Plasma oscillations, also known as Langmuir waves (after
Irving Langmuir), 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 typica ...
s in the
ultraviolet 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 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 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 ferromagn ...
in the 1920s. They are parallel in form to
Jeans instability 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 conv ...
s and negatively charged
electrons. If one displaces by a tiny amount an electron or a group of electrons with respect to the ions, the
Coulomb force
Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that quantifies the amount of force between two stationary, electrically charged particles. The electric force between charged bodies at rest is convention ...
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''
: