The 584 Aring; photoelectron spectrum of ethylene was reexamined at higher resolution using a supersonic molecular beam source for rotational cooling. The vibrational frequencies of the C2H+4Xtilde;thinsp;2B3state were measured with improved accuracy, which permitted the torsional potential energy curve to be characterized. The measured equilibrium torsional angle is 27deg;ast;2deg;, and the inversion barrier is 270plusmn;150 cmminus;1. TheAtilde;thinsp;2B3state 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. TheBtilde;thinsp;2Astate correlation function indicates a rapid decay mechanism is operative. The broadened vibrational structure in the fourth band results from a saddle point in theCtilde;thinsp;2B2potential energy surface from which the ion relaxes to theXtilde; state by moving along a repulsive degree of freedom, which in this case is internal rotation. TheCtilde; state vibrational broadening as well as the decay of the correlation function yield a lifetime for this process of 7 fs.
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