The ν5 + ν9 carbon-deuterium stretching combination band of ethylene-D4: a case of almost exact accidental degeneracy of vibration levels

The ν5 + ν9 A-type infrared combination band of C2D4 at 4627 cm-1 has the superficial appearance of an unperturbed absorption. Assignment, however, reveals the effects of Coriolis interaction with an inactive vibration level, ν2 + ν8 + ν9, which lies within 0·4 cm-1 of ν5 + ν9. Through the K dependence of the interaction, the K a = 0 subband J structure of ν5 + ν9 is completely unperturbed, while higher K a subband J structures assigned are displaced varyingly up to 3·5 cm-1. Due to almost exact degeneracy of interacting rovibration levels, for the K a = 3–7 subbands, two series of J structures are identified in the spectrum, and two series of subbands are identified in the overall Q branch for K a = 1–8, the ‘inactive’ transitions stealing intensity from those of ν5 + ν9 through the Coriolis interaction. Analysis of the assignments made yield unperturbed upper state parameters and the magnitude of the Coriolis interaction parameter.