American physicists have created a photocathode from a single crystal of copper, the brightness of the radiation which is four times higher than the existing counterparts. For this purpose it is cooled to a temperature of 35 Kelvin and irradiated with photons with wavelengths close to the threshold of photoemission. In addition, the width of the emission spectrum of the surface was found to be only 11.5 millielectronvolt, which is significantly lower than previous results. Article published in Physical Review Letters.
One of the methods of producing electron beams — photoemission — is actively used for free electron lasers, ultrafast electron diffraction and microscopy. Photoemission — the process of knocking out of electrons by photons from the surface of the photocathode. Such electrons are called photoelectrons and their energy is determined by the difference between the energy of incident photons and the work function of the cathode E = ℏω – W. If the photon energy is so small that this difference tends to zero, this mode is called threshold photoemission.
For active use such sources of electrons in the research it is necessary that the emitted electron beams were bright — the brightness of the source shows how directionally emitted electrons from the surface of the source. If, for example, there are two sources, and one emits a hundred electrons per second, which scatter in different directions, and the second the same hundred electrons per second are flying along the same straight line, the brightness of the second will be more. If scientists can learn how to do a. with high brightness electron beams, there will be the opportunity to explore the lattice of the crystals of large sizes and you can get more information about their electronic structure. The brightness of the beam photocathode is inversely proportional to the square from which emitted electrons and the average energy in the cross section (MTE). This energy is equivalent to the temperature of the emitted photoelectrons, therefore, the temperature decrease of the photocathode leads to an increase in the brightness of the source. Another important characteristic of the photo cathodes is the dispersion of the electron energy. The smaller it is, the easier it is to investigate, for example, a very fast oscillation of the lattice.
The sources used (1, 2) now, MTE is a few hundred millielectronvolt and a rough approximation is considered equal to one third of the energy of the electrons. On the threshold of the generation of photoelectrons can be observed lower values of MTE due to the fact that the radiation comes from the “tail” of the Fermi and of MTE, you can limit the product of the Boltzmann constant on temperature. In this mode, at room temperature MTE does not exceed 25 millielectronvolt.
In 2015, physicists from Cornell University have shownthat the cooling of the photo cathodes to 90 Kelvin on the border of photoemission it is possible to achieve values of MTE in 20 millielectronvolt, while theoretical calculations have predicted 7.5 millielectronvolt at this temperature. The fact that the cathode surface is not ideal, it can be rough or heterogeneous, and the work function of electrons can vary in time. It is therefore very important to make the cathodes of the single crystals with an ordered atomic structure. Such crystals are more convenient because they are much easier to model. It is sufficient to use a simple model that takes into account the transition of electrons from the crystal to the vacuum and is consistent with the experimental data.