l-Type Doubling in C2H2 and HCN

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Abstract
A high‐resolution infrared grating spectrometer has been used to investigate l‐type doubling of energy levels of the linear molecules C2H2 and HCN. Resolving power of 145 000 was obtained at 4100 cm—1 which permitted the resolution of the Q branch of ν1+ν51(Σg+→πu) of C2H2 to J″ = 6. The difference between the B′—B″ value determined from the Q branch and the same quantity obtained from the P and R branches of this band yields a value of q = 4.72×10—3 cm—1. The doublet structure in the difference band 0110→0112(π→π) of HCN has been resolved for J″≥6 in the R branch and J″≥10 in the P branch. This π→π band is treated as two bands with slightly different molecular constants. Evaluation of rotational constants shows that different α values are obtained for the + and — components of these π states. The difference in frequency between lines of equal J″ in the 000→002 and 0110→0112 bands is shown to be a quadratic function of J″. The value obtained for q′ was 7.68×10—3 cm—1.