Interaction of atomic electrons with electromagnetic radiation

1 Interaction of atomic electrons with electromagnetic radiation 

Until now we have studied the methods of theoretical description of the energy of separate, non-interacting atoms or ions. Interaction was present indirectly while speaking about many states or excited configurations, saying nothing about the mechanisms of their creation or their lifetimes. 

An atom in a ground state can exist for an infinitely long time, whereas any excited state exists only for a finite time interval. This time interval (lifetime of the excited state), together with the energy spectrum of the excited configuration, is one of the basic spectral characters of the quantum mechanical system considered. There exist various channels of decay of the excited states, and the realization of each of them depends on concrete physical conditions. 

The finite lifetime of each excited state is the reflection of a fundamental law of nature - tendency towards minimum total energy of a system. The quantum mechanical system tends to occupy the state in which its total energy would be minimal. However, the transition of an atom to the lowest (ground) state depends on many circumstances (first of all, on the sort of excited configuration, on the presence of external fields, on the character of the matter itself - density of gas, vapours or plasma, etc.). There are two main channels of decay of the excited states radiative and radiationless. In the first case the electronic transition from the higher to the lower state is connected with the radiation of one or several quanta of 

Excited states may decay also in the process of autoionization, with the detachment of an excited electron and its transition to the state of a continuous spectrum (free electron). 

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