Spectroscopy, predissociation dynamics, and reactions of free-radical complexes

Leanna C Giancarlo, University of Pennsylvania


Information on the potential energy surfaces between two partners can be elucidated from the intermolecular vibrational levels of a weakly bound complex formed from these components. We have provided information on the attractive well region for OH A $\sp2\Sigma\sp+$ + Ar by investigating OH-Ar complexes using electronic spectroscopy: identification of bound intermolecular vibrational levels have enabled a refinement to a semi-empirical potential to be made. We have also explored the predissociation dynamics of these OH-Ar complexes prepared in levels lying above the OH A $\sp2\Sigma\sp+$ + Ar asymptote. These studies which examined both the internal rotational and vibrational predissociation dynamics for OH-Ar yield information on the repulsive wall region of the potential. A novel variation of stimulated emission pumping has allowed us to obtain quantitative product rotational state distributions for the OH A $\sp2\Sigma\sp+$ fragments resulting from OH-Ar predissociation. The energies, lifetimes, and OH A $\sp2\Sigma\sp+$ product distributions are compared with current theoretical calculations. Recent studies of the OH-N$\sb2$ complex in the OH $\rm A\ \sp2\Sigma\sp+ - X\ \sp2\Pi\ 1 - 0$ spectral region have explored the vibrational predissociation and quenching dynamics occurring on the OH A $\rm\sp2\Sigma\sp+\ (v\sp\prime = 1) + N\sb2$ potential energy surface. Using laser-induced fluorescence, we have identified 28 features which are attributed to excitation of the OH-N$\sb2$ complex to intermolecular vibrational levels supported with OH A $\rm\sp2\Sigma\sp+\ (v\sp\prime = 1) + N\sb2$. Dispersed fluorescence measurements reveal that our ability to detect OH-N$\sb2$ features in the 1 $-$ 0 region is due to the fact that the complex undergoes vibrational predissociation on a timescale which is rapid compared to radiative decay; this predissociation process yields OH A $\sp2\Sigma\sp+$ fragments which then fluoresce. In similar fluorescence excitation studies in the OH $\rm A - X\ 0 - 0$ region, no bound complex features were detected, presumably due to rapid electronic quenching. Using a double resonance fluorescence depletion technique, 95 OH-N$\sb2$ features have been identified in this spectral region. Comparison between the features observed in both regions enables us to determine that the timescale for vibrational predissociation is $<$200 fs. This information is meant to stimulate theoretical calculations on this interesting system.

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Recommended Citation

Giancarlo, Leanna C, "Spectroscopy, predissociation dynamics, and reactions of free-radical complexes" (1996). Dissertations available from ProQuest. AAI9627925.