The first part of the talk reviews two classic experiments that probe dynamics through conical intersections in the excited states of polyatomic molecules.  One used parent vibrational excitation to access bent geometries at the conical intersection, altering the product branching.  The other uses polarized emission spectroscopy to probe the evolution of molecular dissociation through a conical intersection.  The latter part of the talk focuses on ongoing experiments to elucidate the chemical mechanism by which an energetic material releases energy.  The initial chemical reaction, which may or may not be exothermic, is followed by a complicated sequence of elementary reactions that sum to the net exothermic reaction. Bernstein and co-workers have given strong empirical and computational evidence that for a series of nitro-containing energetic materials the nature of the initial step is different if the molecule is excited electronically than if it is excited thermally.  If the molecules are excited thermally, NO2 loss is the dominant initial step in the decomposition, but if decomposition is initiated by the molecule absorbing a UV photon, Bernstein has showed that dissociation via a conical intersection accesses a different region of the ground state potential energy near the nitrite isomer, so NO loss is the dominant initial elementary reaction. The experiments in my group focus on the next steps in the overall mechanism, after the thermally excited energetic material has undergone C-NO2 bond fission.  We use velocity map imaging and molecular beam scattering methods to directly probe the decomposition pathways of highly excited radical and molecular intermediates. We present our results on the unimolecular dissociation reactions of a key radical intermediate and a highly vibrationally excited molecular intermediate relevant to the thermal decomposition of 1,3,3-trinitroazetidine.

Photodissociation dynamics through conical intersections

243rd National Meeting (2012)

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