Knowledge Bank

A mechanism has been devised to account for the anomalous intensity distribution of the rotation lines in certain fundamental bands in the infrared spectra of triatomic molecules, notably in the $3.8\mu$ band of the $H2S$ spectrum. The theory assumes it is necessary to take account of the mixing of the wave function for a given rotation state in a vibration level with the wave functions of rotation states in other vibration levels when calculating the intensities. The mixing, to give a first-order effect, must be between the wave functions of the first excited states of the three vibration frequencies. Such a mixing may be achieved by taking account of the coupling of these vibrations by the Coriolis operator. The intensity anomaly is strongly influenced by the ratio of the coefficients multiplying the dipole moments induced by the oscillations $\nu 2$ and $\nu 3$. The lines due to the transitions K - K-1 will be strongly enhanced at the expense of the transitions K - K+1 if it is assumed that the coefficient of the dipole moment induced by the oscillation $\nu 2$ is of the order of ten to fifteen times as great as the coefficient multiplying the dipole moment induced by $\nu 3$, and that the Coriolis coupling coefficient between $\nu 2$ and $\nu 3$ is very nearly unity (as is the case in $H2O$). It must be assumed that in molecules like $H2O$ and $H2Se$, where this intensity anomaly is much less accentuated, the above ratio is more nearly of the order of five. No significant intensity anomaly is to be expected in $\nu 2$ in such instances.