The 1.66 μm spectrum of the ethynyl radical, CCH.

• New high resolution spectra of ethynyl radical in the 1.65 μm region. • Improved parameters for the lowest few levels of the radical are obtained. • The spectra will potentially be useful for monitoring CCH in chemical systems Frequency-modulated diode laser transient absorption spectra of the ethynyl radical have been recorded at wavelengths close to 1.66 μ m. The observed spectrum includes strong, regular, line patterns. The two main bands observed originate in the ground X ̃ 2 Σ + state and its first excited bending vibrational level of 2 Π symmetry. The upper states, of 2 Σ + symmetry at 6055.6 cm−1 and 2 Π 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 Π level and J = 29/2 for the 2 Σ one. Additionally, a weaker series of lines have been assigned to absorption from the second excited bending, (0 2 0), level of 2 Σ symmetry, to a previously observed state of 2 Π 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. [ABSTRACT FROM AUTHOR]