142 ❯ STEP 4. Review the Knowledge You Need to Score High
Photoelectron (Photoemission) Spectroscopy (PES)
Photoelectron spectroscopy is one of a group of related techniques where high-energy pho-
tons remove an electron from an atom in a photoelectric effect process. The method relies
on a measurement of the kinetic energy of the emitted electron. The kinetic energy is equal
to the energy of the photon minus the binding energy of the electron. The binding energy
is the energy holding the electron in the atom and can be rather difficult to measure.
X-ray photons can excite core electrons. For example, it is possible to focus on the 1s
electrons of an oxygen atom. The binding energy is in part related to the effective nuclear
charge experienced by the electron. In compounds, other atoms bonded to the atom of inter-
est can influence the effective nuclear charge. Atoms donating electron density to the atom
of interest decrease the effective nuclear charge, while electron-withdrawing atoms lead to
an increase in the effective nuclear charge. An important factor in whether an atom donates
or withdraws electron density is the relative electronegativity of the two atoms. This experi-
mental method can be used to give information on which atoms are bonded to each other.z y x z y x z y xzxz y x z y x
z y x z y x z y xs orbital
(a)pz orbital px orbital py orbital
(b)dxz orbital dxy orbital dyz orbitaldz 2 orbital dx (^2) −y 2 orbital
(c)
Figure 10.3 The shapes of the s, p, and d atomic orbitals.