Let’s assume the molecule initially occupies the lowest vibrational energy level of its electronic ground state, which is the singlet state labeled S 0 in Figure 10.6.2 To appreciate the origin of fluorescence and phosphorescence we must consider what happens to a molecule following the absorption of a photon.

Although the discovery of phosphorescence preceded that of fluorescence by almost 200 years, qualitative and quantitative applications of molecular phosphorescence did not receive much attention until after the development of fluorescence instrumentation.Īs you might expect, the persistence of long-lived phosphorescence made it more noticeable. Instrumentation for fluorescence spectroscopy using a filter or a monochromator for wavelength selection appeared in, respectively, the 1930s and 1950s. The use of molecular fluorescence for qualitative analysis and for semi-quantitative analysis dates to the early to mid 1800s, with more accurate quantitative methods appearing in the 1920s. Because the average lifetime for phosphorescence ranges from 10 –4–10 4 s, phosphorescence may continue for some time after we remove the excitation source. Emission between a triplet excited state and a singlet ground state-or between any two energy levels that differ in their respective spin states–is called phosphorescence. In some cases an electron in a singlet excited state is transformed to a triplet excited state (Figure 10.6.1Ĭ) in which its spin no is longer paired with the ground state. Fluorescence, therefore, rapidly decays once the source of excitation is removed.

The probability of fluorescence is very high and the average lifetime of an electron in the excited state is only 10 –5–10 –8 s. Emission of a photon from a singlet excited state to the singlet ground state-or between any two energy levels with the same spin-is called fluorescence. When an analyte absorbs an ultraviolet or a visible photon, one of its valence electrons moves from the ground state to an excited state with a conservation of the electron’s spin (Figure 10.6.1ī). Electron configurations for (a) a singlet ground state (b) a singlet excited state and (c) a triplet excited state. A pair of electrons that occupy the same electronic ground state have opposite spins and are in a singlet spin state (Figure 10.6.1 Photoluminescence is divided into two categories: fluorescence and phosphorescence.

Intensity equation light chemistry