Analysis of the $\tilde{A}-\tilde{X}$ bands of the Ethynyl Radical near 1.48$\mu$m and Re-evaluation of $\tilde{X}$ State Energies

TL;DR

This study analyzes near-infrared spectra of the C2H radical to refine molecular constants, observe new vibronic levels, and explore how vibrational excitation affects its electronic structure and reactivity.

Contribution

It provides improved molecular constants for C2H's ground and excited vibrational states and reports the first observation of two upper vibronic levels.

Findings

Refined molecular constants for C2H's ground and excited vibrational states.

Observation of two previously unobserved vibronic levels.

Indications of changing electronic wavefunction character with vibrational excitation.

Abstract

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 $\tilde{X}\,^2\Sigma^+$ 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 $^2\Sigma^+$ 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.