View source for HR 3803

'''Ultraviolet photoelectron spectroscopy''' (UPS) refers to the measurement of kinetic energy [[spectroscopy|spectra]] of [[photoelectric effect|photoelectrons]] emitted by molecules which have absorbed [[ultraviolet]] photons, in order to determine [[molecular orbital]] energies in the valence region.

==Basic Theory==
If [[Einstein]]’s photoelectric law is applied to a free molecule, the [[kinetic energy]] (<math> E_K</math>) of an emitted photoelectron is given by 

:<math> E_K = h\nu - I\,</math>, 

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]] (XPS) 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 ([[electronvolt]]).<ref>Carlson T.A., "Photoelectron and Auger Spectroscopy" (Plenum Press, 1975) ISBN 0-306-33901-3</ref>

The ultraviolet method (UPS) was developed to study the photoelectron spectra 
of free molecules in the gas phase 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.<ref>Rabalais J.W. "Principles of Ultraviolet Photoelectron Spectroscopy" (Wiley 1977) ISBN 0-471-70285-4</ref><ref>Turner D. W. Molecular Photoelectron Spectroscopy (Wiley, 1970)</ref> As a photon source, he used a [[helium]] discharge lamp which emits a wavelength of 58.4&nbsp;nm (corresponding to an energy of 21.2 eV) 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.<ref>Baker A.D. and Betteridge D. "Photoelectron Spectroscopy. Chemical and Analytical Aspects." (Pergamon Press 1972) p.ix</ref>

==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 spectroscopy|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]] (PES). It is particularly sensitive to the surface region (to 10&nbsp;nm depth), due to the short range of the emitted photoelectrons (compared to X-rays). It is therefore used to study [[adsorbed]] species and their binding to the surface, as well as their orientation on the surface.<ref>Peter W. Atkins and Julio de Paula "Physical Chemistry" (Seventh edition, W.H.Freeman, 2002), p.980 ISBN 0-7167-3539-3</ref> 

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.<ref> Y. Park et al., Appl. Phys. Lett 68(19), 2699-2701 (1996) Work function of indium tin oxide transparent conductor measured by photoelectron spectroscopy</ref> Briefly, the full width of the photoelectron spectrum (from the highest kinetic energy/lowest binding energy point to the low kinetic energy cutoff) 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 for UPS===
{| class="wikitable"
|-
! Gas !! Emission Line !! Energy (eV) !! Wavelength (nm) !! Relative Intensity (%)
|-
| H || Lyman α || 10.20 || 121.57 || 100
|-
| || Lyman β || 12.09 || 102.57 || 10
|-
| He || 1 α || 21.22 || 58.43 || 100
|-
| || 1 β|| 23.09 || 53.70 || approx 1.5
|-
| || 1 γ || 23.74 || 52.22 || 0.5
|-
| || 2 α || 40.81 || 30.38 || 100
|-
| || 2 β || 48.37 || 25.63 || <10
|-
| || 2 γ || 51.02 || 24.30 || negligible 
|-
| Ne || 1 α || 16.67 || 74.37 || 15
|-
| || 1 α || 16.85 || 73.62 || 100
|-
| || 1 β || 19.69 || 62.97 || < 1
|-
| || 1 β || 19.78 || 62.68 || < 1
|-
| || 2 α|| 26.81 || 46.24 || 100
|-
| || 2 α|| 26.91 || 46.07 || 100
|-
| || 2 β || 27.69 || 44.79 || 20
|-
| || 2 β || 27.76 || 44.66 || 20
|-
| || 2 β || 27.78 || 44.63 || 20
|-
| || 2 β || 27.86 || 44.51 || 20
|-
| || 2 γ || 30.45 || 40.71 || 20
|-
| || 2 γ || 30.55 || 40.58 || 20
|-
| Ar || 1 || 11.62 || 106.70 || 100
|-
| || 1 || 11.83 || 104.80 || 50
|-
| || 2 || 13.30 || 93.22 || 30
|-
| || 2 || 13.48 || 91.84 || 15
|}

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

==See also==
*[[Angle resolved photoemission spectroscopy]] ARPES
*[[X-ray photoelectron spectroscopy]] XPS
*[[Photoelectron photoion coincidence spectroscopy]] PEPICO

==References==
{{reflist}}

[[Category:Molecular physics]]
[[Category:Spectroscopy]]
[[Category:Surface chemistry]]
[[Category:Scientific techniques]]