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

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 C(2)H(4)and CH(4)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(3)A '' 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(1)A(1)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(1)A '' excited state are also given.