Thermal de Broglie wavelength


In physics, the thermal de Broglie wavelength is roughly the average de Broglie wavelength of the gas particles in an ideal gas at the specified temperature. We can take the average interparticle spacing in the gas to be approximately where is the volume and is the number of particles. When the thermal de Broglie wavelength is much smaller than the interparticle distance, the gas can be considered to be a classical or Maxwell–Boltzmann gas. On the other hand, when the thermal de Broglie wavelength is on the order of or larger than the interparticle distance, quantum effects will dominate and the gas must be treated as a Fermi gas or a Bose gas, depending on the nature of the gas particles. The critical temperature is the transition point between these two regimes, and at this critical temperature, the thermal wavelength will be approximately equal to the interparticle distance. That is, the quantum nature of the gas will be evident for
i.e., when the interparticle distance is less than the thermal de Broglie wavelength; in this case the gas will obey Bose–Einstein statistics or Fermi–Dirac statistics, whichever is appropriate. This is for example the case for electrons in a typical metal at T = 300 K, where the electron gas obeys Fermi–Dirac statistics, or in a Bose–Einstein condensate. On the other hand, for
i.e., when the interparticle distance is much larger than the thermal de Broglie wavelength, the gas will obey Maxwell–Boltzmann statistics. Such is the case for molecular or atomic gases at room temperature, and for thermal neutrons produced by a neutron source.

Massive Particles

For massive, non-interacting particles, the thermal de Broglie wavelength can be derived from the calculation of the partition function. Assuming a 1-dimensional box of length, the partition function is :
Since the energy levels are extremely close together, we can approximate this sum as an integral:
Hence,
where is the Planck constant, is the mass of a gas particle, is the Boltzmann constant, and is the temperature of the gas.
This can also be expressed using the reduced Planck constant as:

Massless particles

For a massless particle, the thermal wavelength may be defined as:
where c is the speed of light. As with the thermal wavelength for massive particles, this is of the order of the average wavelength of the particles in the gas and defines a critical point at which quantum effects begin to dominate. For example, when observing the long-wavelength spectrum of black body radiation, the "classical" Rayleigh–Jeans law can be applied, but when the observed wavelengths approach the thermal wavelength of the photons in the black body radiator, the "quantum" Planck's law must be used.

General definition of the thermal wavelength

A general definition of the thermal wavelength for an ideal quantum gas in
any number of dimensions and for a generalized relationship between energy and
momentum has been given by Yan. It is of practical importance, since there are many experimental situations with different dimensionality and dispersion relationships. If is the number of dimensions, and the relationship
between energy and momentum is given by:
where and are constants, then the thermal wavelength is defined as:
where Γ is the Gamma function. For example, in the usual case of massive particles in a 3-D gas we have , and which gives the above results for massive particles. For massless particles in a 3-D gas, we have, and which gives the above results for massless particles.

Examples

Some examples of the thermal de Broglie wavelength at 298 K are given below.
Molecule
H23.3474E-277.1228E-11
N24.6518E-261.91076E-11
O25.31352E-261.78782E-11
F26.30937E-261.64105E-11
Cl21.1614E-251.2093E-11
HCl5.97407E-261.68586E-11