Characterisation of the first electronically excited state of protonated acetylene C₂H₃⁺ by coincident imaging photoelectron spectroscopy

ABSTRACT

We present a combined experimental and theoretical study of the threshold photoelectron spectroscopy of the vinyl radical encompassing the first triplet excited state of the vinyl cation. The radicals were produced in a flow-tube reactor by hydrogen abstraction of C₂H₄ and CH₄ using fluorine atoms generated in a microwave discharge. Vinyl was ionised with synchrotron vacuum ultraviolet radiation. A double imaging coincidence setup was used to record the threshold photoelectron spectrum. The experimental and simulated spectra show a marked adiabatic transition to the ${\widetilde{\mathbf{\text{a}}}}^{+}$ ³A′′ state with a short vibrational progression dominated by the C = C stretching mode. The adiabatic ionisation energy to this state is measured precisely at 10.747 ± 0.008 eV. In combination with the adiabatic ionisation energy to the ${\widetilde{\mathbf{\text{X}}}}^{+}$ ¹A₁ state from the Active Thermochemical Tables (ATcT), we find a singlet–triplet gap of 2.27 ± 0.01 eV (219 ± 1 kJ mol⁻¹). Calculated ionisation energies and Franck–Condon factors for the singlet ${\tilde{\mathbf{A}}}^{+}$ ¹A′′ excited state are also given.

GRAPHICAL ABSTRACT

KEYWORDS:

• Protonated acetylene
• vinyl radical
• photoelectron spectroscopy
• synchrotron

Acknowledgements

GAG, BG, JC, FH, CA and LN acknowledge SOLEIL for provision of synchrotron radiation facilities under project number 20140832 and the DESIRS beamline staff for their assistance, in particular J.-F. Gil for his help in the mounting of the reactor radical sources. The authors would also like to thank J. Krüger, A. Röder and A. Lopes for their invaluable help in performing the CH₄ + F experiments. Calculations were carried out at the Paul Scherrer Institute.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work received financial support from the French Agence Nationale de la Recherche (ANR) [grant number ANR-12- BS08-0020-02 (project SYNCHROKIN)].