Origin and properties of the absorption spectra

The profile of the absorption spectrum will depend extensively on the relative position of the value of Ei which depends on the different vibrational states. The intensity of absorption spectrum will depend, among others, on the population of molecules reaching the excited state. More this population is important, more the intensity of the corresponding absorption spectrum is high. Therefore, recording an absorption spectrum of the same molecule at different temperatures should in principle yield altered or modified absorption spectra. 

An absorption spectrum is characterized by the position of its peak (the maximum), and the full width at half maximum (FWHM) which is equal to the difference 

Vl and V2 correspond to the frequencies that are equal to the half of the maximal intensity. 

If the studied molecules do not display any motions, the spectral distribution will display a Lorenzien type profile. In this case, the probability of the electronic transition Ei — Ef is identical for all the molecules that belongs to the Ei level. 

Thermal motion of the molecules will induce different speed of displacement of the molecules and thus different transition probabilities. These facts change from one molecule to another and from a population of molecules to another. In this case, the spectral distribution will be a Gaussian type. The full width at half maximum of a Gaussian spectrum is larger than that of a Lorenzian one. 

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