Communication: On the first ionization threshold of the C₂H radical

Area of Science:

• Physical Chemistry
• Molecular Spectroscopy
• Quantum Chemistry

Background:

• The ethynyl radical (C₂H) is a key species in combustion and astrochemistry.
• Understanding its electronic structure is crucial for modeling these processes.
• Previous studies lacked direct experimental data on its ionization energy.

Purpose of the Study:

• To experimentally determine the adiabatic ionization energy of the ethynyl radical.
• To investigate the electronic states of the ethynyl radical cation.
• To provide benchmark data for theoretical calculations.

Main Methods:

• Slow photoelectron spectroscopy using the DESIRS beamline at SOLEIL synchrotron.
• Generation of ethynyl radicals via microwave discharge flow tube.
• Ab initio electronic structure calculations.

Main Results:

• First recording of the slow photoelectron spectrum of the ethynyl radical.
• Direct measurement of the adiabatic ionization threshold (E_I = 11.641(5) eV).
• Identification of the X⁺Π3←XΣ+2 transition.
• Prediction of a significant Renner-Teller effect in the cation's ground state.

Conclusions:

• The study provides the first experimental value for the adiabatic ionization energy of the ethynyl radical.
• The results validate theoretical calculations and offer new insights into the ethynyl radical cation's electronic structure.
• The predicted Renner-Teller effect warrants further investigation.