Absorbance and Emission Spectroscopy

The illustration here is a simplified energy diagram showing the absorption and emission of a photon by an atom or a molecule. When a photon of energy  strikes the atom or molecule, absorption may occur if the difference in energy, ΔE, between the ground state and the excited state is equal to the photon’s energy. An atom or molecule in an excited state may emit a photon and return to the ground state. The photon’s energy, , equals the difference in energy, ΔE, between the two states.


When an atom or molecule absorbs electromagnetic radiation the number of photons passing through the sample decreases. The measurement of this decrease in photons, which we call absorbance, is a useful analytical signal. A plot of absorbance as a function of the photon’s energy is called an absorbance spectrum. The illustration below, for example, shows the absorbance spectrum of cranberry juice. The anthocyanin dyes in cranberry juice absorb visible light with blue, green, and yellow wavelengths; as a result, the juice appears red.


When an atom or molecule in an excited state returns to a lower energy state, the excess energy often is released as a photon, a process we call emission. There are several ways in which an atom or molecule may end up in an excited state, including thermal energy, absorption of a photon, or by a chemical reaction. Emission following the absorption of a photon is also called photoluminescence, and that following a chemical reaction is called chemiluminescence.

A typical photoluminescence emission spectrum is shown in the illustration below for the dye coumarin 343. The dye’s absorbance spectrum (which is not shown) has a broad peak centered around 400 nm. The sharp peak at 409 nm is from the laser source used to excite coumarin 343. The broad band at approximately 500 nm is the dye’s emission band. Because the dye absorbs blue light, a solution of coumarin 343 appears yellow in the absence of photoluminescence. Its photoluminescent emission is blue-green.


(Source: data courtesy of Bridget Gourley, Department of Chemistry & Biochemistry, DePauw University).

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