Spectroscopy Based on Emission
An analyte in an excited state possesses an energy, E2, that is greater than that when it is in a lower energy state, E1. When the analyte returns, or relaxes to a lower en- ergy state the excess energy, ∆E,
∆E = E2 – E1
must be released. Figure 10.5 shows a simplified picture of this process.
The lifetime of an analyte in the excited state, A*, is short; typically 10–5–10–9 s for electronic excited states and 10–15 s for vibrational excited states. Relaxation oc- curs through collisions between A* and other species in the sample, by photochemi- cal reactions, and by the emission of photons. In the first process, which is called vi- brational deactivation, or nonradiative relaxation, the excess energy is released as heat; thus
A* → A + heat
Relaxation by a photochemical reaction may involve a decomposition reaction in which A* splits apart
A* → X+Y
or a reaction between A* and another species
A* +Z → X+Y
In either case the excess energy is used up in the chemical reaction or released as heat.
In the third mechanism excess energy is released as a photon of electromagnetic radiation.
A* → A + hv
The release of a photon following thermal excitation is called emission, and that fol- lowing the absorption of a photon is called photoluminescence. In chemilumines- cence and bioluminescence, excitation results from a chemical or biochemical reac- tion, respectively.
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