Molecular beam

A molecular beam or molecular beam (English molecular beam, MB) is a directed beam of molecules matter. Molecular beams can find many applications in atomic, molecular, cluster and surface physics, surface chemistry and physical chemistry. A technically relevant application is the molecular beam epitaxy.

Generation

To generate molecular beams, there are various methods, which have a strong influence on the speed, the temperature, density, and the divergence of the molecular beam.

Effusivquelle

In this ( also referred to as Knudsen cell ) structure, a gas is expanded from a reservoir through a small orifice into a vacuum chamber. The nozzle diameter of this method is much less than the mean free path of the molecules ( Knudsen number greater than 1 ) so that the escape of molecules does not influence the status of the gas prior to opening. The velocity distribution and the energy distribution of the internal degrees of freedom ( vibration, rotation) the beaten molecules correspond to the Maxwell -Boltzmann distribution of the gas in the reservoir. So you depend only on the molecular mass and the temperature of the gas. The average speed of small molecules at room temperature is in the range of several hundred meters per second.

The resulting molecular beam is sometimes called a Knudsen effusive beam or jet. A beam requires a narrower distribution than the speed of the Maxwell-Boltzmann distribution, the molecules with the desired velocity distribution can be filtered out of the Effusivstrahl by using a velocity selector.

Jet method

Similar to the Effusivquelle a gas from a reservoir is expanded in a vacuum chamber and in the jet method, but the use of a sufficiently high pressure prevailing qualitatively different expansion conditions: the mean free path of the gas molecules must be much smaller than the nozzle diameter in order for the non-directed heat movement of the molecules can be transformed by impacts with each other in a directed movement. Here, the axial velocities of the molecules in the expansion zone of the nozzle the same to each other, there occurs an adiabatic cooling of all the degrees of freedom of the molecules. The kinetic energy of the molecules is almost completely converted into translational kinetic energy, there arises 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 beam depend on the pressure in the reservoir and of the temperature of the nozzle. The achievable temperature is bounded below by the formation of clusters and the associated generation of heat of condensation. The cluster formation can be suppressed by addition of an inert gas ( seeded beam). At a sufficiently low concentration of the molecules of the beam characteristics are determined by the carrier gas. With minor carrier gases to high velocities to be achieved (for example, helium at room temperature), for low speeds, a heavy carrier gas such as xenon is used ().

A sensor mounted behind the nozzle Strahlabschäler (English: skimmer ) and a suitable arrangement of diaphragms allow the formation of a collimated beam profile. To ensure the quality of the jet stream is not affected by collisions with residual gas, a pressure must be maintained small 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 beam and thus the necessary suction.

Applications

A large field of application of molecular beams are scattering experiments, in which the rays are scattered by gaseous, liquid or solid targets.

In the molecular beam method molecular or atomic beams crosswise be arranged. During the collision of the molecules they can react chemically with each other, continue to purchase or Abregungsprozesse of vibration and rotation movements take place. By examining the velocity distributions, the chemical composition and the internal excitations of the scattered particles wealth of information on the intermolecular interactions and the reaction processes of the molecules involved can be obtained.

In analog form molecular beams to be scattered at interfaces of solids and liquids.

Another field of application is the molecular beam epitaxy for depositing thin films on surfaces.