The Absorption Spectrum

If we monitor a beam of light shining through a sample containing a substance that can absorb one of the beam's wavelengths, we can obtain a plot of the amount of light absorbed versus the wavelength, as shown in Figure 1a. This plot is known as an absorption spectrum, and shows which particular wavelengths of light a chemical species can absorb. The energy associated with the absorbed wavelength corresponds to the energy difference between the electronic levels involved in the excitation of the electrons in the absorber. In solution, these energy levels are modified by the properties of the solvent. Subsequently, they can have a number of different energy values. Thus, when we repeat the experiment with a molecule or ion in solution, many different wavelengths adjacent to the main transition wavelength are also absorbed. This is shown in Figure 1b.

Figure 1: Absorbed Light vs. Wavelength

The absorption spectrum for a substance can be used to identify the presence of that substance, since every chemical species has a specific set of energy levels that it can absorb, depending on its unique electronic configuration. Unfortunately, the levels of these species in solution are modified so greatly that many different substances assume similar levels. Thus, the absorption spectra for many compounds and ions look the same.