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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.

The vacuum UV photoabsorption spectrum of CH 3 F has been recorded between 7 and 24 eV and has been analyzed in detail. Broad and structureless peaks are observed over the entire photon energy range. They are all assigned to transitions to Rydberg states, members of series converging successively to the 2e -1 , 5a-1 and 1e -1 ionization limits. These features are underlying very long series of narrow and weak structures in the range of 10-13.2 eV. Through a continuum subtraction procedure, about 70 lines could clearly be identified. These have been assigned to long vibrational progressions belonging to Rydberg states corresponding to the 2e → 3p and 2e → 6s/5d configurations.

In this paper, we report the He(I) photoelectron spectrum (PES) and the threshold-photoelectron (TPES) spectrum of C 2 H 3 Br. The fine structure observed in the first two ionic states in the He(I) spectrum is assigned to progressions belonging partially to previously unobserved vibrational normal modes. The TPES has been measured between 9.0 and 25.0 eV, and the photon energy range of 9.8-12.0 eV has been investigated in more detail. Extensive calculations with the GAUSSIAN set of programs have been performed to help in the assignment of the observed features. Furthermore, a conical intersection between the à 2 A" and the 2 A" states was found to take place along the C-Br stretching coordinate. Intramolecular dynamics of the 2 A" state, initially prepared in the Frank-Condon region, was probed by the Fourier transform of the spectrum. The 2 A" state is almost readily depleted, most probably due to a very effective internal conversion through the conical intersection.