the vibration–rotation interaction on the intensities of pure rotation lines of diatomic molecules has been calculated for two different molecular models, the anharmonic oscillator and the rotating Morse or Pekeris oscillator. The intensity correction for the anharmonic oscillator has been obtained by adapting the contact transformation formalism for calculating second-order corrections to the energy to the calculation of first-order corrections to the matrix elements of the electric moment as suggested by H. H. Nielsen. The correction to the line intensity for vibrationless transitions of the anharmonic oscillator is found to be[Formula: see text]The results obtained here are also in complete agreement, to first order, with the results obtained earlier by Herman and Wallis for the 1–0 and 2–0 vibration–rotation line intensities. In the case of the Pekeris or rotating Morse oscillator the correction to the pure rotation line intensity is of the same form as above, namely,[Formula: see text]but exhibits minor differences which can be explained in terms of the difference in the vibrational potential energy function in the two cases.