On the Validity of the P–R Separation Method in the Presence of l-Type Resonance

A resonance calculation computer program has been applied to a quantitative investigation of the effects of strong l‐type resonances on the envelopes of degenerate $\mathrm{(E)}$ infrared bands of symmetric tops. The immediate object of these calculations has been to establish whether l‐type resonance is likely to be a serious source of error in the $\mathrm{P–R}$ separation method of determining $\eta$ constants. Attention has been directed mainly to bands where $\mathrm{(C\hspace{0.17em}-\hspace{0.17em}C\eta \hspace{0.17em}-\hspace{0.17em}B)}$ is close to zero so that the mutually perturbing levels are nearly degenerate. The effect of assuming l‐type doubling constants, $q$ , much larger than those observed has been examined. All of our calculations suggest that uncertainties in the $\mathrm{P–R}$ separation method arising from this source are small and comparable to those deriving from other known sources, such as the neglect of $\alpha$ constants in the method. An explanation for these results in terms of line by line shifts is advanced.