Vibrational Intensity Progression in the V ← N Transition of Ethylene

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Abstract
The relative intensities of the vibrational subbands of the V ← N transitions of C2H4 and C2D4 have been measured and compared with the squares of the vibrational overlap integrals of two choices of one-dimensional oscillators—one hindered by a harmonic barrier and one hindered by a sinusoidal barrier. The harmonic oscillator is found to more adequately describe the experimental data. The value of the upper-state A′ rotational constant deduced from the experimental data, 5.45 cm−1, precludes structures of the V state in which the carbon atoms lie off the minimum axis of inertia. The difference between A and A′ can be explained by reducing the distance between the geminal hydrogens 5%. The electronic transition moment is shown to be independent of torsional quantum number from υ′ = 5–12 (C2H4 or υ′ = 12–18 (C2D4). In this range of υ′ for C2H4 the oscillator strength is estimated to be 0.1.