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In X-ray photoelectron spectroscopy, the electron binding energy relative to the chosen reference follows $E_B=h\nu -E_K-{\varphi }_{\mathrm{analyzer}}$. A core-level chemical shift is a change in a core peak's binding energy between environments. Initial-state contributions include charge redistribution, bonding, the local electrostatic potential and chemical-potential changes. Final-state contributions include relaxation and screening of the newly created core hole. The measured shift contains both; it is not a direct meter of an integer oxidation state.

For a related chemical series, electron withdrawal often raises a core binding energy, but screening and referencing can change that trend. A surface core-level shift can separate surface atoms from bulk atoms because coordination and screening differ. Adsorption can introduce another component for chemically bound surface atoms. Spin-orbit doublets, multiplet or satellite features must be distinguished from separate chemical environments.

Reliable comparison needs a consistent Fermi-level or other justified energy reference, compensation or diagnosis of sample charging, adequate energy resolution and a stated line-shape/background model. Angle or photon-energy variation changes surface weighting through electron escape depth, helping distinguish surface and bulk assignments. A shifted, broader or overlapping peak alone does not uniquely identify a chemical species or its concentration.

Ultraviolet photoelectron spectroscopy probes occupied valence states through photoemitted electron energies. Angle-resolved photoemission spectroscopy adds emission direction and therefore parallel crystal momentum, revealing occupied band dispersions and Fermi contours. For a clean periodic surface, $k_{\parallel }=\sqrt{2m_e{E}_{\mathrm{kin}}}\sin \theta /\hslash$ modulo a surface reciprocal-lattice vector. A two-dimensional surface band has no intrinsic perpendicular dispersion; varying photon energy helps distinguish it from bulk bands, while allowing for matrix-element changes. A simple surface-state example is a parabolic band $E({\mathbf{k}}_{\parallel })=E_0+{\hslash }^2{k}_{\parallel }^2/(2m_{\ast })$.

Scanning tunneling spectroscopy measures local differential conductance. With a suitable featureless tip and weak tunneling, $dI/dV$ samples the local density of states, giving occupied and unoccupied features on opposite bias sides. It provides spatial localization, defects and standing-wave patterns, while ordinary spectroscopy at one point does not directly give the complete momentum-resolved band structure. For the parabolic two-dimensional band, the ideal density of states has a step at its onset, while spatial interference can supply information about allowed wavevectors. Inverse photoemission spectroscopy accesses unoccupied states by injecting electrons and measuring emitted photons. Optical absorption and electron energy loss spectroscopy instead probe allowed excitation energies and collective modes, with their own selection rules and momentum ranges. These methods supply complementary information; none automatically returns the full interacting band structure without matrix elements, resolution and surface effects.

For the Pt core-level spectra, electrons suffering inelastic losses emerge with reduced kinetic energy and appear at larger inferred binding energies. Loss tails and secondary-electron contributions therefore generate the background, principally on the high-binding-energy side of a core peak. The two clean-surface components correspond to different environments of the same Pt core-level branch. The stated values imply a surface core-level shiftReduced coordination at a surface narrows the $d$ band. In a simple almost-filled-band picture, maintaining its occupation moves the band center upward relative to the Fermi level; associated local-potential changes can move core levels upward too, reducing their binding energy. This is a plausible initial-state explanation of the negative shift. Final-state screening of the core hole also matters, so the measured shift alone does not uniquely determine a local charge or separate these mechanisms.

After adsorption, Pt atoms directly bonded to CO have a different local potential, hybridization and core-hole screening. In carbon monoxide adsorption on platinum, carbon-end donation and metal-to-molecule back-donation change the Pt–CO bond and hence the Pt core binding energy. The additional weak high-binding-energy component is naturally assigned to these chemically modified surface Pt atoms. Its small weight is compatible with only a minority of surface sites being occupied; a quantitative coverage estimate requires fitted areas and appropriate attenuation assumptions.

The new component is centered at approximately $71.9$–$72.0\mathrm{eV}$ in the supplied plot. Relative to bare surface Pt at $70.5\mathrm{eV}$, the core-level chemical shift is thereforeUsing the bulk peak as the reference would instead give about $+1.0$–$1.1\mathrm{eV}$, which answers a different comparison. The plotted axis runs toward decreasing binding energy to the right, so the additional peak on its left is a positive binding-energy shift. The estimate is a graphical reading, with no claim to sub-tenth-eV precision or a unique charge-transfer value.