# 9.2: Absorbing Species


As shown in Figure 9.2.1, when a molecule, M, absorbs light it transitions to a higher energy, excited, electronic state, M*.  The molecule resides in the excited state for a short time before dissipating the energy along one of three pathways.  The most common relaxation pathway is radiationless relaxation where the excess energy is quickly transferred as heat through collisions with surrounding molecules in the bath.  A small fraction of molecules will relax by releasing the energy as light through the processes of fluorescence or phosphorescence.  Another relaxation pathway that is very much dependent on the nature of the electronic state leads to fragmentation through dissociation or predissociation.  The timescales for absorption, radiationless relaxation and dissociation are short, 10-13 s - 10-15 s , while fluorecence, 10-9 s and phosphorescence, 10-6 s are much slower.

Figure$$\PageIndex {1}$$: An illustration of the three major relaxation pathways for a molecule that has absorbed light in the UV - Vis.

In the next four subsections of section 9.2, the absorption characteristics of organic molecules, inorganic molecules, charge transfer systems and lanthanide containing species are presented.

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