Fluorescence theory photon emission

Although Einstein s theory accounts well for the relative amplitudes of absorption, fluorescence and stimulated emission, the notion that fluorescence occurs spontaneously is fundamentally inconsistent with the assertion we made in Chap. 2 that an isolated system is stable indefinitely in any one of its eigenstates. If the latter principle is correct, fluorescence must be caused by some perturbation we have neglected. The quantum theory of radiation provides a way out of this conundrum. As we discussed in Chap. 3, a radiation field has an eigenstate in which the number of photons is zero. Spontaneous fluorescence can be ascribed to perturbation of the excited molecule by the zero-point radiation field [26,27]. Let s examine this rather xmsettling idea. 

Internal conversion, thermal reversion of the excited singlet to the ground state with the release of heat to surrounding molecules such as the solvent (Sj So + heat). Because these two states are of like multiplicity, the transformation is allowed in terms of quantum theory and is often a very favorable process with a rate constant close to diffusion control Fluorescence, emission of visible or ultraviolet radiation (Sj Sq -l- hi ). The emitted photon is always of a longer wavelength than the absorbed photon. This is also a quantum mechanically allowed process and usually occurs rapidly. 

The term upconversion describes an effect [1] related to the emission of anti-Stokes fluorescence in the visible spectral range following excitation of certain (doped) luminophores in the near infrared (NIR). It mainly occurs with rare-earth doped solids, but also with doped transition-metal systems and combinations of both [2, 3], and relies on the sequential absorption of two or more NIR photons by the dopants. Following its discovery [1] it has been extensively studied for bulk materials both theoretically and in context with uses in solid-state lasers, infrared quantum counters, lighting or displays, and physical sensors, for example [4, 5]. Substantial efforts also have been made to prepare nanoscale materials that show more efficient upconversion emission. Meanwhile, numerous protocols are available for making nanoparticles, nanorods, nanoplates, and nanotubes. These include thermal decomposition, co-precipitation, solvothermal synthesis, combustion, and sol-gel processes [6], synthesis in liquid-solid-solutions [7, 8], and ionothermal synthesis [9]. Nanocrystal materials include oxides of zirconium and titanium, the fluorides, oxides, phosphates, oxysulfates, and oxyfluoiides of the trivalent lanthanides (Ln ), and similar compounds that may additionally contain alkaline earth ions. Wang and Liu [6] have recently reviewed the theory of upconversion and the common materials and methods used. 

The first two chapters of this work cover theoretical and practical aspects of the emission process, the spectroscopic techniques and the equipment used to characterize the emission. Chapter 3 introduces and reviews the property of circularly polarized emission, while Chapter 4 reviews the use of lanthanide ion complexes in bioimaging and fluorescence microscopy. Chapter 5 covers the phenomenon of two-photon absorption, its theory as well as applications in imaging, while Chapter 6 reviews the use of lanthanide ions as chemo-sensors. Chapter 7 introduces the basic principles of nanoparticle upconversion luminescence and its use for bioimaging and Chapter 8 reviews direct excitation of the lanthanide ions and the use of the excitation spectra to probe the metal ion s coordination environment in eoordination compounds and biopolymers. Finally, Chapter 9 describes the formation of heterobimetallic complexes, in whieh the lanthanide ion emission is promoted through the hetero-metal. 

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