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10.5: Emission Spectroscopy

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  • An analyte in an excited state possesses an energy, E2, that is greater than its energy when it is in a lower energy state, E1. When the analyte returns to its lower energy state—a process we call relaxation—the excess energy, ∆E

    \[∆E = E_2 − E_1 \tag{10.5.1}\]

    must be released. Figure 10.4 shows a simplified picture of this process.

    The amount of time the analyte spends in the excited state—its lifetime—is short, typically 10–5–10–9 s for electronic excited states and 10–15 s for vibrational excited states. Relaxation of an analyte’s excited-state, A*, occurs through several mechanisms, including collisions with other species in the sample, by photochemical reactions, and by the emission of photons. In the first process, which is called vibrational relaxation, or nonradiative relaxation, the excess energy is released as heat.

    \[\mathrm{A^* \rightarrow A + heat} \tag{10.5.2}\]

    Relaxation by a photochemical reaction may involve a decomposition reaction

    \[\mathrm{A^* \rightarrow X + Y} \tag{10.5.3}\]

    or a reaction between A* and another species

    \[\mathrm{A^* + Z \rightarrow X + Y} \tag{10.5.4}\]

    In both cases the excess energy is used up in the chemical reaction or released as heat.

    In the third mechanism, the excess energy is released as a photon of electromagnetic radiation.

    \[\ce{A}^* \rightarrow \ce{A} + h\nu \tag{10.5.5}\]

    The release of a photon following thermal excitation is called emission and that following the absorption of a photon is called photoluminescence. In chemiluminescence and bioluminescence, excitation results from a chemical or biochemical reaction, respectively. Spectroscopic methods based on photoluminescence are the subject of the next section and atomic emission is covered in Section 10.7.