DOUBLE-IONIZATION OF THE ETHYNE MOLECULE

When previously measured double-ionization energies of the ethyne (C2H 2) molecule to singlet and triplet electronic states of its dication a re compared with previously calculated values, it becomes apparent tha t some of the predicted values are not matched with equivalent experim ental data. In the present investigation, the results of ADC(2) Green' s function calculations indicate that some of the transitions are to s atellite states and so too weak to allow double-ionization energies to those states to be measured. The (1) Delta(g) and (1) Sigma(g)(+) sta tes are, however, main states and transitions to them should be suffic iently strong to give two separate double-ionization energies. Only on e peak was seen in the appropriate region using both Auger-electron sp ectroscopy 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 doub le-ionization energies of C2H2 to singlet states of C2H22+ have been m easured using a double-charge-transfer spectrometer which is capable o f resolving spectral peaks 0.7 eV apart. Four peaks were observed, the first two corresponding to double-ionization energies of 33.6+/-0.3 e V and 34.3+/-0.2 eV, These are identifiable with transitions to the (1 ) Delta(g) and (1) Sigma(g)(+) states, corresponding to calculated ene rgies of 33.8 eV and 34.6 eV, respectively. The two other peaks corres pond to double-ionization energies of 38.7+/-0.4 eV and 40.7+0.5 eV, T hey identify with transitions to the (1) Pi(a), state and (1) Pi(g), s tate, the double-ionization energies to, which are predicted to be 39. 0 eV and 41.0 eV, respectively.