Ramsauer-Townsend effect
Encyclopedia
The Ramsauer–Townsend effect, also sometimes called the Ramsauer effect or the Townsend effect, is a physical phenomenon involving the scattering
of low-energy electron
s by atom
s of a noble gas
. Since its explanation requires the wave theory of quantum mechanics
, it demonstrates the need for physical theories more sophisticated than those of Newtonian physics
.
if they cause excitation
or ion
ization of the atom to occur and elastic if they do not.
The probability of scattering in such a system is defined as the number of electrons scattered, per unit electron current, per unit path length, per unit pressure at 0°C, per unit solid angle
. The number of collisions equals the total number of electrons scattered elastically and inelastically in all angles, and the probability of collision is the total number of collisions, per unit electron current, per unit path length, per unit pressure at 0°C.
Because noble gas atoms have a relatively high first ionization energy
and the electrons do not carry enough energy to cause excited electronic states, ionization and excitation of the atom are unlikely, and the probability of elastic scattering over all angles is approximately equal to the probability of collision.
(1879-1955) and John Sealy Townsend
(1868-1957), who each independently studied the collisions between atoms and low-energy electrons in the early 1920s.
If one tries to predict the probability of collision with a classical model that treats the electron and atom as hard spheres, one finds that the probability of collision should decrease monotonically
with increasing electron energy. However, Ramsauer and Townsend observed that for slow-moving electrons in argon, krypton, or xenon, the probability of collision between the electrons and gas atoms obtains a minimum value for electrons with a certain amount of kinetic energy (about 0.7 electron volts for xenon gas). This is the Ramsauer-Townsend effect.
No good explanation for the phenomenon existed until the introduction of quantum mechanics, which explains that the effect results from the wave-like properties of the electron. A simple model of the collision that makes use of wave theory can predict the existence of the Ramsauer-Townsend minimum. Bohm presents one such model that considers the atom as a finite square potential well
.
Predicting from theory the kinetic energy that will produce a Ramsauer-Townsend minimum is quite complicated since the problem involves relativistic, electron exchange, and spin polarisation effects. However, the problem has been extensively investigated both experimentally and theoretically and is well understood (see Johnson).
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
of low-energy electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s by atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s of a noble gas
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
. Since its explanation requires the wave theory of quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
, it demonstrates the need for physical theories more sophisticated than those of Newtonian physics
Classical mechanics
In physics, classical mechanics is one of the two major sub-fields of mechanics, which is concerned with the set of physical laws describing the motion of bodies under the action of a system of forces...
.
Definitions
When an electron moves through a gas, its interactions with the gas atoms cause scattering to occur. These interactions are classified as inelasticElasticity (physics)
In physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain....
if they cause excitation
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
or ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
ization of the atom to occur and elastic if they do not.
The probability of scattering in such a system is defined as the number of electrons scattered, per unit electron current, per unit path length, per unit pressure at 0°C, per unit solid angle
Solid angle
The solid angle, Ω, is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large that object appears to an observer looking from that point...
. The number of collisions equals the total number of electrons scattered elastically and inelastically in all angles, and the probability of collision is the total number of collisions, per unit electron current, per unit path length, per unit pressure at 0°C.
Because noble gas atoms have a relatively high first ionization energy
Ionization energy
The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...
and the electrons do not carry enough energy to cause excited electronic states, ionization and excitation of the atom are unlikely, and the probability of elastic scattering over all angles is approximately equal to the probability of collision.
Description
The effect is named for Carl RamsauerCarl Ramsauer
Carl Wilhelm Ramsauer was an internationally notable professor of physics and research physicist, famous for the discovery of the Ramsauer-Townsend effect...
(1879-1955) and John Sealy Townsend
John Sealy Townsend
John Sealy Edward Townsend, FRS was a mathematical physicist who conducted various studies concerning the electrical conduction of gases and directly measured the electrical charge...
(1868-1957), who each independently studied the collisions between atoms and low-energy electrons in the early 1920s.
If one tries to predict the probability of collision with a classical model that treats the electron and atom as hard spheres, one finds that the probability of collision should decrease monotonically
Monotonic function
In mathematics, a monotonic function is a function that preserves the given order. This concept first arose in calculus, and was later generalized to the more abstract setting of order theory....
with increasing electron energy. However, Ramsauer and Townsend observed that for slow-moving electrons in argon, krypton, or xenon, the probability of collision between the electrons and gas atoms obtains a minimum value for electrons with a certain amount of kinetic energy (about 0.7 electron volts for xenon gas). This is the Ramsauer-Townsend effect.
No good explanation for the phenomenon existed until the introduction of quantum mechanics, which explains that the effect results from the wave-like properties of the electron. A simple model of the collision that makes use of wave theory can predict the existence of the Ramsauer-Townsend minimum. Bohm presents one such model that considers the atom as a finite square potential well
Potential well
A potential well is the region surrounding a local minimum of potential energy. Energy captured in a potential well is unable to convert to another type of energy because it is captured in the local minimum of a potential well...
.
Predicting from theory the kinetic energy that will produce a Ramsauer-Townsend minimum is quite complicated since the problem involves relativistic, electron exchange, and spin polarisation effects. However, the problem has been extensively investigated both experimentally and theoretically and is well understood (see Johnson).