Quantum liquids, fluorescence

Calibration. In general, standards used for instrument calibration are physical devices (standard lamps, flow meters, etc.) or pure chemical compounds in solution (solid or liquid), although some combined forms could be used (e.g., Tb + Eu in glass for wavelength calibration). Calibrated lnstr iment parameters include wavelength accuracy, detection-system spectral responsivity (to determine corrected excitation and emission spectra), and stability, among others. Fluorescence data such as corrected excitation and emission spectra, quantum yields, decay times, and polarization that are to be compared among laboratories are dependent on these calibrations. The Instrument and fluorescence parameters and various standards, reviewed recently (1,2,11), are discussed briefly below. 

In photoirradiated solid cyclohexane (freezing point 6.5°C), much higher fluorescence quantum yields and longer fluorescence lifetimes were observed than in the liquid phase [89]. In solid Ar matrices, the fluorescence characteristics, energy dependence of the lifetime and intensity, were found to be very similar to these characteristics in the gas phase. This points to the importance of cyclohexane-cyclohexane interactions to determine the excited-state characteristics in the liquid phase [76]. 

The fluorescence of liquid alkanes is supposed to originate entirely from the relaxed Si state. Walter and Lipsky [154], by measuring the fluorescence yields of alkane solutions irradiated with 165 nm photons or Kr beta particles ( niax = 0-67 MeV) relative to benzene fluorescence, determined the following yields 2.3-dimethylbutane G Si) < 1.3, cyclohexane 1.4-1.7, methylcyclohexane 1.9-2.2, dodecane 3.3-3.9, hexadecane 3.3-3.9, d5-decalin 3.4, and bicyclohexyl 3.5. After reinvestigating the intrinsic quantum yield of cyclohexane fluorescence, Choi et al. published G(5 i) = 1.45 for this alkane in Ref. 155. For tra 5-decalin a G Si) value of 2.8-3.1 has been accepted [65,128,132]. The uncertainties in the values reflect the uncertainties in the intrinsic fluorescence quantum yields. 

Fluorescence spectra and quantum yields of pyrene in supercritical CO2 have been determined systematically as functions of temperature, CO2 density, and pyrene concentration. Under near-critical conditions, contributions of the pyrene excimer emission in observed fluorescence spectra are abnormally large. The results cannot be explained in the context of the classical photophysical mechanism well established for pyrene in normal liquid solvents. The photophysical behavior of pyrene in a supercritical fluid is indeed unusual. The experimental results can be rationalized with a proposal that the local concentration of pyrene monomer in the vicinity of an excited pyrene molecule is higher than the bulk in a supercritical solvent environment. It is shown that the calculated ratios between the local and bulk concentrations deviate from unity more significantly under near-critical conditions (Sun and Bunker, 1995). 

In fluorescence microscopy (FM) a small amount of a fluorescent dye is added to the mono-layer. To be incorporated into the monolayer the dye must be amphiphilic. The film is illuminated and the lateral distribution of the fluorescent molecules is observed with an optical microscope [589], Depending on the phase condition of the monolayer, the fluorescent molecules distribute unevenly or have a different quantum yield. Usually the dyes are expelled from condensed liquid and solid phases. With this technique the coexistence of different phases in monolayers on water was demonstrated for the first time [590,591],... 

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. 

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. 

Three of the experiments are completely new, and all make use of optical measurements. One involves a temperature study of the birefringence in a liquid crystal to determine the evolution of nematic order as one approaches the transition to an isotropic phase. The second uses dynamic laser light scattering from an aqueous dispersion of polystyrene spheres to determine the autocorrelation function that characterizes the size of these particles. The third is a study of the absorption and fluorescence spectra of CdSe nanocrystals (quantum dots) and involves modeling of these in terms of simple quantum mechanical concepts. 

Cyclopentanone emits both fluorescence and phosphorescence in the liquid phase ° The results of Dunion and Trumbore show that the quantum yield of 4-pentenal formation, in the photolysis of cyclopentanone, is decreased by O2 or piperylene . Since both substances are known to be triplet state quenchers, it may be suggested that the triplet state participates in the photolytic decomposition of cyclic ketones in the liquid phase and in solution. 

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