Frequency-modulated diode laser transient absorption spectra of the ethynyl radical have been recorded at wavelengths close to 1.66 $\mu$m. The observed spectrum includes strong, regular, line patterns. The two main bands observed originate in the ground $\tilde{X}\,^2\Sigma^+$ state and its first excited bending vibrational level of $^2\Pi$ symmetry. The upper states, of $^2\Sigma^+$ symmetry at 6055.6 cm$^{-1}$ and $^2\Pi$ 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 $^2\Pi$ level and J = 29/2 for the $^2\Sigma$ one. Additionally, a weaker series of lines have been assigned to absorption from the second excited bending, (020), level of $^2\Sigma$ symmetry, to a previously observed state of $^2\Pi$ 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.

中文翻译：

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乙炔基 (CCH) 的 1.66 μm 光谱

已在接近 1.66 μm 的波长处记录了乙炔基的调频二极管激光瞬态吸收光谱。观察到的光谱包括强烈的、规则的、线条图案。观察到的两个主要带起源于地面 $\tilde{X}\,^2\Sigma^+$ 状态及其 $^2\Pi$ 对称性的第一个激发弯曲振动能级。先前未观察到分别在 6055.6 cm$^{-1}$ 处的 $^2\Sigma^+$ 对称和 6413.5 cm$^{-1}$ 处的 $^2\Pi$ 对称的上部状态并根据表示其旋转和精细结构水平的有效哈密顿量对数据进行分析，以推导出参数，这些参数可用于计算 $^2\Pi$ 水平和 J = 29/ 的最高 J = 37/2 的旋转水平2 表示 $^2\Sigma$ 一。此外，一系列较弱的谱线被指定为从第二次激发弯曲 (020)、$^2\Sigma$ 对称水平到先前观察到的 $^2\Pi$ 对称状态在 6819 cm$^{- 1}$。这些在方便的近红外激光波长处的强吸收带将有助于监测化学系统中的 CCH 自由基。