Characteristics of an Absorption Spectrum

An absorption spectrum is the result of electronic, vibrational, and rotational transitions. The spectrum maximum (the peak) corresponds to the electronic transition line, and the rest of the spectrum is formed by a series of lines that correspond to rotational and vibrational transitions. Therefore, absorption spectra are sensitive to temperature. Raising the temperature increases the rotational and vibrational states of the molecules and induces the broadening of the recorded spectrum. 

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

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

If the molecules studied do not display any motions, the spectral distribution will display a Lorenzian-type profile. In this case, the probability of the electronic transition ) — f is identical for all molecules that belong to the ) level. 

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

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