Being very high intensity, this focus induces a population inversion in the oxygen atoms, ensuring that there are more excited than non-excited atoms.
Some of the excited atoms undergo radiative deexcitation.
The boundary radii of excited atoms are defined by the classical turning point of the electron moving in an atom.
Hence the spectrum is called a line spectrum; it is also called an atomic emission spectrum since it is produced by emission of light from excited atoms.
Such states can be produced by the multiphoton interaction between the excited atoms and the field in squeezed vacuum state.
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