Photoelectron spectra of ethylene and of the six deuterated derivatives

Abstract

The photoelectron spectra of C 2H 4 and of six deuterated molecules of ethylene — C 2D 4, C 2D 3H, C 2H 3D, cis-C 2H 2D 2, trans-C 2H 2D 2 and gem-C 2H 2D 2 — have been recorded with the 584-Å resonance line of He. The adiabatic ionization potentials of the X 2 B 3u and the 2 B 3g states of the seven isotopic components have been determined with an accuracy of about 7 meV. The ionization potentials of the other excited electronic states have been measured with a lower accuracy owing to a less well defined onset. The measured ionization potentials of C 2H 4 are 10.514 eV, 12.431 eV, 14.4 3 eV, 15.7 4 eV and 18.7 eV. The vibrational structure of the first electronic band shows that the two normal modes ν 2 (symmetric CC stretching) and ν 3 (symmetric HCH bending) are excited simultaneously. The measured vibrational frequencies show no abnormal isotopic effect if the assignment given in the literature for the ν 2 and ν 3 modes in C 2H 4 + are reversed and if a stronger excitation of the ν 3 symmetric bending mode in the least deuterated compounds is assumed. The vibrational modes most strongly excited in the second and third electronic bands are ν 1 (symmetric CH stretching) and ν 3, and in the fourth band ν 3.