Although highly vibrationally excited (HVE) radicals are ubiquitous in natural environments, the effect of collisional energy transfer (ET) on their reactivity has yet to be fully characterized. We have used time-resolved IR emission spectroscopy to characterize the vibrational-to-translational quenching of a small HVE radical, ketenyl (HCCO), by inert gases. Photolysis of ethyl ethynyl ether at 193 nm provides HVE HCCO in the X̃(2)A″ electronic ground-state, with a nascent internal energy of 2.2 ± 0.6 eV. IR emission is monitored as an indicator of vibrational energy, and spectral modeling allows direct determination of the average energy lost per collision as a function of the internal energy. Collisional deactivation of HVE HCCO is shown to be minimally an order of magnitude more efficient than closed-shell molecules of comparable size. Schwartz-Slawsky-Herzfeld-Tanczos (SSHT) theory, modified for HVE molecules, suggests that this ET enhancement is due to a strong attractive intermolecular interaction.
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