Molecular Structure and Dynamics Beyond the Born-Oppenheimer Approximation

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The Experimental Physical Chemistry Program is supporting Edward R. Grant of Purdue University to continue his research on state-to-state photoionization and the dynamics of intramolecular relaxation in highly excited states of small molecules. Systems to be studied include a set of fundamentally important oxy-hydrocarbon and metal-centered free radicals, relevant as intermediates in air pollution, combustion and electronic materials processing. Multiresonant excitation techniques will be used to make high-resolution spectroscopic measurements of resonant lineshapes and positions for final states coupled to ionization and dissociation continua. These observations will lay an experimental foundation for collaborative theoretical work, designed to achieve a fundamental understanding of the interaction between core-vibrational and extravalent-electronic degrees of freedom, and to determine the source of mode selectivity in the vibrational autoionization of polyatomic molecules. Photoselection and specialized detection methods will also be used to study rovibrational structure in the electronic ground states of cations and precursor free-radicals: zero-kinetic-energy threshold photoionization measurements will establish vibrational-rotational level spectra of closed-shell oxy-hydrocarbon cations, characterizing potential energy surfaces and anharmonic coupling from zero-points to thresholds for intramolecular proton transfer. Similar measurements on Group III hydride cations will follow progressions of vibrational resonances up to and beyond thresholds for diatomic hydrogen elimination. Vibration-rotation spectra obtained by state-to-state photoionization - bleaching methods will address outstanding issues of structure and symmetry in free-radical ground states. The detailed dynamics of energy transfer between electronic and internuclear degrees of freedom has significant practical consequences. In nature, for example, photosynthesis begins with selective intramolecular energy migration, followed by charge separation and electron transfer. In an extension of his previous research effort, Grant will now include studies on main-group free radical systems, chosen for their relevance to thermochemical issues in combustion, atmospheric chemistry and electronic materials processing. As an example, the group III radical hydrides mentioned above are important intermediates in the metallization and p-doping of silicon devices by chemical vapor deposition.