New experimental tests of existing interpretations of electronic transitions of ethylene

The absorption spectra of the gases ethylene-d0, d1, 1,1-d2, cis-d2, trans-d2, d3, and d4 have been photographed using the first order of a 21-ft vacuum spectrograph. Analyses of both the Rydberg system at 1750 Å and the π*-π system between 1500 and 2100 Å have been extended. Band contour analysis has confirmed the assignment of the Rydberg system as 1B3u-1Ag. The resulting rotational constants of the 42 level of the excited state of C2D4 lead to an inertial defect indicative of a nonplanar molecule. Assuming point group D2, we have combined the constants with Franck-Condon calculations in a complete structure determination for the 42 level of the Rydberg state: rCC = 1.41 Å, rCH = 1.08 Å, [right parenthesis lessthan] HCH = 124.4° and an azimuthal angle of 37°. Linewidths in the successful simulation of the C2D4 0–0 band indicate a lifetime greater than 10−13 sec for molecules in the zero level of the Rydberg state. For the π*-π system, all observed bands for all molecules studied have been assigned. Most striking is the relative inactivity of the C–C stretching vibration; except in C2D4, only the torsional mode is active. It is concluded that rCC for the π*-π state is not very different than it is for the Rydberg state. Finally, the extrapolation to the (unobserved) origin of the π*-π system is revised to give 5.5 eV (2285 Å).