The Ramsauer–Townsend effect, also sometimes called the Ramsauer effect or the Townsend effect, is a physical phenomenon involving the

scattering In physics, scattering is a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including particles and radiat ...

of low-energy

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 by

atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...

s of a

noble gas The noble gases (historically the inert gases, sometimes referred to as aerogens) are the members of Group (periodic table), group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn) and, in some ...

. This effect is a result of

quantum mechanics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Addison-Wesley, 1989, It is ...

. The effect is named for

Carl Ramsauer Carl Wilhelm Ramsauer (; 6 February 1879 – 24 December 1955) was a German physicist known for the discovery of the Ramsauer–Townsend effect. He pioneered the field of electron and proton collisions with gas molecules.Mehra, Volume 1, Part 2, ...

and

John Sealy Townsend Sir John Sealy Edward Townsend (; 7 June 1868 – 16 February 1957) was an Irish physicist who conducted various studies concerning the electrical conduction of gases (concerning the kinetics of electrons and ions) and directly measured the el ...

, who each independently studied the collisions between atoms and low-energy electrons in 1921.

Definitions

When an electron moves through a gas, its interactions with the gas atoms cause scattering to occur. These interactions are classified as inelastic if they cause

excitation Excitation, excite, exciting, or excitement may refer to: * Excitation (magnetic), provided with an electrical generator or alternator * ''Exite'', a series of racing video games published by Nintendo starting with ''Excitebike'' * Excite (web port ...

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

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 In geometry, a solid angle (symbol: ) is a measure of the amount of the field of view from some particular point that a given object covers. That is, it is a measure of how large the object appears to an observer looking from that point. The poin ...

. 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 In physics and chemistry, ionization energy (IE) is the minimum energy required to remove the most loosely bound electron of an isolated gaseous atom, positive ion, or molecule. The first ionization energy is quantitatively expressed as :X(g) ...

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

If one tries to predict the probability of collision with a classical model that treats the electron and atom as

hard spheres Hard spheres are widely used as model particles in the statistical mechanical theory of fluids and solids. They are defined simply as impenetrable spheres that cannot overlap in space. They mimic the extremely strong ("infinitely elastic bouncing" ...

, one finds that the probability of collision should be independent of the incident electron energy. However, Ramsauer and Townsend, independently observed that for slow-moving electrons in

argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...

krypton Krypton (from 'the hidden one') is a chemical element; it has symbol (chemistry), symbol Kr and atomic number 36. It is a colorless, odorless noble gas that occurs in trace element, trace amounts in the Earth's atmosphere, atmosphere and is of ...

, or

xenon Xenon is a chemical element; it has symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...

, the probability of collision between the electrons and gas atoms obtains a minimum value for electrons with a certain amount of kinetic energy (about 1 electron volts for xenon gas). No good explanation for the phenomenon existed until the introduction of

, 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.

Niels Bohr Niels Henrik David Bohr (, ; ; 7 October 1885 – 18 November 1962) was a Danish theoretical physicist who made foundational contributions to understanding atomic structure and old quantum theory, quantum theory, for which he received the No ...

presented a simple model for the phenomenon 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 understanding the wave nature of particles. However, the problem has been extensively investigated both experimentally and theoretically and is well understood.

References

{{DEFAULTSORT:Ramsauer-Townsend effect Scattering Physical phenomena