CONDENSED-PHASE ELECTRON-ENERGY-LOSS SPECTROSCOPY OF THE LOW-LYING TRIPLET-STATES OF ACETYLENE

Low-energy electron-energy-loss spectra of solid acetylene were record ed in the range of the low-lying valence excited triplet states at a t emperature of 17 K. The spectra show that the first band, which was pr eviously assigned to a single transition, actually contains several ov erlapping singlet-triplet transitions. The transition (1) Sigma(g)(+) --> (3)A(u) starts with an extrapolated lowest vibronic transition at 4.89 eV and is dominated by the bending vibrational mode with frequenc y 86 meV (694 cm(-1)). Below this band the spectra show a short progre ssion with vibrational spacings of 100 meV (806 cm(-1)) starting at 4. 62 eV and a weak band with irregular vibrational spacings close to the energy-loss threshold. Both are assigned to the transition to the low est tripler potential energy surface, which correlates to the linear ( 3) Sigma(u)(+) State and the two bent B-2 and B-u isomer states. (C) 1 997 American Institute of Physics.