Rotationally resolved pulsed-field-ionization zero-kinetic-energy photoelectron spectra of the X̃→X̃+ transition in ethylene and ethylene-d4 have been recorded at a resolution of 0.09 cm−1. The spectra provide new information on the large amplitude torsional motion in the cationic ground state. An effective one-dimensional torsional potential was determined from the experimental data. Both C2H4+ and C2D4+ exhibit a twisted geometry, and the lowest two levels of the torsional potential form a tunneling pair with a tunneling splitting of 83.7(5) cm−1 in C2H4+ and of 37.1(5) cm−1 in C2D4+. A model was developed to quantitatively analyze the rotational structure of the photoelectron spectra by generalizing the model of Buckingham, Orr, and Sichel [Philos. Trans. R. Soc. London, Ser. A 268, 147 (1970)] to treat asymmetric top molecules. The quantitative analysis of the rotational intensity distributions of allowed as well as forbidden vibrational bands enabled the identification of strong vibronic mixing between the + and Ã+ states mediated by the torsional mode ν4 and a weaker mixing between the + and + states mediated by the symmetric CH2 out-of-plane bending mode ν7. The vibrational intensities could be accounted for quantitatively using a Herzberg–Teller-type model for vibronic intensity borrowing. The adiabatic ionization energies of C2H4 and C2D4 were determined to be 84 790.42(23) cm−1 and 84 913.3(14) cm−1, respectively.

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