THE DISSOCIATION DYNAMICS OF LOW-LYING TRIPLET-STATES IN BUTANE AND BUTENE MOLECULES

Butane, methylpropane, 1-,2-butene and isobutene are studied as a complete set of doubly methylated ethane and ethene molecules in order to probe the dissociation dynamics of their excited states. The butane and butene molecules are formed in electronically highly excited states by neutralization of their respective radical cations in collisions with alkali metal atoms. Processes that lead to dissociations into two fragments are measured with differential translational spectroscopy. It is found that the first triplet state is dominantly populated. Butanes exhibit direct dissociation of a carbon-carbon (C-C) bond via a repulsive state. Intramolecular vibrational energy redistribution (IVR) prior to dissociation is observed for the unsaturated butenes. For the butenes the C-C bond cleavage is even preceded by a hydrogen shift, resulting in molecular losses like methane and ethene. In this paper we stress the relations between the present results and optical excitation and electron scattering experiments. For butane the proposed dissociation via a repulsive triplet state is corroborated in ab initio calculations.