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The near-infrared spectrum of ethynyl radical

Transient diode laser absorption spectroscopy has been used to measure three strong vibronic bands in the near infrared spectrum of the C$_2$H, ethynyl, radical not previously observed in the gas phase. The radical was produced by ultraviolet excimer laser photolysis of either acetylene or (1,1,1)-trifluoropropyne in a slowly flowing sample of the precursor diluted in inert gas, and the spectral resolution was Doppler-limited. The character of the upper states was determined from the rotational and fine structure in the observed spectra and assigned by measurement of ground state rotational combination differences. The upper states include a $^2Σ^+$ state at 6696 cm$^{-1}$, a second $^2Σ^+$ state at 7088 cm$^{-1}$, and a $^2Π$ state at 7110 cm$^{-1}$. By comparison with published calculations (R. Tarroni and S. Carter, \textit{J. Chem. Phys} \textbf{119}, 12878 (2003) and \textit{Mol. Phys}. \textbf{102}, 2167 (2004)), the vibronic character of these levels was also assigned. The observed states contain both $X^2Σ^+$ and $A^2Π$ electronic character. Several local rotational level perturbations were observed in the excited states. Kinetic measurements of the time-evolution of the ground state populations following collisional relaxation and reactive loss of the radicals formed in a hot, non-thermal, population distribution were made using some of the strong rotational lines observed. The case of C$ _{2} $H may be a good place to investigate the behavior at intermediate pressures of inert colliders, where the competition between relaxation and reaction can be tuned and observed to compare with master equation models, rather than deliberately suppressed to measure thermal rate constants.