Electron spectrometer
Encyclopedia
In an Electron spectrometer, an incoming beam of electrons is bent with electric or magnetic fields. As higher energy electrons will be bent less by the beam, this produces a spatially distributed range of energies.
Electron spectrometers are used on a range of scientific equipment, including particle accelerator
s, transmission electron microscopes, and astronomical satellites.
An electrostatic electron spectrometer uses the electric field, which cause electrons to move along field gradients, whereas magnetic devices cause electrons to move at right angles to the field. Magnetic fields will act in a direction perpendicular to the electron propagation, thereby conserving velocity, whereas electrostatic fields will cause electrons to move along the field gradient, which may change electron energies if the component of the direction of propagation and field gradients are not perpendicular. Owing to these effects, sector based designs are commonly used in electron spectrometers.
The effective potential
in the solution of motion in a magnetic or electric system with rotational symmetry leads to radial focusing onto a mean radius. By superposition of a quadrupole field axial focusing is possible while weakening the radial focusing, until the astigmatism
vanishes. By breaking the rotational symmetry a bit and varying the electrostatic potential along the mean path of the spherical aberration
is minimized.
The spectrometer can use entrance and exit slits or use a small source, which only emits into specific angle and a small detector. Photoelectron spectra from single crystals exhibit a dependency on the emission angle, so that the entrance slit is needed. All the electrons from an isotopic source may be sucked off and focused into a directed beam (much like in an electron gun
), which can then be analyzed. A position sensitive detector can detect the energy along one direction and depending on the additional optics lateral resolution or one angle along the other direction.
Electrostatic spectrometers preserve the spin
, which can be resolved afterwards.
Electron spectrometers are used on a range of scientific equipment, including particle accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
s, transmission electron microscopes, and astronomical satellites.
An electrostatic electron spectrometer uses the electric field, which cause electrons to move along field gradients, whereas magnetic devices cause electrons to move at right angles to the field. Magnetic fields will act in a direction perpendicular to the electron propagation, thereby conserving velocity, whereas electrostatic fields will cause electrons to move along the field gradient, which may change electron energies if the component of the direction of propagation and field gradients are not perpendicular. Owing to these effects, sector based designs are commonly used in electron spectrometers.
The effective potential
Effective potential
The effective potential, or effective potential energy is a mathematical expression combining the centrifugal potential energy with the potential energy of a dynamical system. Commonly used in calculating the orbits of planets , the effective potential allows one to reduce a problem to fewer...
in the solution of motion in a magnetic or electric system with rotational symmetry leads to radial focusing onto a mean radius. By superposition of a quadrupole field axial focusing is possible while weakening the radial focusing, until the astigmatism
Astigmatism
An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances...
vanishes. By breaking the rotational symmetry a bit and varying the electrostatic potential along the mean path of the spherical aberration
Spherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...
is minimized.
The spectrometer can use entrance and exit slits or use a small source, which only emits into specific angle and a small detector. Photoelectron spectra from single crystals exhibit a dependency on the emission angle, so that the entrance slit is needed. All the electrons from an isotopic source may be sucked off and focused into a directed beam (much like in an electron gun
Electron gun
An electron gun is an electrical component that produces an electron beam that has a precise kinetic energy and is most often used in television sets and computer displays which use cathode ray tube technology, as well as in other instruments, such as electron microscopes and particle...
), which can then be analyzed. A position sensitive detector can detect the energy along one direction and depending on the additional optics lateral resolution or one angle along the other direction.
Electrostatic spectrometers preserve the spin
Spin polarization
Spin polarization is the degree to which the spin, i.e., the intrinsic angular momentum of elementary particles, is aligned with a given direction. This property may pertain to the spin, hence to the magnetic moment, of conduction electrons in ferromagnetic metals, such as iron, giving rise to...
, which can be resolved afterwards.
See also
- Mass spectrometryMass spectrometryMass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...
- Electron energy loss spectroscopyElectron energy loss spectroscopyIn electron energy loss spectroscopy a material is exposed to a beam of electrons with a known, narrow range of kinetic energies. Some of the electrons will undergo inelastic scattering, which means that they lose energy and have their paths slightly and randomly deflected...
- Time-of-flight mass spectrometryTime-of-flight mass spectrometryTime-of-flight mass spectrometry is a method of mass spectrometry in which an ion's mass-to-charge ratio is determined via a time measurement. Ions are accelerated by an electric field of known strength. This acceleration results in an ion having the same kinetic energy as any other ion that has...
- PEEMPEEMPhotoemission Electron microscopy is a widely used type of emission microscopy. PEEM utilizes local variations in electron emission to generate image contrast. The excitation is usually produced by UV light, synchrotron radiation or X-ray sources...
- Energy filtered transmission electron microscopyEnergy filtered transmission electron microscopyEnergy-filtered transmission electron microscopy is a technique used in Transmission electron microscopy, in which only electrons of particular kinetic energies are used to form the image or diffraction pattern...