levels of and are higher than that of . When electrons in states and : subsequently decay to state , two kinds of emission are radiated.

In -type atoms, there are also 3 states: , , and :. However, in this type, is at the highest energy level, while and : are at lower levels. When two electrons in state decay to states and :, respectively, two kinds of emission are also radiated.

The derivation below follows the reference Quantum Optics.

Calculation based on semiclassical theory

In the semiclassical picture,

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