Light and general radiation laws. Coherence and incoherence. Emission, absorption and amplification of radiation. Units and physical constants

Emission, absorption and amplification of radiation

It is known that atomic systems, such as atoms, ions and molecules, can be in certain stationary states, each of which corresponds to a certain value of energy. These states are characterized by quantum numbers. The energy values ​​are called the levels of the atomic system. If two or more states have the same energy, then they speak of a degenerate level, and the number of states with the same energy is equal to the multiplicity of the level degeneracy. Transitions between stationary states can occur during emission or absorption of the energy of electromagnetic radiation or during transmission or reception of energy when interacting with another system. If the transition is radiative, the frequency of the emitted or absorbed radiation system is determined by the Bohr frequency rule:

(1.4)
where E₁ and E₂ are the energies of the states between which the transition occurs, and h is the Planck constant.

The lowest energy level of the system is called the ground level, and all others - the excited levels. In the ground state, the atom can only absorb radiation. If the atomic system is not at the ground level, it can go to a lower level without any connection with the existing external radiation. This phenomenon is called spontaneous emission. The probability that, per unit time, an atom from level n will pass spontaneously to a lower level m, is called the probability of a spontaneous transition. It is denoted by A_nm. Spontaneous emission is emitted by an ensemble of atoms with a disordered phase, hence an ensemble of independent atoms will emit as an incoherent source.

Between different atomic or molecular energy levels there are not only spontaneous but also forced transitions caused by electromagnetic radiation of the corresponding frequency. The total probability that the atomic system will go from level n to a lower energy level, m per unit time, is defined as
(1.5)
where is the radiation density at a frequency corresponding to the transition energy; A_nm and are constants characterizing the atomic system. In the presence of radiation of the corresponding frequency, the atomic system can also move from the lower one to a higher energy level. The probability of such a process (absorption) is
(1.6)
The radiation emitted by the atomic system in the presence of external radiation consists of two parts. The part whose intensity is proportional to A_nm is spontaneous emission; its phase does not depend on the phase of external radiation. A part whose intensity is proportional is forced radiation; its phase is the same as that of the external radiation that causes it.

Relations between the values of A and B are known as the Einstein relations:

, (1.7)

These relations are valid in a vacuum for particles that have only nondegenerate energy levels.