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The hydroxyl radical (OH) is a key oxidant in atmospheric and combustion chemistry. Recently, a sensitive and state-selective ionization method has been developed for detection of the OH radical that utilizes UV excitation on the A²Σ+-X²π transition followed by fixed 118 nm vacuum ultraviolet (VUV) radiation to access autoionizing Rydberg states [J. M. Beames et al., J. Chem. Phys. 134, 241102 (2011)]. The present study uses tunable VUV radiation generated by four-wave mixing to examine the origin of the enhanced ionization efficiency observed for OH radicals prepared in specific A²Σ+ intermediate levels. The enhancement is shown to arise from resonant excitation to distinct rotational and fine structure levels of two newly identified ²π Rydberg states with an A³π cationic core and a 3d electron followed by ionization. Spectroscopic constants are derived and effects due to uncoupling of the Rydberg electron are revealed for the OH ²π Rydberg states. The linewidths indicate a Rydberg state lifetime due to autoionization on the order of a picosecond. [ABSTRACT FROM AUTHOR]

In the atmosphere, a dominant loss process for carbonyl oxide intermediates produced from alkene ozonolysis is also an important source of hydroxyl radicals. The rate of appearance of OH radicals is revealed through direct time-domain measurements following vibrational activation of prototypical methyl-substituted Criegee intermediates under collision-free conditions. Complementary theoretical calculations predict the unimolecular decay rate for the Criegee intermediates in the vicinity of the barrier for 1,4 hydrogen transfer that leads to OH products. Both experiment and theory yield unimolecular decay rates of ca. 108 and 107 s-1 for syn-CH3CHOO and (CH3)2COO, respectively, at energies near the barrier. Tunneling through the barrier, computed from high level electronic structure theory and experimentally validated, makes a significant contribution to the decay rate. Extension to thermally averaged unimolecular decay of stabilized Criegee intermediates under atmospheric conditions yields rates that are six orders of magnitude slower than those evaluated directly in the barrier region. [ABSTRACT FROM AUTHOR]