Fluorescence of excited ethynyl radicals produced by pulsed vacuum ultraviolet photolyses of C2H2, C2D2, and C2HBr

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Abstract
Electronically excited ethynyl radicals were produced by pulsed vacuum ultraviolet photolyses of C2H2, C2D2, and C2HBr. The broad fluorescence spectra of the ethynyl radical extending from 410 to wavelengths longer than 750 nm were found to be insensitive to parent molecules photolyzed, their pressures, and also pressures of an added gas, He. Time profiles of the fluorescence intensity were found to be a superposition of two components. The dominant component is due to excited ethynyl radicals formed within the excitation pulse duration of ∼20 ns. At higher pressures, however, some minor contribution from a long‐lived precursor to the fluorescence was detected. Zero‐pressure decay rates and quenching rate constants were obtained. Formation mechanism of fluorescent ethynyl radicals is discussed briefly.