Ultraviolet Absorption Spectrum and Cross-Sections of Ethynyl (C2H) Radicals
This work reports the ultraviolet absorption detection and cross-sections of ethynyl (C2H) radicals in the wavelength range of 235 nm to 270 nm at T=298 K. Ethynyl radicals were produced from the single photon 193 nm excimer laser photolysis of dilute mixtures of C2HCF3 in He or C2H2 in He at a total pressure of about 20 kPa (150 Torr). Gas chromatographic analysis of the photolyzed samples showed diacetylene (C4H2) as the major stable product. Addition of methane in the photolysis mixtures resulted in reduction of diacetylene and production of methylacetylene and ethane. A transient absorption, within the spectral range of 235 nm to 270 nm, using real-time UV-absorption spectroscopy, was detected. The absorption spectra obtained from the two C2HCF3/He and C2H^2^/He radical sources were nearly identical indicating that the transient absorption can be attributed to the ethynyl radical. The observed ultraviolet spectrum exhibits a relatively broad absorption feature with some structures and a peak absrption at about 243.5 nm. The cross-section at the maximum absorption is about 7 x 10-19 cm2 molecule-1. The UV absorption band, detected in this work, can be assigned to the transitions from the ground electronic state (Χ2Σ+) and also possibly from coupled ground (Χ2Σ+) and lowest electronic (A2Π) states to the electronic state B (or 32A') of C2H. This assignment is based on previously reported high level ab initio molecular orbital calculations and spectroscopic information derived from recent laser-induced fluorescence studies of the ethynyl radical.