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Frequency-modulated diode laser transient absorption spectra of the ethynyl radical have been recorded at wavelengths close to 1.66 $��$m. The observed spectrum includes strong, regular, line patterns. The two main bands observed originate in the ground $\tilde{X}\,^2��^+$ state and its first excited bending vibrational level of $^2��$ symmetry. The upper states, of $^2��^+$ symmetry at 6055.6 cm$^{-1}$ and $^2��$ symmetry at 6413.5 cm$^{-1}$, respectively, had not previously been observed and the data were analyzed in terms of an effective Hamiltonian representing their rotational and fine structure levels to derive parameters which can be used to calculate rotational levels up to J = 37/2 for the $^2��$ level and J = 29/2 for the $^2��$ one. Additionally, a weaker series of lines have been assigned to absorption from the second excited bending, (020), level of $^2��$ symmetry, to a previously observed state of $^2��$ symmetry near 6819 cm$^{-1}$. These strong absorption bands at convenient near-IR laser wavelengths will be useful for monitoring CCH radicals in chemical systems.

We report the observation and analysis of spectra in part of the near-infrared spectrum of C$_2$H, originating in rotational levels in the ground and lowest two excited bending vibrational levels of the ground $\tilde{X}\,^2��^+$ state. In the analysis, we have combined present and previously reported high resolution spectroscopic data for the lower levels involved in the transitions to determine significantly improved molecular constants to describe the fine and hyperfine split rotational levels of the radical in the zero point, $v_2=1$ and the $^2��^+$ component of $v_2=2$. Two of the upper state vibronic levels involved had not been observed previously. The data and analysis indicate the electronic wavefunction character changes with bending vibrational excitation in the ground state and provide avenues for future measurements of reactivity of the radical as a function of vibrational excitation., 16 pages including references, 2 figures and 2 tables