The ionization energy of C2

Resonant two-photon threshold ionization spectroscopy is employed to determine the ionization energy of C-2 to 5 meV precision, about two orders of magnitude more precise than the previously accepted value. Through exploration of the ionization threshold after pumping the 0-3 band of the newly discovered 4(3)Pi(g) <- a(3)Pi(u) band system of C-2, the ionization energy of the lowest rovibronic level of the a(3)Pi(u) state was determined to be 11.791(5) eV. Accounting for spin-orbit and rotational effects, we calculate that the ionization energy of the forbidden origin of the a(3)Pi(u) state is 11.790(5) eV, in excellent agreement with quantum thermochemical calculations which give 11.788(10) eV. The experimentally derived ionization energy of X-1 Sigma(+)(g) state C-2 is 11.866(5) eV. (C) 2016 AIP Publishing LLC.