The 1.66 μm spectrum of the ethynyl radical, CCH

Abstract

Frequency-modulated diode laser transient absorption spectra of the ethynyl radical have been recorded at wavelengths close to 1.66 μm. The observed spectrum includes strong, regular, line patterns. The two main bands observed originate in the ground X̃2Σ+ state and its first excited bending vibrational level of 2Π symmetry. The upper states, of 2Σ+ symmetry at 6055.6 cm−1 and 2Π 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 2Π level and J = 29/2 for the 2Σ one. Additionally, a weaker series of lines have been assigned to absorption from the second excited bending, (0 2 0), level of 2Σ symmetry, to a previously observed state of 2Π 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.