The electronic structure of late transition metals fundamentally govern their key material properties including catalytic activity, magnetic behavior, superconductivity, and correlated electron phenomena which are the backbone for modern technological applications starting from energy conversion and storage to quantum materials design. An in depth understanding of electronic structure is thus essential for advancement in materials science and condensed matter physics. However, the complex many-body interactions in transition metals such as electron-electron correlations, hybridization effects, and charge transfer processes, influence material properties in ways that are beyond the conventional single-particle theoretical frameworks. Traditional spectroscopic methods such as X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) are fundamentally limited in characterizing these electron-electron correlations because they probe single-particle excitations within a mean-field approximation, cannot distinguish between correlated and uncorrelated electronic processes, suffer from overlapping contributions between different chemical environments, and lack the selectivity to isolate specific multi-electron final states that encode essential correlation information such as on-site Coulomb repulsion energies and hybridization strengths. This thesis focusses on extensive investigation of surface properties and electron-electron correlation interactions in transition metal surfaces, their surface oxides, molecular adsorbate systems and transition metal oxide-based cuprates using Auger Photoelectron Coincidence Spectroscopy (APECS). APECS is an advanced technique that overcomes the fundamental limitations of conventional XPS and AES by simultaneously measuring photoelectrons in coincidence with corresponding Auger electrons. It offers unprecedented capabilities in probing electronic correlations and surface chemistry with enhanced selectivity and sensitivity through direct access to two-hole final states that reveal correlation effects, completely eliminates uncorrelated background contributions that obscure conventional measurements, and provides exceptional surface and chemical specificity that can definitively separate contributions from different atomic environments including surface versus bulk species and distinct chemical bonding configurations. In this thesis, surface sensitivity of APECS using the 5d transition metal Au as a model system is discussed. The measurements demonstrate APECS’s ability to distinguish between surface and bulk components of Au 4f, with a probing depth of approximately 2-3 Å. This establishes APECS as an extremely surface-sensitive technique capable of providing detailed information about surface electronic states. Then, the investigation is directed towards clean Cu surfaces (100), (110), and (111), where distinct tailing in their two-hole binding energy spectra are identified. Through detailed analysis using (Z+2) Surface Core Level Shifts (SCLS), depth-dependent intensity distributions, and Doniach-Sunjic lineshape analysis, surface specific electronic screening effects that are reflected in their surface-projected band structure are characterized. Extending the investigation to oxidized copper systems, the long-debated oxidation state of the "29" CuxO/Cu(111) surface oxide structure are adressed, which have crucial implications for applications in nuclear waste storage and industrial processes. Through APECS measurements and combined analysis of (Z+2) SCLS, depth-dependent intensity distribution and Voigt lineshapes, the oxidation state of Cu(0.3) for "29" CuxO/Cu(111) is determined, which closely resembles to metallic Cu(0), rather than to Cu(I) of Cu2O. Furthermore, the continuous distribution of oxygen species across the "29" CuxO/Cu(111) are identified and describes as surface, sub-surface, and adatom oxygen species. In another study, molecule-surface interactions relevant to catalytic applications are explored with APECS. The interaction between N2O molecules and Ni(111) surfaces across multiple coverage regimes (multilayer, bilayer, and monolayer) are examined by combining APECS measurements with ab initio calculations and solid-state density functional theory. A Blyholder bonding mechanism is revealed. This mechanism is characterized by electron donation from the terminal nitrogen NT to the Ni surface, accompanied by back-donation from the Ni atom surface to the molecule. In the final study, with the unprecedented chemical selectivity of APECS, the lattice and surface oxygen contributions were seperated alongside eliminating uncorrelated background. With this selectivity, APECS combined with Cini-Sawatzky theoretical framework is employed to directly measure oxygen 2p hole-hole Coulomb repulsion energies in La2CuO4, determining Upp = 6.3 eV for in-plane oxygen and and an upper limit of Upp = 9.2 eV for apical oxygen revealing predominantly localized electronic character due to the material’s two-dimensional layered structure. These findings are crucial benchmarks for understanding electron correlation effects in cuprate superconductors and demonstrate how crystal structure dimensionality directly controls the balance between localized and delocalized electronic behavior in strongly correlated oxide materials. This comprehensive study not only advances understanding of electronic structures across diverse systems:- transition metal surfaces, oxide interfaces, correlated electron materials, and molecular adsorbate interactions, but also establishes both the surface sensitivity and analytical power of APECS for applications in surface science.
Swarnshikha Sinha (Thu,) studied this question.