If Albert Einstein's photoelectric law is applied to a free molecule, the kinetic energy of an emitted photoelectron is given by where h is Planck's constant, ν is the frequency of the ionizing light, and I is an ionization energy for the formation of a singly charged ion in either the ground state or an excited state. According to Koopmans' theorem, each such ionization energy may be identified with the energy of an occupied molecular orbital. The ground-state ion is formed by removal of an electron from the highest occupied molecular orbital, while excited ions are formed by removal of an electron from a lower occupied orbital.
History
Prior to 1960, virtually all measurements of photoelectron kinetic energies were for electrons emitted from metals and other solid surfaces. About 1956 Kai Siegbahn developed X-ray photoelectron spectroscopy for surface chemical analysis. This method uses x-ray sources to study energy levels of atomic core electrons, and at the time had an energy resolution of about 1 eV. The ultraviolet method was pioneered by Feodor I. Vilesov, a physicist at St. Petersburg State University in Russia in 1961 to study the photoelectron spectra of free molecules in the gas phase. The early experiments used monochromatized radiation from a hydrogen discharge and a retarding potential analyzer to measure the photoelectron energies. The PES was further developed by David W. Turner, a physical chemist at Imperial College in London and then at Oxford University, in a series of publications from 1962 to 1967. As a photon source, he used a heliumdischarge lamp which emits a wavelength of 58.4 nm in the vacuum ultraviolet region. With this source Turner's group obtained an energy resolution of 0.02 eV. Turner referred to the method as "molecular photoelectron spectroscopy", now usually "Ultraviolet photoelectron spectroscopy" or UPS. As compared to XPS, UPS is limited to energy levels of valence electrons, but measures them more accurately. After 1967 commercial UPS spectrometers became available.
Application
The UPS measures experimental molecular orbital energies for comparison with theoretical values from quantum chemistry, which was also extensively developed in the 1960s. The photoelectron spectrum of a molecule contains a series of peaks each corresponding to one valence-region molecular orbital energy level. Also, the high resolution allowed the observation of fine structure due to vibrational levels of the molecular ion, which facilitates the assignment of peaks to bonding, nonbonding or antibonding molecular orbitals. The method was later extended to the study of solid surfaces where it is usually described as photoemission spectroscopy. It is particularly sensitive to the surface region, due to the short range of the emitted photoelectrons. It is therefore used to study adsorbed species and their binding to the surface, as well as their orientation on the surface. A useful result from characterization of solids by UPS is the determination of the work function of the material. An example of this determination is given by Park et al. Briefly, the full width of the photoelectron spectrum is measured and subtracted from the photon energy of the exciting radiation, and the difference is the work function. Often, the sample is electrically biased negative to separate the low energy cutoff from the spectrometer response.
Gas discharge lines
Outlook
UPS has seen a considerable revival with the increasing availability of synchrotron light sources which provide a wide range of monochromatic photon energies.
