
Photoemission spectroscopy (PES), also known as photoelectron spectroscopy, refers to energy measurement of
electron
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s emitted from solids, gases or liquids by the
photoelectric effect
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 physi ...
, in order to determine the
binding energies of electrons in the substance. The term refers to various techniques, depending on whether the
ionization
Ionization or ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged at ...
energy is provided by
X-ray
An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...
,
EUV 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.
Types
X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy (XPS) is a surface-sensitive quantitative spectroscopic technique that measures the very topmost 50-60 atoms, 5-10 nm of any surface. It belongs to the family of photoemission spectroscopies in which electro ...
(XPS) was developed by
Kai Siegbahn starting in 1957 and is used to study the energy levels of atomic core electrons, primarily in solids. Siegbahn referred to the technique as "electron spectroscopy for chemical analysis" (ESCA), since the core levels have small
chemical shifts depending on the chemical environment of the atom that is ionized, allowing chemical structure to be determined. Siegbahn was awarded the
Nobel Prize
The Nobel Prizes ( ; ; ) are awards administered by the Nobel Foundation and granted in accordance with the principle of "for the greatest benefit to humankind". The prizes were first awarded in 1901, marking the fifth anniversary of Alfred N ...
in 1981 for this work. XPS is sometimes referred to as PESIS (photoelectron spectroscopy for inner shells), whereas the lower-energy radiation of UV light is referred to as PESOS (outer shells) because it cannot excite core electrons.
Ultraviolet photoelectron spectroscopy
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 regio ...
(UPS) is used to study valence energy levels and chemical bonding, especially the bonding character of molecular orbitals. The method was developed originally for gas-phase molecules in 1961 by
Feodor I. Vilesov and in 1962 by
David W. Turner, and other early workers included David C. Frost, J. H. D. Eland and K. Kimura. Later,
Richard Smalley modified the technique and used a UV laser to excite the sample, in order to measure the binding energy of electrons in gaseous molecular clusters.
Angle-resolved photoemission spectroscopy
Angle-resolved photoemission spectroscopy (ARPES) is an experimental technique used in condensed matter physics to probe the allowed energies and momenta of the electrons in a material, usually a crystalline solid. It is based on the photoel ...
(ARPES) has become the most prevalent electron spectroscopy in condensed matter physics after recent advances in energy and momentum resolution, and widespread availability of synchrotron light sources. The technique is used to map the band structure of crystalline solids, to study quasiparticle dynamics in highly correlated materials, and to measure electron spin polarization.
Two-photon photoelectron spectroscopy (2PPE) extends the technique to optically excited electronic states through the introduction of a pump-and-probe scheme.
Extreme-ultraviolet photoelectron spectroscopy (EUPS) lies in between XPS and UPS. It is typically used to assess the valence band structure. Compared to XPS, it gives better energy resolution, and compared to UPS, the ejected electrons are faster, resulting in less space charge and mitigated final state effects.
Physical principle
The physics behind the PES technique is an application of the
photoelectric effect
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 physi ...
. The sample is exposed to a beam of UV or XUV light inducing photoelectric ionization. The energies of the emitted photoelectrons are characteristic of their original electronic states, and depend also on vibrational state and rotational level. For solids, photoelectrons can escape only from a depth on the order of nanometers, so that it is the surface layer which is analyzed.
Because of the high frequency of the light, and the substantial charge and energy of emitted electrons, photoemission is one of the most sensitive and accurate techniques for measuring the energies and shapes of electronic states and molecular and atomic orbitals. Photoemission is also among the most sensitive methods of detecting substances in trace concentrations, provided the sample is compatible with ultra-high vacuum and the analyte can be distinguished from background.
Typical PES (UPS) instruments use helium gas sources of UV light, with photon energy up to 52 eV (corresponding to wavelength 23.7 nm). The photoelectrons that actually escaped into the vacuum are collected, slightly retarded, energy resolved, and counted. This results in a spectrum of electron intensity as a function of the measured kinetic energy. Because binding energy values are more readily applied and understood, the kinetic energy values, which are source dependent, are converted into binding energy values, which are source independent. This is achieved by applying Einstein's relation
. The
term of this equation is the energy of the UV light quanta that are used for photoexcitation. Photoemission spectra are also measured using tunable
synchrotron radiation
Synchrotron radiation (also known as magnetobremsstrahlung) is the electromagnetic radiation emitted when relativistic charged particles are subject to an acceleration perpendicular to their velocity (). It is produced artificially in some types ...
sources.
The binding energies of the measured electrons are characteristic of the chemical structure and molecular bonding of the material. By adding a source monochromator and increasing the energy resolution of the electron analyzer, peaks appear with
full width at half maximum
In a distribution, full width at half maximum (FWHM) is the difference between the two values of the independent variable at which the dependent variable is equal to half of its maximum value. In other words, it is the width of a spectrum curve ...
(FWHM) less than 5–8 meV.
See also
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Angle resolved photoemission spectroscopy (ARPES)
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Inverse photoemission spectroscopy (IPS)
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Rydberg ionization spectroscopy, including zero electron kinetic energy spectroscopy (ZEKE)
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Two-photon photoelectron spectroscopy (2PPE)
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Ultra-high vacuum
Ultra-high vacuum (often spelled ultrahigh in American English, UHV) is the vacuum regime characterised by pressures lower than about . UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of ...
(UHV)
*
Ultraviolet photoelectron spectroscopy
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 regio ...
(UPS)
*
Vibronic spectroscopy
*
X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy (XPS) is a surface-sensitive quantitative spectroscopic technique that measures the very topmost 50-60 atoms, 5-10 nm of any surface. It belongs to the family of photoemission spectroscopies in which electro ...
(XPS)
*
Stefan Hüfner
*
William E. Spicer
References
Further reading
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External links
Presentationon principle of
ARPES
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Emission spectroscopy
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Electron spectroscopy