![]() ![]() By a combination of internal conversions and vibrational relaxations, a molecule in an excited electronic state may return to the ground electronic state without emitting a photon. Because vibrational relaxation is so efficient, a molecule in one of its excited state’s higher vibrational energy levels quickly returns to the excited state’s lowest vibrational energy level.Īnother form of radiationless deactivation is an internal conversion in which a molecule in the ground vibrational level of an excited state passes directly into a higher vibrational energy level of a lower energy electronic state of the same spin state. Vibrational relaxation is very rapid, with an average lifetime of <10 –12 s. One example of radiationless deactivation is vibrational relaxation, in which a molecule in an excited vibrational energy level loses energy by moving to a lower vibrational energy level in the same electronic state. When a molecule relaxes without emitting a photon we call the process radiationless deactivation. The absorption, fluorescence, and phosphorescence of photons also are shown. The electronic ground state is shown in black and the three electronic excited states are shown in green. The lowest vibrational energy for each electronic state is indicated by the thicker line. The most likely relaxation pathway is the one with the shortest lifetime for the excited state.įigure 10.48 Energy level diagram for a molecule showing pathways for the deactivation of an excited state: vr is vibrational relaxation ic is internal conversion ec is external conversion and isc is an intersystem crossing. These relaxation mechanisms are shown in Figure 10.48. Relaxation to the ground state occurs by a number of mechanisms, some involving the emission of photons and others occurring without emitting photons. Absorption of a photon excites the molecule to one of several vibrational energy levels in the first excited electronic state, S 1, or the second electronic excited state, S 2, both of which are singlet states. Let’s assume that the molecule initially occupies the lowest vibrational energy level of its electronic ground state, which is a singlet state labeled S 0 in Figure 10.48. 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.ġ0.6.1 Fluorescence and Phosphorescence Spectra ![]() 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 semi-quantitative analysis can be traced 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 removing the excitation source.įigure 10.47 Electron configurations for (a) a singlet ground state (b) a singlet excited state and (c) a triplet excited state. 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 10.47c) in which its spin is no longer paired with the ground state. ![]() ![]() Fluorescence, therefore, decays rapidly 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 the 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 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 10.47b). A pair of electrons occupying the same electronic ground state have opposite spins and are said to be in a singlet spin state (Figure 10.47a). Photoluminescence is divided into two categories: fluorescence and phosphorescence. ![]()
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