Torsional potential and intramolecular dynamics in the C2H+4 photoelectron spectra

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Abstract
The 584 Å photoelectron spectrum of ethylene was reexamined at higher resolution using a supersonic molecular beam source for rotational cooling. The vibrational frequencies of the C2H+4X̃ 2B3 state were measured with improved accuracy, which permitted the torsional potential energy curve to be characterized. The measured equilibrium torsional angle is 27°*2°, and the inversion barrier is 270±150 cm−1. The à 2B3 state is shown to be in reasonable agreement with a recent theoretical calculation which takes into account strong nonadiabatic effects. The correlation function calculated for this state is consistent with this calculation and recent calculations on the unimolecular dissociation of this state. The B̃ 2A state correlation function indicates a rapid decay mechanism is operative. The broadened vibrational structure in the fourth band results from a saddle point in the C̃ 2B2 potential energy surface from which the ion relaxes to the X̃ state by moving along a repulsive degree of freedom, which in this case is internal rotation. The C̃ state vibrational broadening as well as the decay of the correlation function yield a lifetime for this process of 7 fs.