Far Ultraviolet Absorption Spectra of Ethylene and Ethylene-d4

The absorption spectra of ethylene and ethylene-d4 have been reinvestigated in the 1500 A to 2050 A region, using the first order of the Harrison 21-foot vacuum spectrograph. Vibrational constants of the C=C stretching vibration in the R (first Rydberg) state were determined to be: C2H4, ω20=1381.5 cm—1; x220=—11.2 cm—1; C2D4, ω20=1306.8 cm—1; x220 —8.2 cm—1; and (tentatively) in the V state: C2H4, ω20=852 cm—1; x220=—1.9 cm—1; C2D4, ω20=797 cm—1; x220=—1.8 cm—1. Newly observed fine structure in the v′—0 stretching vibration progression in the long wavelength part of the V←N transition of C2D4 is tentatively attributed to a torsional oscillation. The 0—0 bands of the V←N transition, though too weak to be seen, were estimated to lie near 2500 A. The probable dissociation of ethylene into CH2 radicals in the V←N continuum is discussed. The torsional frequency in the R state was redetermined as 141 cm—1 (C2D4) and 236 cm—1 (C2H4), decreasing somewhat with increasing stretching quantum number. An attempt has been made to fit the corresponding vibrational levels, for both isotopes, to those calculated for a torsional oscillator-rotator using a potential function of the form V=12V0(1+cos2θ). Rough agreement is obtained for a reasonable barrier height of about 790 cm—1. (Values of 650 cm—1 for C2D4 and 920 cm—1 for C2H4 are obtained when the two isotopes are treated separately.) The anomalous isotope effect and intensities in the R←N twisting bands are tentatively attributed to perturbation of the R state levels by V state levels and/or the departure of the twisting potential from the simple cosine form. These views are encouraged by the fact that a higher Rydberg state shows similar twisting bands but with much less anomalous isotope effect and intensities.