In photoelectron spectroscopy, PES, the kinetic energies of electrons emitted from a sample when it is irradiated with high energy ultraviolet radiation are measured. The use of ultraviolet radiation is sometimes reflected in the name UV-PES.

The kinetic energy, E_K, of an emitted electron, the so called photoelectron, can be related to the energy of the incident radiation, hν, and the energy required to liberate it from the species, its ionization energy, I, by the relationship

E_K = I - hν

Hence, if we know the frequency of the incident radiation, and measure the energy of the photoelectron, we can calculate the ionization for the electron. According to Koopman's theorem, the ionization energy of an electron from the orbital, i, is related to the energy of that orbital, ε_i, by the relationship

ε_i = -I_i

An analysis of the energies of the photoelectrons will therefore give us the energies of the orbitals from which they were emitted. Photoelectrons with high energies correspond to orbitals with low ionization energies, or those which are the furthest from the core of the species. Similarly, photoelectrons with low energies correspond to orbitals with high ionization energies, or the low energy, core levels.

In the spectrum we see a line (or more accurately a set of lines, see below) for each orbital energy. However, we only see lines corresponding to occupied orbitals (an electron may not be emitted from an unoccupied orbital), and orbitals with energies smaller than the energy of the incident photon (so core levels may sometimes not be observed).

Ultraviolet Photoelectron spectroscopy
The general case	Nitrogen (N2)

Vibrational structure in the Photoelectron Spectrum

If we consider the UV-PES spectrum of N₂ in the table, we see that emitted electrons from the 1π_u orbital give a series of lines in the spectrum at closely spaced energies.

The reason for this is that the incident radiation is of high energy, and so in addition to ionizing an electron, it may also leave the resultant N₂⁺ ion in a vibrationally excited state. There are a series of occupied vibrational states in the excited electronic state, and so there are a range of transition energies for the ionization depending upon which vibrational state becomes occupied. Hence, we see a series of lines separated by the vibrational frequency of the N₂⁺ ion. It is important that we realise, though, that the vibrational structure observed is that of the electronically excited species, N₂⁺, and not the ground state N₂ molecule.

UV-PES can therefore give information not only on the electronic structure of a species, but also the vibrational structure of the excited states of that species. the vibrational structure of the excited state species is, however, often closely related to the vibrational structure of the ground state species.