Molecular Structure and Dynamics Beyond the Born-Oppenheimer Approximation

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In this project of the Physical Chemistry Program in the Chemistry Division Prof. E.R. Grant of Purdue University will pursue a program of research on the state-to-state-dynamics of intramolecular relaxation and electron ejection in highly excited small molecules. Specific systems to be investigated are hydrocarbon and metal-centered free radical molecules. Multiple-resonance laser methods will be used to isolate individual transitions that bring the molecules close to their ionization thresholds. Experiments will also be performed to obtain high-resolution information on ionization potentials, photoionization cross sections of free radicals and the rotational-vibrational structure of the corresponding cation, as well as on the vibrational structure of excited electronic states that lie below the adiabatic ionization thresholds. %%% The conventional view of the properties of a polyatomic molecule is based on the Born-Oppenheimer (B-O) approximation according to which its total energy is the sum of independent energies associated with the translational, rotational, vibrational, and electronic degrees of freedom, the actual non-vanishing couplings between these being assumed to be small and negligible in the first approximation. Chemical reactions, however, often involve highly excited electronic and vibrational degrees of freedom and the B-O approximation may be of questionable validity. Processes that involve energies near or above the first ionization threshold, where the dynamics of intramolecular energy relaxation leads in particular to electron ejection, are not amenable to be treated in the B-O approximation. This research concerns fundamental questions about the processes that occur in excited molecular states near the ionization threshold and will provide information suitable for the development of theoretical models of chemical reactivity.