Introduction-What happens to photons of laser radiation whose energy differs from the energy of the inter-level electronic transitions of atoms of the substance under study. According to Bohr and Einstein, the traditional scheme of interaction between radiation and matter includes the following processes: 1) resonant absorption of radiation, 2) spontaneous radiation, 3) resonant stimulated radiation. In the absence of resonance between the pumping frequency and the frequencies of electronic transitions in atoms alloying a dispersed medium, the interaction of radiation and matter is carried out due to the annihilation of pairs of pump radiation photons, the birth of axions and their subsequent decay into new photons whose energy differs from the energy of the pump photons. If in atomic vapors in the resonant case one photon is required for the transition of an electron from the ground level S to the excited P, then in the non-resonant case such a transition is possible due to the annihilation of a pair of photons.
Introduction-What happens to photons of laser radiation whose energy differs from the energy of the inter-level electronic transitions of atoms of the substance under study. According to Bohr and Einstein, the traditional scheme of interaction between radiation and matter includes the following processes: 1) resonant absorption of radiation, 2) spontaneous radiation, 3) resonant stimulated radiation. In the absence of resonance between the pumping frequency and the frequencies of electronic transitions in atoms alloying a dispersed medium, the interaction of radiation and matter is carried out due to the annihilation of pairs of pump radiation photons, the birth of axions and their subsequent decay into new photons whose energy differs from the energy of the pump photons. If in atomic vapors in the resonant case one photon is required for the transition of an electron from the ground level S to the excited P, then in the non-resonant case such a transition is possible due to the annihilation of a pair of photons.
Based on the modern idea that a photon is a stable, indivisible particle, it is necessary to abandon the generally accepted idea that in the elementary act of Raman scattering of light, a photon splits into two particles. Such a rejection and the transition to a four-photon scheme of the elementary act with the participation of an axion make it possible to preserve the integrity of the photons involved in the elementary act of Raman scattering. The laser beam interacts with vibrations of atoms in molecules, with electrons in atoms, and with phonons or other excitations in the medium under study, as a result of which frequency components appear in the radiation spectrum at the output of the medium under study, shifted relative to the laser frequency toward high or low values. Raman scattering of light is the inelastic scattering of optical radiation in the medium under study, accompanied by a noticeable change in the frequency of radiation. Unlike Rayleigh scattering, which occurs when the refractive indices of the medium and the pumping radiation coincide in phase, in Raman scattering, spectral lines appear in the spectrum of scattered radiation that are not present in the spectrum of primary (exciting) light. Raman scattering of light can be observed in various media: liquids, semiconductors, and gases. It is noteworthy that with the advent of lasers, spontaneous Raman scattering has been supplemented by forced scattering, which allows for a clear result. Using the example of laser radiation scattering in a cell with atomic potassium vapor, it is shown that axions participate in the process of Raman scattering of light. Received: 26 November 2024 | Revised: 29 July 2025 | Accepted: 28 August 2025 Conflicts of Interest The author declares that he has no conflicts of interest to this work. Data Availability Statement The data that support the findings of this study are openly available at https://journalofscience.org/index.php/GJSFR/article/view/102768, reference number [5]. Author Contribution Statement Valeriy Evgenievich Ogluzdin: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data curation, Writing – original draft, Writing – review & editing, Visualization, Supervision, Project administration.
Light emission occurs as a result of interaction between light and matter. Let us assume a two-level atom with levels 1 (ground state) and 2 (excited state). The energies of the ground and excited states are E 1 and E 2 , respectively, and the energy difference is therefore given by the difference ΔE 12 =E 2 −E 1 . When light with photons of energy equal to this difference is absorbed by the atomic or molecular system, an electron will be excited from level 1 to level 2. The energy of the photons is given by ΔE 12 =E 12 =hν 12 (h is Planck's constant and 1→2 is the frequency of light resulting from the level 1→2 transition). This energy is absorbed by an atom or a molecule that has energy levels, separated by ΔE, where these energy levels are the ground and the excited states. This kind of “quantum jump” of an electron between two states occurs in atomic systems between electronic levels; it can be extended to molecular systems, where vibrational and translational energy levels participate in the quantum jump. The interaction between light and matter involves the transition of electrons between different states. This interaction results in the absorption of photons (stimulated absorption) as well as spontaneous and stimulated emission. These processes can be described using Einstein's A and B coefficients, as will be described in the next paragraphs.
Radiation can change the energy state of matter via three optical processes: spontaneous emission, absorption and stimulated emission. In spontaneous emission, an excited atomic or molecular state decays to a lower energy state with the emission of a photon of energy equal to the difference in the quantum state energies. Photon absorption is the reverse process, with a photon causing excitation from a lower to an excited quantum state. Stimulated emission was first proposed by Einstein in a paper published in 1917. He attempted to balance spontaneous emission and photon absorption, but could not get the correct balance without the postulated process of stimulated emission (see Section 4.2). With stimulated emission, an excited quantum state is stimulated to decay to a lower quantum state by an impinging photon. The existing photon survives with another identical photon being created (so that energy is conserved). The rates at which radiative transitions occur can be calculated using quantum mechanics. The time-dependent Schrodinger equation is used with the system wavefunction comprising time-varying linear combinations of the two wavefunctions for the lower and upper quantum states. In semi-classical treatments, an atom is treated quantum mechanically with the radiation field treated classically. There is a perturbation in energy associated with, say, the electric field of the radiation interacting with the electric dipole of the atom, though other interactions can be significant if the electric dipole interaction energy is small. In absorption, for example, a differential equation representing the change in ‘weighting’ of the wavefunction from the lower to upper state is obtained. A ‘time constant’ for the speed of change represents the rate of the radiative process. We examine the quantum mechanical nature of radiation absorption in Section 10.1 before discussing (Section 10.3) another important parameter arising with radiative transitions between discrete bound quantum states: the lineshape function. Quantum Theory of the Atom–Radiation Interaction An expression for the Einstein B-coefficient for photo-absorption is derived using quantum mechanics in this section. Einstein determined the detailed balance relationship between the rates of spontaneous emission, photo-absorption and stimulated emission when light interacts with an atom by considering an atomic system of two energy levels which are in equilibrium with a black-body radiation field. We use a semi-classical treatment where the atom is treated quantum mechanically and the radiation field is treated as a classical oscillating electric field.
The phenomena which are principally responsible for the interaction of radiation with matter in everyday life are absorption and spontaneous emis sion. Light falling on matter is absorbed, leaving atoms in excited states which spontaneously emit radiation with a spread of frequency inversely proportional to the decay time. The situation is modified when the atomic decay times are sufficiently long and the radiation sufficiently intense, for then radiation may fall on excited atoms, resulting in stimulated emission rather than absorption. We saw in Chap. 2 that this third process was required to obtain the Planck blackbody radiation law in Einstein’s derivation. It is stimulated emission which enables an atomic medium to amplify incident radiation, and therefore is an essential ingredient in light amplifiers and oscillators. In fact, the word LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.
We treat the interaction between an electromagnetic radiation bath and a material system in the semi-classical picture: the radiation is treated according to the Maxwell equations, but the process of emission/absorption is described through the concept of a photon; on the other hand, the material system is modelled according to quantum mechanics, assuming a discrete two-level energy spectrum. This very simple model allows one to elaborate the elementary theory of the absorption/emission coefficients, both for the spontaneous and for the stimulated processes which will be described within the electric dipole approximation. Selection rules for allowed transitions are eventually obtained and the nature of high-order transition is briefly outlined. Finally, a qualitative description of the light amplification by stimulated emission of radiation is presented.
Radiative transitions When a system changes its energy as the result of the emission or the absorption of a photon, it is said to undergo radiative transition. There are three kinds of radiative transition. In the presence of an electromagnetic field the system can absorb a photon, so that its energy is raised to a higher level. If the system is initially in a state other than the ground state, it can emit a photon and so shed energy. This can happen without the presence of any external electromagnetic field, in which case a spontaneous emission is said to occur. On the other hand, if the excited system is placed in an electromagnetic field that varies in time with the appropriate frequency, the probability that it will emit a photon can be greatly enhanced. In this case the emission process is known as stimulated emission . Stimulated emission is the basis of maser and laser action, which we describe in this chapter. First, we construct a simple model that allows us to investigate how the internal structure of a quantum system changes as the result of a radiative transition. Although the model is very simple, it will allow us to understand the main features of the proper, more exact treatment. We shall deal mainly with absorption and stimulated emission, the two types of radiative transition where there is an external electromagnetic field.