Absorption techniques harmonic oscillator

In contrast to the harmonic oscillator of our classical model for absorption, real atoms or molecules have many energy levels and therefore also many possible transitions from lower thermally populated levels. This means that there are many absorption lines at different frequencies coi. The magnitude of the absorption coefficient a depends on the population density of the absorbing level and on the transition probabilities for the different transitions. These transition probabilities can be only calculated by using quantum mechanical methods, but there are several experimental techniques for their determination (see next section). 

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