= Solution
<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, $k_\parallel=\sqrt{2m_eE_{\rm kin}}\sin\theta/\hbar$ 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 $E(\mathbf k_\parallel)=E_0+\hbar^2k_\parallel^2/(2m_*)$.
<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 shift>
$$
\boxed{E_B^{\rm surface}-E_B^{\rm bulk}=70.5-70.9=-0.4\,\mathrm{eV}.}
$$
Reduced 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 therefore
$$
\boxed{\Delta E_B^{\rm CO}\simeq+1.4\text{–}1.5\,\mathrm{eV}.}
$$
Using 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.
Back to article page
