Analytical Chem istry - DePauw University

Chapter 10 Spectroscopic Methods613(a) (b) (c)10Fsingletground statesingletexcited statePhotoluminescence Spectroscopytripletexcited statePhotoluminescence is divided into two categories: fluorescence and phosphorescence.A pair of electrons occupying the same electronic ground statehave opposite spins and are said to be in a singlet spin state (Figure 10.47a).When an analyte absorbs an ultraviolet or visible photon, one of its valenceelectrons moves from the ground state to an excited state with a conservationof the electron’s spin (Figure 10.47b). Emission of a photon from thesinglet excited state to the singlet ground state—or between any twoenergy levels with the same spin—is called fluorescence. The probabilityof fluorescence is very high and the average lifetime of an electron in theexcited state is only 10 –5 –10 –8 s. Fluorescence, therefore, decays rapidlyonce the source of excitation is removed.In some cases an electron in a singlet excited state is transformed to atriplet excited state (Figure 10.47c) in which its spin is no longer pairedwith the ground state. Emission between a triplet excited state and a singletground state—or between any two energy levels that differ in their respectivespin states–is called phosphorescence. Because the average lifetimefor phosphorescence ranges from 10 –4 –10 4 s, phosphorescence may continuefor some time after removing the excitation source.The use of molecular fluorescence for qualitative analysis and semiquantitativeanalysis can be traced to the early to mid 1800s, with moreaccurate quantitative methods appearing in the 1920s. Instrumentationfor fluorescence spectroscopy using a filter or a monochromator for wavelengthselection appeared in, respectively, the 1930s and 1950s. Althoughthe discovery of phosphorescence preceded that of fluorescence by almost200 years, qualitative and quantitative applications of molecular phosphorescencedid not receive much attention until after the development offluorescence instrumentation.10F.1 Fluorescence and Phosphorescence SpectraTo appreciate the origin of fluorescence and phosphorescence we must considerwhat happens to a molecule following the absorption of a photon.Let’s assume that the molecule initially occupies the lowest vibrational energylevel of its electronic ground state, which is a singlet state labeled S 0 inFigure 10.47 Electron configurations for(a) a singlet ground state; (b) a singlet excitedstate; and (c) a triplet excited state.