Molecular beam

Molecular beam

Atomic beam and molecular beam are important methods for studying the structure of atoms and molecules and the interaction of atoms and molecules with other substances. The distances between atoms or molecules in solids, liquids and dense gases are small, and there are complex interactions. It is difficult to study the properties of isolated molecules. The distance between molecules in rare gases is large, and the interaction becomes weaker as the pressure decreases. However, the random movement of molecules makes it difficult to detect and study the molecules themselves. In atomic or molecular beams, atoms or molecules move in a well-aligned direction, and the interaction between them can be ignored. Therefore, the beam can be considered as a collection of moving isolated atoms or molecules, which can be used to study molecules, The nature of the atom itself and the interaction of molecules, atoms and other particles. This type of research is very important to some fields of atomic and molecular physics, gas laser dynamics, plasma physics, micro-chemical reaction dynamics, space physics, astrophysics, and biology. In addition, atomic beams and molecular beams can also be used to study the surface and solid structure of objects.


The experimental device for atomic and molecular beams can be roughly divided into three parts: atomic or molecular collimated beam source, experimental area and detector. A simple atom beam or molecular beam source is a sealed gas chamber called a source chamber with a collimating hole, and atoms or molecules are ejected from the collimating hole. At a certain distance facing the small hole of the beam source, another small hole tube is placed to collimate the beam, called a sharpener, and only the molecules passing through the tube hole can enter the experimental area. For a solid substance with a very low vapor pressure at room temperature, it can be heated to vaporize, and the indoor vapor pressure can be controlled by adjusting the temperature of the source chamber. The atoms or molecules emitted from the source chamber generate beams in the adjacent high-vacuum experimental zone. The average velocity of beam atoms and molecules is about 10 cm/s. The ions produced by the ion source can also be accelerated and focused by an electric field, and electrons are added to produce a higher-speed atomic or molecular beam. The speed of the atoms or molecules can reach 10cm/s or higher, and they are in an excited state. However, the vapor pressure in the furnace is not high, and the flow of atomic and molecular beams is not high. If you want to obtain a high-intensity molecular beam, you can make the gas from the high-pressure zone pass through the micro nozzle, adiabaticly expand to the vacuum chamber, and form an ultrasonic molecular beam. Through this process, part of the internal energy of the molecules is converted into kinetic energy for directional translation, the molecules are cooled, and the intensity of the molecular beam is also increased.


The surface ionization method can be used to detect atoms and molecular beams. When the beam is used to bombard the metal surface, the atoms with low ionization potential in the beam lose electrons and become positive ions due to collisions. . The number of atoms or molecules can be detected by measuring the ion current. The number of particles in the beam can also be detected by the secondary electron beam generated when the beam particles of higher energy bombard the solid surface. When the beam current is very weak, pulse counting with an electron multiplier can greatly improve the detection sensitivity. The experimental area and the detector part are generally in a high vacuum.


Because when the laser beam of the frequency modulation laser crosses the atomic and molecular beam, the atoms or molecules in the beam can be selectively excited to a specific excited state, including molecules It is possible to study various types of collision cross sections, interaction potentials and chemical reactions when atoms or molecules are in a certain excited state. This is a new and large research field.

Through the cascade excitation of lasers of different frequencies, the atoms in the beam can also be excited to highly excited states and self-ionized states, so as to study the properties of these states. The probability of field ionization and self-ionization of this kind of atomic state is very high (close to 1), and the ions produced by ionization can be counted. Therefore, it can be detected as long as the atoms can be turned into ions. After taking certain measures to improve the sensitivity and eliminate the background noise in the detection, the detection of a single atom can be realized.

When the molecule has a magnetic or electric dipole moment, the dipole moment orientation can be selected by the interaction of the external magnetic field and electric field and the dipole moment, so that the atoms and molecules with different dipole moments are separated in space . By adopting this measure, precise atomic and molecular beam spectroscopy experiments can be carried out, the magnetic moment of the nucleus can be accurately measured, and the frequency or time measurement standards of atoms and molecules can be developed.

When the low current intensity of atoms, molecular beams and beams are used, the collision broadening of the atomic and molecular spectral lines can be ignored (see spectral line broadening); selective saturated absorption and pairing can also be used The two-photon transition method of the standing wave field further eliminates the Doppler broadening of the atomic and molecular spectral lines, which enables the study of the spectra and energy levels of free atoms and molecules with extremely high precision. With some appropriate arrangements, it can also measure Lamb shifts, verify quantum electrodynamics, and determine some basic physical constants.

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