Ultraviolet absorption spectrum and cross-sections of ethynyl (C 2H) radicals

Abstract

This work reports the ultraviolet absorption spectrum and cross-sections of ethynyl (C 2H) radicals in the wavelength range 235–260 nm, determined at T=298 K and at a total pressure of about 20 kPa (150 Torr). Ethynyl radicals were produced from the single photon 193 nm excimer laser photolysis of dilute mixtures of C 2HCF 3 or C 2H 2 in He. Gas chromatographic/mass spectroscopic analysis of the photolyzed samples showed diacetylene (C 4H 2) as the major stable product. Addition of methane in the photolysis mixtures resulted in reduction of diacetylene and production of methylacetylene and ethane, products of the reaction of ethynyl radicals with methane. Survey of the ultraviolet spectral region, employing time-resolved UV-absorption spectroscopy, resulted in detection of a transient absorption centered at about 243 nm. The spectra obtained from the 193-nm photolysis of both C 2HCF 3/He and C 2H 2/He radical precursors were nearly identical, suggesting that the absorption feature can be attributed to the ethynyl radicals. The observed ultraviolet spectrum exhibits a relatively broad absorption feature with some structures and an absorption peak at about 243.5 nm. The absorption cross-sections for ethynyl radicals have been determined in this work for the first time. The C 2H cross-section, at the maximum absorption, at 243.5 nm is (7.5±0.9)×10 −19 cm 2 molecule −1 . The stated uncertainty includes the random and systematic measurement errors. The UV absorption feature, detected in this work, can be assigned to the transitions from the ground electronic state (X 2 Σ +) and also possibly from coupled ground X 2 Σ + and lowest electronic (A 2 Π) states to the electronic state B (or 3 2A ′ ) of C 2H. This assignment is based on previously reported high level ab initio molecular orbital calculations and results of recent laser-induced fluorescence studies of the ethynyl radical.