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A physical scientist who helped develop X-ray photoelectron spectroscopy. Measuring the energies of electrons emitted after X-ray absorption probes electronic binding and chemical environment. His research extended from laboratory sources to synchrotron radiation, linking improved light sources with more detailed studies of atoms, molecules and solid materials.^[ref]

Kai Siegbahn developed electron spectroscopy as a quantitative probe of chemical and electronic structure. In X-ray photoelectron spectroscopy, incident X-rays eject electrons whose measured kinetic energies reveal their binding energies after the instrument's reference offset is accounted for. Small changes of a core-level binding energy reflect the atom's chemical environment. Resolving and calibrating these shifts makes the spectrum sensitive to electronic surroundings as well as elemental identity, linking the spectrometer's design with a physical inference.^[ref]

Physics.^[ref]

Photoelectron spectroscopy measures orbital-dependent binding energies from which an effective charge can be inferred using a stated model. Ultraviolet photoelectron spectroscopy accesses outer-shell ionizations; X-ray photoelectron spectroscopy accesses deeper core states. For an isolated gas-phase atom and a calibrated threshold, measure emitted-electron kinetic energy and use $I=h\nu -E_{\mathrm{kin}}$, with the appropriate energy reference. Optical atomic term spectra and series limits also constrain valence ionization energies.

A hydrogenic inference would set $I\simeq R_{\infty }{Z}_{\mathrm{eff}}^2/n_{\mathrm{eff}}^2$, hence $Z_{\mathrm{eff}}\simeq n_{\mathrm{eff}}\sqrt{I/R_{\infty }}$, with $R_{\infty }$ here an energy of approximately $13.6\mathrm{eV}$ and $n_{\mathrm{eff}}$ a chosen effective shell parameter. The measured binding energies are experimental; the extracted $Z_{\mathrm{eff}}$ values remain model dependent. They need not equal Slater screening parameters fitted to radial functions.

Unlike the equal $ns,np$ values in Slater's rules, spectroscopic values distinguish s from p because their penetration and shielding differ; the more tightly bound s states usually imply larger binding-energy-derived effective charges at a common shell parameter. Subshell, atomic-term and, especially for heavier atoms, spin-orbit coupling structure give further distinctions. Electron relaxation following ionization and electron correlation also matter. Thus real binding energies vary across the heavier group-15 atoms even where the simple Slater outer-shell estimates plateau; no universal constant correction to the Slater values follows.^[ref]

A plausible explanation is partial dissociation leaving a residual surface species, such as an NH${}_x$ fragment, adsorbed nitrogen or hydrogen, after molecular ammonia has finished desorbing. A small residual population can maintain a surface electric dipole moment and a $0.2$ eV work function offset. Heating farther can remove, recombine or rearrange those species.

Test this hypothesis by recording temperature-programmed desorption signals for hydrogen and nitrogen as well as ammonia, and by measuring residual nitrogen with X-ray photoelectron spectroscopy. A surface-sensitive vibrational spectrum could distinguish NH${}_x$ groups from atomic nitrogen. Isotopically labelled ammonia would help assign desorption products. Correlating disappearance of the residual spectral signal with recovery of the clean-surface work function would be more diagnostic than the absence of an ammonia signal alone. This is a proposed explanation, not a uniquely established identification.

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.

X-ray photoelectron spectroscopy uses X-ray excitation to investigate electron removal energies, including those of core electrons. The common name electron spectroscopy for chemical analysis emphasizes its use in distinguishing chemical surroundings. The whole recorded spectrum can also contain electrons produced by processes other than direct photoionization; a peak assignment must identify the emission process as well as the chemical element.

In a modern implementation, a quartz crystal selects aluminum characteristic X-rays according to Bragg's law and focuses them onto the sample. Vacuum reduces gas collisions and contamination; a separately pumped entry chamber limits exposure to air. Lenses collect photoelectrons; an electrostatic electron energy analyzer transmits a narrow kinetic energy band. Stepping that band and counting detector pulses produces the spectrum. An electron flood source can compensate positive electric charge on insulating samples. Ion cleaning can alter vulnerable samples.^[ref]

For a scan with count $N_j$ and dwell time $t_j$ at setting $j$, compare rates $N_j/t_j$, rather than raw counts when dwell times differ. Under an independent Poisson distribution model, the count standard deviation is approximately $\sqrt{N_j}$ for a sufficiently populated bin. Longer acquisition reduces this relative counting measurement uncertainty; it does not automatically improve separation of overlapping peaks. Photoelectron peak overlap and resolution gives an example in which the same chemical separation produces either two maxima or one.