Molecular beam

A molecular beam or molecular beam ( English molecular beam , MB) is a directed jet of material molecules . Molecular beams have a wide range of applications in atomic , molecular , cluster and surface physics , in surface chemistry and in physical chemistry . A technically relevant application is molecular beam epitaxy .

The method was developed in 1911 by Louis Dunoyer de Segonzac and from 1919 by Otto Stern , who received the Nobel Prize for it.

generation

There are various methods of generating molecular beams that have a strong influence on the speed, temperature , density and divergence of the molecular beam.

Effusive source

In this setup (also known as a Knudsen cell), a gas is expanded from a reservoir through a small opening into a vacuum chamber. With this method, the nozzle diameter is much smaller than the mean free path of the molecules ( Knudsen number greater than 1), so that the exit of molecules does not affect the state of the gas in front of the opening. The speed distribution and the energy distribution of the internal degrees of freedom (vibrations, rotation) of the exited molecules correspond to the Maxwell-Boltzmann distribution of the gas in the reservoir. So they only depend on the molecular mass and the temperature of the gas. The average speed of small molecules at room temperature is in the range of a few hundred meters per second.

The resulting molecular beam is sometimes referred to as the effusive beam or Knudsen beam . If a beam with a narrower velocity distribution than the Maxwell-Boltzmann distribution is required, molecules with the desired velocity distribution can be filtered out of the effusive beam by using a velocity selector.

Jet method

Similar to the effusive source, the jet method also expands a gas from a reservoir into a vacuum chamber, but if a sufficiently high pressure is used, qualitatively different expansion conditions prevail: The mean free path of the gas molecules must be much smaller than the nozzle diameter, so that the non-directional thermal Movement of the molecules can be converted into a directed movement by collisions with one another. The axial velocities of the molecules in the expansion zone of the nozzle are equal to one another, and all degrees of freedom of the molecules are cooled adiabatically . The kinetic energy of the molecules is almost completely converted into translational energy, creating a jet of molecules with a very low temperature of the internal degrees of freedom (rotation, vibration).

Both the speed and the temperature of the jet depend on the pressure in the reservoir and the temperature of the nozzle. The temperature that can be reached is limited by the formation of clusters and the associated generation of condensation heat . The cluster formation can be suppressed by adding a noble gas ( seeded beam ). If the concentration of the molecules is sufficiently low, the beam properties are determined by the carrier gas. With light carrier gases, high speeds can be achieved (for example, helium with at room temperature), low speeds a heavy carrier gas is such. B. Xenon used ( ). ${\displaystyle v\approx 1800\ {\text{m/s}}}$ ${\displaystyle v\approx 330\ {\text{m/s}}}$

A jet peeler (English: skimmer ) attached behind the nozzle and a suitable arrangement of apertures enable the formation of a collimated jet profile. So that the quality of the nozzle jet is not impaired by impacts with residual gas, a lower pressure must be maintained in the vacuum chamber , which requires the use of at least one vacuum pump with high pumping speed . A suitable shape of the nozzle reduces the opening angle of the jet and thus the necessary suction power. ${\displaystyle 10^{-3}\ {\text{mbar}}}$

Applications

A large area of ​​application for molecular beams are scattering experiments , in which the beams are scattered on gaseous, liquid or solid targets .

In the molecular beam method , molecular or atomic beams are arranged crosswise. When the molecules collide, they can react chemically with one another; excitation or de-excitation processes of vibrations and rotational movements can also take place. By examining the velocity distributions, the chemical composition and the internal excitations of the scattered particles, diverse information can be obtained about the intermolecular interactions and the reaction processes of the molecules involved.

In an analogous form, molecular beams can be scattered at the interfaces of solids or liquids.

Another area of ​​application is molecular beam epitaxy for the deposition of thin layers on surfaces.

See also

• Atomic beam

literature

• R. Campargue: Atomic and Molecular Beams: The State of the Art 2000 . Springer, 2003 ( limited preview in Google Book Search).

Individual evidence

1. ^ L. Dunoyer: Sur la réalisation d'un rayonnement matériel d'origine purement thermique. Cinétique expérimentale . In: Le Radium . No. October 8 , 1911, p. 142-146 , doi : 10.1051 / radium: 0191100804014201 .
2. ↑ Horst Schmidt-Böcking, Wolfgang Trageser: The ray of knowledge . 100 years ago Otto Stern developed the groundbreaking molecular beam method. In: Physics Journal . Wiley-VCH Verlag Chemie, October 2019, p. 25–28 ( pro-physik.de [PDF]).
3. ↑ L. Bergmann, C. Schaefer, W. Raith Components of matter: Atoms, molecules, atomic nuclei, elementary particles . Walter de Gruyter, 2003, ISBN 3-11-016800-6 , pp. 185ff. ( limited preview in Google Book search).