This report describes the results of a program of research on the thermochemistry, spectroscopy and intramolecular relaxation dynamics of the combustion intermediate, HCO. We prepare this radical from acetaldehyde as a photo-precursor in a differentially pumped laser-ionization source quadrupole mass spectrometer. Using a multiresonant spectroscopic technique established in our laboratory, we select individual rotational states and overcome Franck-Condon barriers associated with neutral-to-cation geometry changes to promote transitions to individual autoionizing series and state-resolved ionization thresholds. Systematic analysis of rotational structure and associated lineshapes provide experimental insight on autoionization dynamics as input for theoretical modeling. Extrapolation of series, combined with direct thresholdphotoelectron detection, yield precise ionization potentials that constitute an important contribution to the thermochemical base of information on HCO.
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