Double Ionization of the Ethyne Molecule

When previously measured double‐ionization energies of the ethyne (C 2 H 2 ) molecule to singlet and triplet electronic states of its dication are compared with previously calculated values, it becomes apparent that some of the predicted values are not matched with equivalent experimental data. In the present investigation, the results of ADC(2) Green's function calculations indicate that some of the transitions are to satellite states and so too weak to allow double‐ionization energies to those states to be measured. The 1 Δ g and 1 Σ + g states are, however, main states and transitions to them should be sufficiently strong to give two separate double‐ionization energies. Only one peak was seen in the appropriate region using both Auger‐electron spectroscopy and double‐charge‐transfer spectroscopy. This may be due to limitations in resolving power since the states are predicted to lie within 0.8 eV of one another. To investigate this possibility, the double‐ionization energies of C 2 H 2 to singlet states of C 2 H 2+ 2 have been measured using a double‐charge‐transfer spectrometer which is capable of resolving spectral peaks 0.7 eV apart. Four peaks were observed, the first two corresponding to double‐ionization energies of 33.6±0.3 eV and 34.3±0.2 eV. These are identifiable with transitions to the 1 Δ g and 1 Σ + g states, corresponding to calculated energies of 33.8 eV and 34.6 eV, respectively. The two other peaks correspond to double‐ionization energies of 38.7±0.4 eV and 40.7±0.5 eV. They identify with transitions to the 1 Π u state and 1 ± g state, the double‐ionization energies to which are predicted to be 39.0 eV and 41.0 eV, respectively.