Dissociacao Directa e Estatistica de Ioes Negativos por Colisao Atomo-Molecula

The electron transfer process in binary collisions between alkali atoms and several neutral molecular targets, was studied using the molecular beam technique. The collision energy used in the laboratory ranged from 10 eV up to 1 KeV, allowing to study the collision process in their several dynamical regimes and inelastic channels involved. For these purposes, experimental measurements of the ionic yields as a function of the collision energy, total cross sections corresponding to the formation of negative and positive ions, energy loss spectra and differential cross sections with energy loss selection, were performed. The experimental results were analysed using theoretical models, and information about the dynamics of dissociation of the polyatomic negative ions formed in the collision was achieved. The exact analytical solutions which describe the classical nuclear motion of a diatomic type system in a vibronic final state represented by a Morse potential after an electronic transition take place were obtained. Using these solutions, some analytical expressions corresponding to the total cross sections of ion-pair formation in atom-molecule collisions, were deduced. Those solutions, in some cases, can give insight into the timing of the bondstretching in comparison with the autodetachment and collision times. A special experimental remark was given to those molecular targets in which the parent ion has a long enough lifetime to be observed and there is intramolecular rearrangement before the dissociation have occurred. From time-of-flight spectra it was possible to get information on the realaxation times and kinetic energy release (KER) in the dissociation. In some aromatic targets there is even time enough to occur scrambling of energy resulting in a statistical dissociation. It was shown that in the case of collisions of potassium atoms with nitromethane, the valence electron of the alkali atom is promoted to three different states of the parent negative ion, and the dissociation of the two #pi#"* states leads to dominant formation of O"-, either by direct dissociation or by previous internal rearrangement of the parent ion. Besides, the transition of the electron to a #sigma#"* state leads predominantly to the formation of NO_2_- and free electrons, these ones produced by autodetachment of the transient CH_3NO_2_-ion. It seems reasonable to conclude from the several targets studied that for molecules exhibiting electronic delocalization, there is always a next LUMO #pi#"* involved in the electronic transition. In which concerns positive molecular ionization it was clearly shown that ionic of the species formed in the collision are quite different from those formed by direct ionization and at collision energies of the order of few hundreds of eV, the positive ionization competes efficiently with negative ionizationAvailable from Fundacao para a Ciencia e a Tecnologia, Servico de Informacao e Documentacao, Av. D. Carlos I, 126, 1200 Lisboa / FCT - Fundação para o Ciência e a TecnologiaSIGLEPTPortuga