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.

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Hydrogen loss from the toluene molecular ion generates benzylium (Bz + ) and tropylium (Tr + ) ions via two competitive and independent pathways. The corresponding kinetic energy release distributions (KERDs) have been determined under various conditions in the metastable time window for toluene and perdeuterated toluene and have been analyzed by the maximum entropy method (MEM). The isomeric fraction Tr + /Bz + is found to be equal to 0.9 ( 0.3, in good agreement with the values obtained using photodissociation and charge exchange experiments. It is, however, in disagreement with the value 5 ( 2 deduced by Moon, Choe, and Kim (J. Phys. Chem. A 2000, 104, 458) from KERD measurements. The origin of the discrepancy is suggested to be the inadequacy of the orbiting transition state theory (OTST) for the calculation of KERDs in hydrogen loss reactions. For both channels, more translational energy is released in the reaction coordinate than would be expected on statistical grounds because of the presence of a barrier along the reaction path. For the Bz + channel, the barrier entirely results from centrifugal effects. Rotational energy is converted into translation as a result of angular momentum conservation. Deuteration is observed to reduce the importance of the rotational energy flow in the reaction coordinate. The Tr + channel is characterized by the presence of a reverse activation energy barrier of electronic origin. The energy in excess of the dissociation asymptote can be partitioned into two components: the reverse barrier plus a nonfixed energy contribution. About 40% of the reverse barrier is converted into relative translational motion of the fragments. Here again, a lower fraction of the nonfixed energy flows into translation for the deuterated isotopomer.

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The kinetic energy release distributions (KERDs) for the fluorine atom loss from the 1,1-difluoroethene cation have been recorded with two spectrometers in two different energy ranges. A first experiment uses dissociative photoionization with the He(I) and Ne(I) resonance lines, providing the ions with a broad internal energy range, up to 7 eV above the dissociation threshold. The second experiment samples the metastable range, and the average ion internal energy is limited to about 0.2 eV above the threshold. In both energy domains, KERDs are found to be bimodal. Each component has been analyzed by the maximum entropy method. The narrow, low kinetic energy components display for both experiments the characteristics of a statistical, simple bond cleavage reaction: constraint equal to the square root of the fragment kinetic energy and ergodicity index higher than 90%. Furthermore, this component is satisfactorily accounted for in the metastable time scale by the orbiting transition state theory. Potential energy surfaces corresponding to the five lowest electronic states of the dissociating 1,1-C2H2F2+ ion have been investigated by ab initio calculations at various levels. The equilibrium geometry of these states, their dissociation energies, and their vibrational wavenumbers have been calculated, and a few conical intersections between these surfaces have been identified. It comes out that the ionic ground state X2B1 is adiabatically correlated with the lowest dissociation asymptote. Its potential energy curve increases in a monotonic way along the reaction coordinate, giving rise to the narrow KERD component. Two states embedded in the third photoelectron band (B2A1 at 15.95 eV and C2B2 at 16.17 eV) also correlate with the lowest asymptote at 14.24 eV. We suggest that their repulsive behavior along the reaction coordinate be responsible for the KERD high kinetic energy contribution.

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The kinetic energy release distributions (KERDs) associated with the hydrogen loss from the benzene cation and the deuterium loss from the perdeuteriobenzene cation have been remeasured on the metastable time scale and analyzed by the maximum entropy method. The experimental kinetic energy releases are larger than expected statistically, in contradistinction to what has been observed for the C-X fragmentations of the halogenobenzene cations. H(D) loss from C(6)H(6)(+) (C(6)D(6)(+)) occurs via a conical intersection connecting the (2)A(2) and (2)A(1) electronic states. Two models are proposed to account for the experimental data: (i) a modified orbiting transition state theory (OTST) approach incorporating electronic nonadiabaticity; (ii) an electronically nonadiabatic version of the statistical adiabatic channel model (SACM) of Quack and Troe. The latter approach is found to be preferable. It leads to the conclusion that the larger the energy stored in the transitional modes, which partly convert to the relative interfragment motion, the shorter the value of the reaction coordinate at which the adiabatic channels cross, and the larger the probability of undergoing the (2)A(2) --> (2)A(1) transition required for hydrogen loss.

A previous interpretation of the kinetic energy release distribution (KERD) observed in the fragmentation of protonated fluorobenzene is confirmed by a maximum entropy analysis. The KERD is bimodal, with an intense and broad component due to the production of the cyclic phenyl ion. Ab initio calculations indicate the existence of several open-chain isomers, with an energy about 1.0-1.2 eV higher than that of the phenyl ring. A weaker component of the KERD corresponds to the formation of one or several of these acyclic structures. Its abundance represents about 6% of the main component (but drops down to 3% for the perdeuterated isomer). More translational energy than the statistical estimate is released during the dissociation process because of the presence of a barrier along the reaction path leading to the generation of the cyclic phenyl ion. About one-half of the energy of the barrier is released as translation, thereby indicating the operation of strong exit-channel interactions between separating fragments.