In the ionization process, the ions receive an average of 3/5 of the momentum of the initial photon and thus move in opposite to the photon momentum direction. It is found out through careful measurement of the pulses that are acquired charged ions of helium and nitrogen after the photoionization of an electron from the inner K-shell. The results for both types of atoms coincide with the theoretical predictions in a wide photon energy range from 300 electron volts to 40 Kev. Work published in Physical Review Letters, briefly on the results reported by the journal Physics.
As the classical theory of electromagnetic waves Maxwell could not explain many features of the photoelectric effect, his study contributed to the development of modern quantum physics. In particular, to construct a theory of this phenomenon, albert Einstein brought the idea that the energy of electromagnetic wave is absorbed only in certain portions, called particles absorbed photons. For the discovery of the laws of the photoelectric effect, Einstein in 1921 received the Nobel prize in physics.
Photoionization — similar to the photoelectric effect the process by which an electron departs from the atom when absorbed by the atom, the incoming photon. This electron is called a photoelectron and an atom after it becomes a charged ion. The theory of photoionization of individual atoms actively developed in the 1920-ies the efforts of Pierre-Victor auger (Pierre Victor Auger) and Jean-Baptiste Perrin (Jean Baptiste Perrin): in 1927 they published a paper on the distribution of the pulses of photoelectrons. In particular, they noted that the photoelectrons are emitted predominantly in the direction of the initial motion of the photon, and that their momentum “…more than 50 percent higher than the momentum of the photon” — leaving, however, is a phenomenon without explanation. Similar phenomenon was reported by other authors, noting that the implementation of the law of conservation of momentum to the ion, which turns the atom after the departure of the photoelectron must inevitably move in the direction whence came the photon.
According to the quantum mechanical description of photoionization, the average reverse pulse of ions occurs due to interferences between dipole and quadrupole transitions, which individually give a symmetrical distribution of momentum of the ions. This is in some contradiction with the effect of light pressure in the direction of propagation of photons. This effect was only briefly mentioned in the context of the influence of light pressure to vnutricostna processes, and only in 2014 a group of theoreticians from the University of Sherbrooke (Canada) more calculated pulses of ion and electron under different scenarios photoionization, in particular showing that in the process of single photon ionization ion acquires backward momentum value of 3/5 of the initial momentum of the photon.
To confirm an existing theory, scientists from the Institute of nuclear physics, Goethe under the direction of Reinhard Dörner (Reinhard Dörner) measured the distribution of impulses of ions of nitrogen N+ and helium He+ after ionization under the influence of synchrotron radiation. The measurements were carried out at the synchrotron PETRA III (Hamburg, Germany) for circularly polarized radiation with energies up to 1775 electron volts, and also at the European Synchrotron Radiation Source (ESRF, Grenoble, France) for linearly polarized light with photon energies from 12 to 40 Kev. To measure the charge and all three components of the momentum of the ions used ion the part of the installation of the ion pulse spectroscopy with cold target (COLTRIMS), which allows you to collect information about the ions in a full solid angle 4π. In the case of low (high) energies, a beam of helium atoms (the nitrogen molecules) were crossed with the photon beam. Scattered ions are collected by the spectrometer, with the point of impact of the ions with zero momentum were determined using Compton scattering.
The average values of pulses of ions, obtained in the measurement result, clearly fall on a straight line corresponding to 3/5 of the momentum of the initial photon. Momentum of electrons in the work was not directly measured, but according to the law of conservation of momentum, it is on average equal to 8/5 of the photon momentum. The authors also measured the angular distribution of the pulses for low-energy case (helium atoms). Due to the angular symmetry of the pulses of ions are located in the area of the circles with a different radius, but the center of these circles is shifted forward by an amount corresponding to the initial momentum of the photon. This means that the center of mass of the system (which almost coincides with the position of the ion) acquires the momentum of the photon. The pulses of ions are distributed along these rings asymmetrically relative to the vertical axis of symmetry of the ring, and the average value of the projection of the ion momentum on the direction of propagation of the photon as -3/5 of the photon momentum.