Neutral dissociation and non Franck-Condon effects in valence excited N and O molecules

Sammanfattning: Neutral photodissociation processes and non Franck-Condon effects in the N2 and O2 molecules induced by excitation with synchrotron radiation photons have been studied in the 15-35 eV region. Neutral dissociation processes have been studied by monitoring dispersed fluorescence from excited N and O atoms. The excited atoms are created through excitations to members of the molecular Rydberg series, which predissociate yielding excited atomic fragments. From comparison with calculations performed using many-body perturbation theory it follows that predissociations of N2 Rydberg series are caused by a number of non-Rydberg doubly excited resonances whereas predissociation of O2 Rydberg series occur after interaction with close-lying singly and doubly excited valence states of the appropriate symmetry. Photoelectron spectroscopy has been used to study photoionization of the N2 molecules to the N2+ A 2Πu state and photoionization of the O2 molecules to the O2+ X 2Πg state. Dispersed fluorescence emitted from the O2+ molecules has been measured to study the photoionization to the b 4Σ-g state of O2+. All three photoionization channels exhibit strong non Franck-Condon effects in their experimental vibrational branching ratios. In the case of the b 4Σ-g state of O2+, these effects seems to be originated by the narrow 3σg σu shape resonance coupled to the 1πu πg channel. Conversely, the σu shape resonance does not control the ionization to the O2+ X 2Πg and to the N2+ A 2Πu states. Photoionization of the O2 πg electron is not fully explained by autoionization from known Rydberg series and valence states, which suggest the existence of unknown process affecting this photoionization channel. On the contrary, the Franck-Condon breakdown in the ionization to the N2+ A 2Πu state in the studied region is attributed to autoionization from Rydberg and valence states of N2.

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