Core Level Energy Splitting in the C 1s Photoelectron Spectrum of C2H2

The partially resolved vibrational fine structure on the C 1s photoelectron line of C2H2 clearly differs from that of other small hydrocarbon molecules such as CH4, C2H4, and C2H6. This is attributed to photoionization from the two 1?g and 1?u core orbitals of C2H2, which can be described as the symmetric and antisymmetric linear combinations of the C 1s orbitals. According to our analysis they are split by 105±10meV, a result which is in good agreement with calculation.