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Using synchrotron radiation, the threshold photoelectron (TPES) spectrum of C 2 H 3 Cl and constant ion state (CIS) spectroscopy of C 2 H 3 Cl, are reported. For comparison, the He(I) photoelectron spectrum has also been measured and reexamined. The threshold photoelectron spectrum has been measured between 9.0 and 25.0 eV, and the photon energy range of 9.9-12.0 eV has been investigated in detail. Many features have been identified and tentatively assigned with the help of the photoabsorption spectroscopic results [1]. These data were compared with a well-resolved He(I) photoelectron spectrum. The fine structure observed in the two first ionic states is assigned to progressions belonging partially to previously unobserved vibration normal modes. State-selected CIS spectra have been recorded for the first vibronic states between 10.0 and 11.67 eV. They exhibit fine structure assigned to autoionization of Rydberg states.

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A retarding field technique coupled with a quadrupole mass analyzer has been used to obtain the kinetic energy release distributions ͑KERDs͒ for the C 2 H 3 Br ϩ →͓C 2 H 3 ͔ ϩ ϩBr dissociation as a function of internal energy. The KERDs obtained by dissociative photoionization using the He͑I͒, Ne͑I͒, and Ar͑II͒ resonance lines were analyzed by the maximum entropy method and were found to be well described by introducing a single dynamical constraint, namely the relative translational momentum of the fragments. Ab initio calculations reveal the highly fluxional character of the C 2 H 3 ϩ ion. As the energy increases, several vibrational modes are converted in turn into large-amplitude motions. Our main result is that, upon increasing internal energy, the fraction of phase space sampled by the pair of dissociating fragments is shown to first decrease, pass through a shallow minimum around 75%, and then increase again, reaching almost 100% at high internal energies ͑8 eV͒. This behavior at high internal energies is interpreted as resulting from the conjugated effect of intramolecular vibrational redistribution ͑IVR͒ and radiationless transitions among potential energy surfaces. Our findings are consistent with the coincidence data of Miller and Baer, reanalyzed here, and with the KERD of the metastable dissociation.