The 1.66 $\mu$m Spectrum of the Ethynyl Radical, CCH

TL;DR

This study records and analyzes the near-infrared spectrum of the ethynyl radical CCH, identifying new excited states and providing detailed rotational parameters to aid in monitoring this radical in chemical environments.

Contribution

First observation and detailed spectral analysis of new excited states of CCH near 1.66 μm, expanding understanding of its energy levels and structure.

Findings

Identified two new excited states of CCH at 6055.6 and 6413.5 cm$^{-1}$.

Derived rotational and fine structure parameters for these states.

Assigned weaker lines to a second excited bending level.

Abstract

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.