Fluorescence and phosphorescence ar

Fluorescence and phosphorescence are analytically importantemission processes in which species are excited by absorptionof a beam of electromagnetic radiation; radiant emission thenoccurs as the excited species return to the ground state. Fluorescence occurs more rapidly than phosphorescence and is generally complete after about 1025 s from the time of excitation.Phosphorescence emission takes place over periods longer than1025 s and may indeed continue for minutes or even hours afterirradiation has ceased. Fluorescence and phosphorescence aremost easily observed at a 90° angle to the excitation beam.Resonance fluorescence describes the process in which theemitted radiation is identical in frequency to the exciting radiation. The lines labeled 1 and 2 in Figure 6-24c are examplesof resonance fluorescence. Resonance fluorescence is mostcommonly produced by atoms in the gaseous state, which donot have vibrational energy states superimposed on electronicenergy levels.Nonresonance fluorescence is caused by irradiationof molecules in solution or in the gas phase. As shown inFigure 6-24a, absorption of radiation promotes the moleculesinto any of the several vibrational levels associated with the twoexcited electronic levels. The lifetimes of these excited vibrational states are, however, only on the order of 10215 s, which ismuch smaller than the lifetimes of the excited electronic states(1028 s). Therefore, on the average, vibrational relaxationoccurs before electronic relaxation. As a result, the energy ofthe emitted radiation is smaller than that of the absorbed by anamount equal to the vibrational excitation energy. For example, for the absorption labeled 3 in Figure 6-24a, the absorbedenergy is equal to 1E2 2 E0 1 es4 2 es02, whereas the energy ofthe fluorescence radiation is again given by 1E2 2 E02. Thus,the emitted radiation has a lower frequency, or longer wavelength, than the radiation that excited the fluorescence. Thisshift in wavelength to lower frequencies is sometimes calledthe Stokes shift as mentioned in connection with Raman scattering in Figure 6-18.Phosphorescence occurs when an excited molecule relaxesto a metastable excited electronic state (called the triplet state),which has an average lifetime of greater than about 1025 s. Thenature of this type of excited state is discussed in Chapter 15.