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Chapter: Modern Analytical Chemistry: Spectroscopic Methods of Analysis

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.

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 = E2E1

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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