The vibronic structure of crystalline ethylene

The absorption spectrum of polycrystalline ethylene was studied in the region of 250–150 nm.The Rydberg π → 3s lines disappeared in the solid and the band structure of the N → V transition extended beyond the maximum. This supports the assignment of the N → V transition in ethylene to an intravalence transition. The observed structure of the transition in the crystal was studied theoretically. The calculations showed that the equilibrium geometry of excited ethylene in the solid is the same as in the free molecule. That is the planes of the two methylene groups are perpendicular to each other despite the crystal forces. It was found that the vibronic progression is due to the torsional mode and to the combinations of torsion and stretching modes. It is suggested that the rigid lattice approximations, which were used in this study are suitable for the evaluation of the vibronic structure in case of large conformational change upon excitation of molecular crystals.