Photoelectron Spectroscopy (PES)

Each electron is held by the nucleus with a characteristic binding energy. The ionization potential is the least energy required to remove this electron from the atom/molecule when high energy radiation  (hν)  is incident on a molecule,  electrons are ejected from molecules with a large kinetic energy (KE) which is given by

hν  =  binding energy  + Kinetic Energy of ejected electrons

This kinetic energy is not quantized.  The KE of ejected electrons is analysed by wing hemispherical plates, filters, electron multipliers and analysers.  The plot of the number of electrons against their kinetic energy is converted to a plot of intensity versus binding energy to get the photoelectron spectrum.  For   hydrogen, we get only one binding energy (13.6 ev).  For Li,  58ev (1s electron) and 5.4 ev (2s electron); for F, 694 ev (1s),  37.9  ev (2s) and 17.4 ev (2p), and more lines with fine structure (corresponding to spin orbit interactions) for higher atoms.  Using these methods of X-ray PES (XPES) and ultraviolet PES (UPES),  elements in a molecule can be detected by using their “fingerprints” generated in atomic PES.  While inner electrons in molecules provide good fingerprints, the outer valence electrons are affected by chemical binding and they bear a “molecular” character  rather than an atomic character.