Study of correlation states of acetylene by synchrotron photoelectron spectroscopy

The inner valence photoelectron spectra of acetylene (C2H2) and isotopically labeled acetylene (1,213C–C2H2) are obtained using high resolution synchrotron photoelectron spectroscopy. Four distinct correlation (satellite) peaks, consistent with previous x-ray photoelectron spectroscopy measurements, are resolved. The photon energy dependence of the intensity ratios of these satellites to the 2σ−1g main peak is observed over a wide photon energy range (32–72 eV). Three of these satellites (26.6, 28.0, and 29.8 eV binding energy) exhibited constant photon energy dependence while the fourth satellite (31.2 eV binding energy) showed enhancement of intensity towards the threshold. The photon energy dependence of correlation (satellite) peak 4 can be explained in either of two ways: (1) Peak 4 is a dynamic correlation peak associated with the 2σ−1g ionization process or (2) peak 4 is an intrinsic correlation peak associated with the 3σ−1g ionization process. A multireference singles and doubles configuration interaction (MRSDCI-ANO) calculation of the theoretical photoelectron inner valence spectrum using average natural orbitals indicates that the latter explanation (2) is more likely. Semiquantitative agreement (in terms of the peak positions and intensities) is also obtained between the experimental photoelectron spectrum and the MRSDCI(ANO) calculation.