The unimolecular reactions of the fluoromethane ion have been investigated by the threshold photoelectron photoion coincidence technique (TPEPICO). The breakdown curves have been measured in the energy range between the adiabatic ionization potential of 12.53 eV and 21 eV. In this energy range the formation of the CH 2 F + , CHF + , CH 2 + , CH 3 + , and CF + fragment ions is observed. The appearance energies (AE) for these ions at 300 K are 13.2, 13.91, 13.93, 14.51 and < 17.7 eV respectively. The 0 K threshold energy for the formation of CH 2 F + ions is 13.37 eV. Two different pathways for the unimolecular reaction of the fluoromethane ion can be distinguished. The reaction proceeding through the electronic ground state of the ion is entirely statistical. In contrast to this the reaction occurring through the first electronic excited state shows nonstatistical behaviour. The two pathways are distinguished via the kinetic energy released in the respective dissociation channels. While all thermochemically allowed reaction channels are observed on the ion ground state potential, the first excited state seems to decay predominantly by F loss reaction.

Abstract. The formation of Ar + ions has been investigated by means of the threshold photoelectron photoion coincidence (TPEPICO) technique. Two pathways for the formation of Ar 2 ions are important. One is a direct path via excitation of Rydberg states of Ar 2 with consecutive autoionization. The other path is dissociative ionization of larger argon clusters, in this case argon trimers. These two pathways lead to Ar] ions with different internal energy. The pathways are easily distinguished in the TPEPICO-TOF spectra by the kinetic energy released (KER) in the dissociative ionization. The KER for the reaction Ar~--+Ar2 v +Ar was measured as a function of the photon energy and compared to the KER expected from statistical theory. The agreement is satisfying and confirms that Ar3 + ions do indeed dissociate at the thermochemical threshold. At higher photon energy the excited 2H (3/2)g state of Ar + is also detected from a second component in the KER. By applying a kinetic energy discrimination it is possible to measure cluster ion spectra in the presence of larger dusters but essentially without interference from the latter.

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A threshold photoelectron photoion coincidence experiment has been setup which allows to measure breakdown curves of energy selected ions at different sample temperatures. These studies reveal the rotational energy of the transition state in unimolecular reactions, which in turn is used as a probe for the transition state structure. According to a model recently suggested this should allow the distinction of tight and loose transition states. The breakdown curves of methane and ethane ions in the region of the lowest reaction channel have been measured at 300 K and close to 0 K. In the case of the H loss from methane ions the shift between these two breakdown curves reveals that the dissociation energy Eo varies significantly with the rotational temperature as expected for a simple bond fission (loose transition state). In the case of H2 loss from ethane ions the shift between the breakdown curves at 300 and close to 0 K shows that E0 (J) does not vary significantly with the rotational temperature. This is to be expected for a concerted elimination reaction (tight transition state).