VIBRATIONAL INTENSITY PROGRESSION IN V[-N TRANSITION OF ETHYLENE

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 v'=5-12 (C2H4) or v'= 12-18 (C 2D4). In this range of v' for C2H4 the oscillator strength is estimated to be 0.1.