Ultraviolet photoelectron spectroscopy

[2][3] 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.

[4][5] As a photon source, he used a helium discharge lamp that emits a wavelength of 58.4 nm (corresponding to an energy of 21.2 eV) in the vacuum ultraviolet region.

[7] The UPS measures experimental molecular orbital energies for comparison with theoretical values from quantum chemistry, which was also extensively developed in the 1960s.

It is particularly sensitive to the surface region (to 10 nm depth), due to the short range of the emitted photoelectrons (compared to X-rays).

UPS has seen a considerable revival with the increasing availability of synchrotron light sources that provide a wide range of monochromatic photon energies.