Liquids fluorescence quantum efficiency

Scintillators Materials used for the measurement of radioactivity, by recording the radioluminescence. They contain compounds (chromophores) which combine a high fluorescence quantum efficiency, a short fluorescence lifetime, and a high solubihty. These compounds are employed as solutes in aromatic Hquids and polymers to form organic liquid and plastic scintillators, respectively. 

The fluorescence quantum efficiency of excited lanthanides in most liquids is very low. To reduce fluorescence quenching due to interactions with high-frequency vibrations in liquids, solvent molecules should have no tightly bonded atoms of low atomic mass (16). Solvents containing hydrogen or other light atoms are therefore undesirable. Aprotic liquid laser materials consist of solutions of a rare-earth salt and an inorganic aprotic solvent. 

The quantum yields of fluorescence of the different systems have also been determined relative to a single crystal of neodymium-doped YAG for which a quantum yield of unity has been assumed (Heller, 1968a). The quantum yields obtained, even if they are accurate only within a factor of two, follow the same trend as for the lifetimes, with the highest values for the acidic solutions 0.70 and >0.75 in presence of S11CI4 and SbCls, respectively. Neutral and basic solutions are less luminescent and have quantum yields of 0.5 and 0.4, respectively. Identical measurements performed on a sodium-compensated neodymium-doped calcium tungstate crystal lead to a value of 0.5. The high quantum efficiency and the low threshold (between 2 and 40 J) of these Nd3+ SeOCl2 systems clearly demonstrate that liquids are not inherently inferior to solids as laser materials. 

The spectroscopic properties and chemistry of aprotic Nd + laser liquids plus references to earlier studies are discussed by Brecher and French (V7). The oscillator strengths and fluorescence lifetimes are comparable to those in solids with quantum efficiencies near unity. Since fluorescence line-widths are smaller than in glasses, the stimulated emission cross sections are larger (1 8), although still less than in crystals. Aprotic liquid laser materials and references are listed in Ref. 19. Thus far only Nd3+ has been used as the laser ion although other lanthanide ions could also be used. 

For temperature measurement by single-dye fluorescence, the temperature sensitivity of a dye, specifically its quantum efficiency, effectively defines the temperature resolution of the measurement itself. Rhodamine B is the most common temperature-dependent fluorescent dye used in both macro- and microscale liquid applications because of its relatively strong temperature sensitivity of 2.3 % in water over a temperature range of 0-120 °C. This dye is also soluble in many other organic solvents, like ethanol, making it a practical choice in a variety of microfluidic applications. Moreover, its absorption spectrum is rather broad (470-600 nm with a peak at 554 nm), meaning it can be readily excited with conventional illumination sources like mercury-arc lamps as well as argon-ion (continuous) and Nd YAG (pulsed) lasers. Further, its emission spectrum is also... 

The spectroscopic properties of rare earths in liquids are characterized by broad absorption and emission bands with linewidths which can approach those in glasses (figs. 35.5 and 35.6). Fluorescence is less prevalent than in solids because high frequency vibrations associated with the solvent cause nonradia-tive relaxation of excited electronic states. The thrust of liquid laser research has been directed toward finding solutions in which rare earth fluorescence is not quenched and the quantum efficiency is sufficiently high to achieve laser action. 

The photoluminescence of polyaniline has been studied as a function of the polymer redox state. It was stated that each of the three PANI species have fluorescent emissions with different quantum yields. When conductive domains are present, the emission fi-om excitons located either inside these domains or near to them is efficiently quenched [40], Organic electroluminescent devices (LED s) are a possible alternative to liquid crystal displays and cathodic tubes, especially for the development of large displays. The principal setup for a polymeric LED is ITO/light-emitting polymer/metal. A thin ITO electrode on a transparent glass or polymeric substrate serves as the anode, while metals such as Al, Ca or Mg are used as cathode materials. After applying an electric field, electrons and holes are injected into the polymer. The formation of e /h" " pairs leads to the emission of photons. One of most important opportunities to follow from the use of poly-... 

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. 

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