Photoemission spectroscopy
Encyclopedia
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 a substance. The term refers to various techniques, depending on whether the ionization
energy is provided by an X-ray
photon
, an EUV
photon, or an ultraviolet
photon. Regardless of the incident photon beam however, all photoelectron spectroscopy revolves around the general theme of surface analysis by measuring the ejected electrons.
X-ray photoelectron spectroscopy
(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 which is ionized, allowing chemical structure to be determined. Siegbahn was awarded the Nobel Prize
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.
In the ultraviolet region, the method is usually referred to as photoelectron spectroscopy for the study of gases, and photoemission spectroscopy for solid surfaces.
Ultra-violet photoelectron spectroscopy (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 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.
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 a better spectrum signal.
. 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, energy resolved, slightly retarded and counted, which 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 due to the energy (frequency) of the UV light that bombards the sample. Photoemission spectra are also measured using synchrotron radiation
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
(FWHM) less than 5–8 meV.
Photoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...
, in order to determine the binding energies of electrons in a substance. The term refers to various techniques, depending on whether the ionization
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
energy is provided by an X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
, an EUV
Extreme ultraviolet
Extreme Ultraviolet radiation is high-energy ultraviolet radiation, generally defined to be electromagnetic radiation in the part of the electromagnetic spectrum spanning wavelengths from 120 nm down to 10 nm, and therefore having photons with energies from 10 eV up to 124 eV...
photon, or an ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
photon. Regardless of the incident photon beam however, all photoelectron spectroscopy revolves around the general theme of surface analysis by measuring the ejected electrons.
X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material...
(XPS) was developed by Kai Siegbahn
Kai Siegbahn
Kai Manne Börje Siegbahn was a Swedish physicist.He was born in Lund, Sweden, and his father Manne Siegbahn also won the Nobel Prize in Physics, in 1924. Siegbahn earned his doctorate at the University of Stockholm in 1944...
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 which is ionized, allowing chemical structure to be determined. Siegbahn was awarded the Nobel Prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...
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.
In the ultraviolet region, the method is usually referred to as photoelectron spectroscopy for the study of gases, and photoemission spectroscopy for solid surfaces.
Ultra-violet photoelectron spectroscopy (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 1962 by David W. Turner
David W. Turner
David Warren Turner is a physical chemist known for the development of ultra-violet photoelectron spectroscopy , a technique for the measurement of molecular orbital energies in gas-phase molecules....
, and other early workers included David C.Frost, J.H.D. Eland and K. Kimura. Later, Richard Smalley
Richard Smalley
Richard Errett Smalley was the Gene and Norman Hackerman Professor of Chemistry and a Professor of Physics and Astronomy at Rice University, in Houston, Texas...
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.
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 a better spectrum signal.
Physical principle
The physics behind the PES technique is an application of the photoelectric effectPhotoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...
. 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, energy resolved, slightly retarded and counted, which 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 due to the energy (frequency) of the UV light that bombards the sample. Photoemission spectra are also measured using synchrotron radiationSynchrotron radiation
The electromagnetic radiation emitted when charged particles are accelerated radially is called synchrotron radiation. It is produced in synchrotrons using bending magnets, undulators and/or wigglers...
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
Full width at half maximum
Full width at half maximum is an expression of the extent of a function, given by the difference between the two extreme values of the independent variable at which the dependent variable is equal to half of its maximum value....
(FWHM) less than 5–8 meV.
See also
- Angle resolved photoemission spectroscopy AR-PES
- Inverse photoemission spectroscopyInverse photoemission spectroscopyInverse photoemission spectroscopy is a surface science technique used to study the unoccupied electronic structure of surfaces, thin films and adsorbates...
IPS - Ultra high vacuumUltra high vacuumUltra-high vacuum is the vacuum regime characterised by pressures lower than about 10−7 pascal or 100 nanopascals . UHV requires the use of unusual materials in construction and by heating the entire system to 180°C for several hours to remove water and other trace gases which adsorb on the...
UHV - X-ray photoelectron spectroscopyX-ray photoelectron spectroscopyX-ray photoelectron spectroscopy is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material...
XPS
External links
- Presentation on principle of ARPESARPESAngle-resolved photoemission spectroscopy , also known as ARUPS , is a direct experimental technique to observe the distribution of the electrons in the reciprocal space of solids...