Ultraviolet photoelectron spectroscopy

Last updated July 10, 2024

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

Basic theory

If Albert Einstein's photoelectric law is applied to a free molecule, the kinetic energy ( $E_{\text{k}}$ ) of an emitted photoelectron is given by

E_{\text{k}}=h\nu -I\,,

where h is the Planck 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

Before 1960, virtually all measurements of photoelectron kinetic energies were for electrons emitted from metals and other solid surfaces. In 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).^[1]

The ultraviolet photoelectron spectroscopy (UPS) was pioneered by Feodor I. Vilesov, a physicist at St. Petersburg (Leningrad) State University in Russia (USSR) in 1961 to study the photoelectron spectra of free molecules in the gas phase.^[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. 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.^[6] One of the latest commercial devices was the Perkin Elmer PS18. For the last twenty years, the systems have been homemade. One of the latest in progress – Phoenix II – is that of the laboratory of Pau, IPREM developed by Dr. Jean-Marc Sotiropoulos.^[7]

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 (PES). 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). It is therefore used to study adsorbed species and their binding to the surface, as well as their orientation on the surface.^[8]

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.^[9] 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

Gas	Emission line	Energy	Wavelength	Relative intensity
H	Lyman α	10.20 eV	121.57 nm	100%
H	Lyman β	12.09 eV	102.57 nm	10%
He	1 α	21.22 eV	58.43 nm	100%
	1 β	23.09 eV	53.70 nm	approx 1.5%
	1 γ	23.74 eV	52.22 nm	0.5%
	2 α	40.81 eV	30.38 nm	100%
	2 β	48.37 eV	25.63 nm	< 10%
	2 γ	51.02 eV	24.30 nm	negligible
Ne	1 α	16.67 eV	74.37 nm	15%
	1 α	16.85 eV	73.62 nm	100%
	1 β	19.69 eV	62.97 nm	< 1%
	1 β	19.78 eV	62.68 nm	< 1%
	2 α	26.81 eV	46.24 nm	100%
	2 α	26.91 eV	46.07 nm	100%
	2 β	27.69 eV	44.79 nm	20%
	2 β	27.76 eV	44.66 nm	20%
	2 β	27.78 eV	44.63 nm	20%
	2 β	27.86 eV	44.51 nm	20%
	2 γ	30.45 eV	40.71 nm	20%
	2 γ	30.55 eV	40.58 nm	20%
Ar	1	11.62 eV	106.70 nm	100%
	1	11.83 eV	104.80 nm	50%
	2	13.30 eV	93.22 nm	30%
	2	13.48 eV	91.84 nm	15%

Outlook

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

Related Research Articles

The photoelectric effect is the emission of electrons from a material caused by electromagnetic radiation such as ultraviolet light. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, solid state, and quantum chemistry to draw inferences about the properties of atoms, molecules and solids. The effect has found use in electronic devices specialized for light detection and precisely timed electron emission.

Auger electron spectroscopy is a common analytical technique used specifically in the study of surfaces and, more generally, in the area of materials science. It is a form of electron spectroscopy that relies on the Auger effect, based on the analysis of energetic electrons emitted from an excited atom after a series of internal relaxation events. The Auger effect was discovered independently by both Lise Meitner and Pierre Auger in the 1920s. Though the discovery was made by Meitner and initially reported in the journal Zeitschrift für Physik in 1922, Auger is credited with the discovery in most of the scientific community. Until the early 1950s Auger transitions were considered nuisance effects by spectroscopists, not containing much relevant material information, but studied so as to explain anomalies in X-ray spectroscopy data. Since 1953 however, AES has become a practical and straightforward characterization technique for probing chemical and compositional surface environments and has found applications in metallurgy, gas-phase chemistry, and throughout the microelectronics industry.

A quantum mechanical system or particle that is bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical particles, which can have any amount of energy. The term is commonly used for the energy levels of the electrons in atoms, ions, or molecules, which are bound by the electric field of the nucleus, but can also refer to energy levels of nuclei or vibrational or rotational energy levels in molecules. The energy spectrum of a system with such discrete energy levels is said to be quantized.

<span class="mw-page-title-main">X-ray photoelectron spectroscopy</span> Spectroscopic technique

X-ray photoelectron spectroscopy (XPS) is a surface-sensitive quantitative spectroscopic technique that measures the very topmost 200 atoms, 0.01 um, 10 nm of any surface. XPS is based on the photoelectric effect that can identify the elements that exist within a material or are covering its surface, as well as their chemical state, and the overall electronic structure and density of the electronic states in the material. XPS is a powerful measurement technique because it not only shows what elements are present, but also what other elements they are bonded to. The technique can be used in line profiling of the elemental composition across the surface, or in depth profiling when paired with ion-beam etching. It is often applied to study chemical processes in the materials in their as-received state or after cleavage, scraping, exposure to heat, reactive gasses or solutions, ultraviolet light, or during ion implantation.

Photoemission spectroscopy (PES), also known as photoelectron spectroscopy, refers to energy measurement of electrons emitted from solids, gases or liquids by the photoelectric effect, in order to determine the binding energies of electrons in the substance. The term refers to various techniques, depending on whether the ionization energy is provided by X-ray, XUV or UV photons. Regardless of the incident photon beam, however, all photoelectron spectroscopy revolves around the general theme of surface analysis by measuring the ejected electrons.

Photoemission electron microscopy is a type of electron microscopy that utilizes local variations in electron emission to generate image contrast. The excitation is usually produced by ultraviolet light, synchrotron radiation or X-ray sources. PEEM measures the coefficient indirectly by collecting the emitted secondary electrons generated in the electron cascade that follows the creation of the primary core hole in the absorption process. PEEM is a surface sensitive technique because the emitted electrons originate from a shallow layer. In physics, this technique is referred to as PEEM, which goes together naturally with low-energy electron diffraction (LEED), and low-energy electron microscopy (LEEM). In biology, it is called photoelectron microscopy (PEM), which fits with photoelectron spectroscopy (PES), transmission electron microscopy (TEM), and scanning electron microscopy (SEM).

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References

↑ Carlson T.A., "Photoelectron and Auger Spectroscopy" (Plenum Press, 1975) ISBN 0-306-33901-3
↑ Vilesov, F. I.; Kurbatov, B. L.; Terenin, A. N. (1961). "Electron Distribution Over Energies In Photoionization Of Aromatic Amines in Gaseous Phase". Soviet Physics Doklady. 6: 490. Bibcode:1961SPhD....6..490V.
↑ Price, W. C. (1974). "Photoelectron Spectroscopy". Advances in Atomic and Molecular Physics. 10: 131. Bibcode:1974AdAMP..10..131P. doi:10.1016/S0065-2199(08)60348-6. ISBN 9780120038107.
↑ Rabalais J.W. "Principles of Ultraviolet Photoelectron Spectroscopy" (Wiley 1977) ISBN 0-471-70285-4
↑ Turner, David W. (1970). Molecular photoelectron spectroscopy. London: Wiley Interscience. ISBN 0-471-89320-X. OCLC 108745.
↑ Baker, Arthur D.; Betteridge, David (1972). Photoelectron Spectroscopy. Chemical and Analytical Aspects (First ed.). Oxford: Pergamon Press. ISBN 0-08-016910-4. OCLC 539873.
↑ mdomingo#utilisateurs (2020-09-10). "Jean-Marc Sotiropoulos". iprem.univ-pau.fr (in French). Retrieved 2021-05-17.
↑ Peter W. Atkins and Julio de Paula "Physical Chemistry" (Seventh edition, W.H.Freeman, 2002), p. 980 ISBN 0-7167-3539-3
↑ Park, Y.; Choong, V.; Gao, Y.; Hsieh, B. R.; Tang, C. W. (1996-05-06). "Work function of indium tin oxide transparent conductor measured by photoelectron spectroscopy". Applied Physics Letters. 68 (19): 2699–2701. Bibcode:1996ApPhL..68.2699P. doi: 10.1063/1.116313 . ISSN 0003-6951.

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[1] Carlson T.A., "Photoelectron and Auger Spectroscopy" (Plenum Press, 1975) ISBN 0-306-33901-3

[2] Vilesov, F. I.; Kurbatov, B. L.; Terenin, A. N. (1961). "Electron Distribution Over Energies In Photoionization Of Aromatic Amines in Gaseous Phase". Soviet Physics Doklady. 6: 490. Bibcode:1961SPhD....6..490V.

[3] Price, W. C. (1974). "Photoelectron Spectroscopy". Advances in Atomic and Molecular Physics. 10: 131. Bibcode:1974AdAMP..10..131P. doi:10.1016/S0065-2199(08)60348-6. ISBN 9780120038107.

[4] Rabalais J.W. "Principles of Ultraviolet Photoelectron Spectroscopy" (Wiley 1977) ISBN 0-471-70285-4

[5] Turner, David W. (1970). Molecular photoelectron spectroscopy. London: Wiley Interscience. ISBN 0-471-89320-X. OCLC 108745.

[6] Baker, Arthur D.; Betteridge, David (1972). Photoelectron Spectroscopy. Chemical and Analytical Aspects (First ed.). Oxford: Pergamon Press. ISBN 0-08-016910-4. OCLC 539873.

[7] #utilisateurs (2020-09-10). "Jean-Marc Sotiropoulos". iprem.univ-pau.fr (in French). Retrieved 2021-05-17.

[8] Peter W. Atkins and Julio de Paula "Physical Chemistry" (Seventh edition, W.H.Freeman, 2002), p. 980 ISBN 0-7167-3539-3

[9] Park, Y.; Choong, V.; Gao, Y.; Hsieh, B. R.; Tang, C. W. (1996-05-06). "Work function of indium tin oxide transparent conductor measured by photoelectron spectroscopy". Applied Physics Letters. 68 (19): 2699–2701. Bibcode:1996ApPhL..68.2699P. doi: 10.1063/1.116313 . ISSN 0003-6951.

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