The K-shell excitation and ionization of metastable (O 2 a 1 D g ) molecule has been studied using direct photoelectron spectroscopy and resonant Auger spectroscopy. The K-edge photoionization spectra of metastable (O 2 a 1 D g ) molecule were investigated at high resolution. These spectra show a better resolution in comparison with the previously published data. Metastable molecules were prepared on–line using a microwave discharge. The 2 D state of the core hole ion has been identified following ionization from the a 1 D g state. From the analysis of the spectra, the energetic and structural information of the 2 D, 4 S - , 2 S - states of the core hole ion was obtained and the vibrational frequency of the 2 D state of the core hole ion was determined for the first time. The first resonant Auger spectra of molecular oxygen following core-excitation of the metastable (O 2 a 1 D g ) molecule to the 1 ? u core excited state have been recorded. The high beamline and analyzer resolution has allowed the separation of the singlet decay from the dominant triplet excitation of ground state oxygen. Fundamental interference phenomena were observed and are discussed in the framework of the “X-ray Raman Scattering Theory”. The resonant Auger spectra have been used to spectroscopically characterize the core excited 1 ? u state. In this study the structural information of the core excited 1 ? u state was obtained for the first time. When compared with its triplet counterpart the singlet state shows a significant lengthening of the equilibrium bond length. Direct variable photon energy X-ray photoelectron spectroscopy along with the total ion yield spectroscopy has been used to measure the ionization cross-section of the 3 d spin-orbit components of the Cs atom for the first time. The measured ionization cross-section qualitatively confirmed the theoretical calculation [ M. Ya. Amusia et al., Phys. Rev. Lett 88 ,093002 (2002)] based on the “Spin-Orbit Activated Interchannel Coupling”.