The photoionization spectrum of vinyl radical is reported, from its observed threshold to 1160 Aring;. Two methods of preparation have been employed; (a) the abstraction reaction of F atoms with C2H4, and (b) the pyrolysis of divinyl mercury at 1200 K. In both experiments, relatively sharp autoionization structure is observed, and interpreted as a Rydberg series converging to the excited3Alsquo;state of vinyl cation. The analysis leads to an adiabatic ionization energy of sim;10.7 eV for this state, with a structure similar to that of vinyl radical but with an increased Cndash;C distance. The observed ionization threshold for the ground state of vinyl cation is 8.59plusmn;0.03 eV with the F atom reaction, and 8.43plusmn;0.03 eV with the pyrolysis method. The lower value in the latter experiment is interpreted as a hot band. The relatively low value of the photoionization cross section near threshold implies a large geometry change between vinyl radical and ground state vinyl cation. A progression in the inhyphen;plane Cndash;H bending vibration is indicated in the photoionization spectrum; it is quite possible that the vibrational 0ndash;0 transition lies one quantum lower than our detected limit. With this bracketed adiabatic ionization potential and the appearance potential of C2H+3(C2H4), a Cndash;H bond energy in ethylene of 107ndash;110 kcal/mol (0 K) is deduced.
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