Communication: On the first ionization threshold of the C2H radical.

The slow photoelectron spectrum of the ethynyl radical has been recorded for the first time by using the DESIRS beamline of the SOLEIL synchrotron facility. Ethynyl was generated using a microwave discharge flow tube. The observation of the X+3Π ← X²Σ+ transition allowed the first direct measurement of the adiabatic ionization threshold of this radical (EI = 11.641(5) eV). The experimental results are supported by ab initio calculations. Our preliminary investigation of the cationic ground state potential energy surfaces predicts a non-negligible Renner-Teller effect which has not been discussed previously. [ABSTRACT FROM AUTHOR]