Studies of C 1s* triplet states of carbon monoxide, benzene, ethylene, and acetylene

The (C 1s −1 ,π*) triplet states of carbon monoxide, benzene, ethylene, and acetylene have been studied using electron energy loss spectroscopy with low-energy (near-threshold) excitation. The singlet–triplet splitting for the (C 1s −1 ,π*) states of the hydrocarbon species were measured for the first time. Intermediate neglect of differential overlap (spectroscopic) (INDO/S) calculations within the equivalent cores approximation were used to calculate (1s −1 , π*) singlet–triplet splittings. The calculated values are in reasonable agreement with the experimental values for the hydrocarbon species but are significantly underestimated for CO. A strong correlation between singlet–triplet splittings and oscillator strengths for the 1s → π* transitions was noted and rationalized in terms of the dependence of both factors on the spatial overlap of 1s and π* wave functions.