Ultraviolet and Visible Light Spectra

When organic molecules in solution are exposed to light in the region of the spectrum of visible and ultraviolet light, then they absorv light of certain wavelengths depending on the type of electronic transition that is associated with the absorption. The transitions depend on the type of bonding between the electrons within the molecule. The extent of the absorption of light at a particular wavelength can be calculated by using Beer's law. Beer stated that the amount of light absorbed (A) to the concentration of absorbing substance (c) and the length of the path of radiation passing through the sample (b).

A = a x b x c The a is the constant which stands for the absorptivity of the particular absorbing species. The absoptivity depends not only on the molecule whose absorbance is being determined, but also on the temperature and wavelength of light used in the analysis.

Fluorescence Spectrum

A molecule that initially absorbs ultraviolet light to get to the excited state and then emit UV or visible light to return to the ground state is said to have undergone photoluminescence. The emission of light is either fluorescence or phosphorescence depending on how the electron returns to the ground state. It is fluorescence if it is a singlet excited state and phosphorescence if it is a triplet excited state.

Fluorescence normally has a longer wavelength than the radiation used for the excitation. This is because some of the internal energy is lost before the fluorescent emission occurs.

Many molecules do not have any photoluminescence even though they can absorb ultraviolet light. The return to the ground state in these molecules occurs through the internal conversion of the excitation energy into vibrational energy. The result is heat rather than photoluminescence. Generally a molecule that phosphoresces or fluoresces contain at least one aromatic ring. Photoluminescent analysis is more sensitive and selective than absorption spectrophotometry.