Plasma oscillations, also known as Langmuir waves (after
Irving Langmuir
Irving Langmuir (; January 31, 1881 – August 16, 1957) was an American chemist, physicist, and engineer. He was awarded the Nobel Prize in Chemistry in 1932 for his work in surface chemistry.
Langmuir's most famous publication is the 1919 art ...
), are rapid oscillations of the
electron density
In quantum chemistry, electron density or electronic density is the measure of the probability of an electron being present at an infinitesimal element of space surrounding any given point. It is a scalar quantity depending upon three spatial va ...
in conducting media such as
plasmas or
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
s in the
ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
region. The oscillations can be described as an instability in the
dielectric function of a free electron gas. The frequency only depends weakly on the wavelength of the oscillation. The
quasiparticle
In physics, quasiparticles and collective excitations are closely related emergent phenomena arising when a microscopically complicated system such as a solid behaves as if it contained different weakly interacting particles in vacuum.
For exa ...
resulting from the
quantization of these oscillations is the
plasmon
In physics, a plasmon is a quantum of plasma oscillation. Just as light (an optical oscillation) consists of photons, the plasma oscillation consists of plasmons. The plasmon can be considered as a quasiparticle since it arises from the quantiz ...
.
Langmuir waves were discovered by American
physicist
A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe.
Physicists generally are interested in the root or ultimate caus ...
s
Irving Langmuir
Irving Langmuir (; January 31, 1881 – August 16, 1957) was an American chemist, physicist, and engineer. He was awarded the Nobel Prize in Chemistry in 1932 for his work in surface chemistry.
Langmuir's most famous publication is the 1919 art ...
and
Lewi Tonks
Lewi Tonks (1897–1971) was an American quantum physicist noted for his discovery (with Marvin D. Girardeau) of the Tonks–Girardeau gas.
Tonks was employed by General Electric for most of his working life, researching microwaves and ferromagne ...
in the 1920s. They are parallel in form to
Jeans instability
In stellar physics, the Jeans instability causes the collapse of interstellar gas clouds and subsequent star formation, named after James Jeans. It occurs when the internal gas pressure is not strong enough to prevent gravitational collapse of ...
waves, which are caused by gravitational instabilities in a static medium.
Mechanism
Consider an electrically neutral plasma in equilibrium, consisting of a gas of positively charged
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 conve ...
s and negatively charged
electrons
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
. If one displaces by a tiny amount an electron or a group of electrons with respect to the ions, the
Coulomb force pulls the electrons back, acting as a restoring force.
'Cold' electrons
If the thermal motion of the electrons is ignored, it is possible to show that the charge density oscillates at the ''plasma frequency''
: