Efficiency of excitation

Besides the conventional Grimm-type dc source, which has dominated the GD-OES scene for approximately 30 years, other discharge sources are well known. Among those are various boosted sources which use either an additional electrode to achieve a secondary discharge, or a magnetic field or microwave power to enhance the efficiency of excitation, and thus analytical capability none of these sources has, however, yet been applied to surface or depth-profile analysis. 

The choice of new complexes was guided by some simple considerations. The overall eel efficiency of any compound is the product of the photoluminescence quantum yield and the efficiency of excited state formation. This latter parameter is difficult to evaluate. It may be very small depending on many factors. An irreversible decomposition of the primary redox pair can compete with back electron transfer. This back electron transfer could favor the formation of ground state products even if excited state formation is energy sufficient (13,14,38,39). Taking into account these possibilities we selected complexes which show an intense photoluminescence (0 > 0.01) in order to increase the probability for detection of eel. In addition, the choice of suitable complexes was also based on the expectation that reduction and oxidation would occur in an appropriate potential range. 

Haas, E. and Steinberg, I. (1984). Intramolecular dynamics of chain molecules monitored by fluctuations in efficiency of excitation energy transfer. Biophys. J. 46, 429-37. 

Compound Chemiluminescence1) 0 Fluorescence of the corresponding acids2) (in 0.1 M K2CO3) es (efficiency of excited product formation)... 

A comparison between the efficiency of excitation with lasers and mercury lamps has been undertaken by Evans etal. and Brandmuller etal. Since that time, lasers have improved considerably and a later comparison would be even more in favor of laser applications. Since several commercial laser Raman spectrometers are now available 190 ), with He-Ne lasers, Ar" or Kr -ion lasers and neodymium lasers, most current investigations employ lasers as light sources, j)... 

According to (5.35), the most fortunate circumstance for the present scheme is a system with heavy mass and parallel potential energy surfaces (A(x) const.). The steepness of the potential difference A(x) is the most crucial parameter it not only affects the validity of this level approximation (5.8) but it also changes the efficiency of excitation according to (5.32). It is obvious that a narrow wavepacket can be relatively easily excited by a quadratically chirped pulse (cf. 5.32). However, a narrow one can easily break the level approximation (5.35) because of the broad distribution in momentum space. The optimal width of a wavepacket can be roughly estimated as... 

Possible examples of the importance of the coincidence of transition-state and excited-state geometries and also the importance of the location of the essential reaction or localization of reaction energy are the chemiluminescent rearrangements of Dewar benzene [ 1 ] and Dewar acetophenone [2] to benzene and acetophenone, respectively (Lechtken et al., 1973 Turro et al., 1974c Turro et al., 1975). The efficiency of excited state production though quite low is approximately the same for both systems. Yet the triplet state of... 

Fig. 27. Efficiency of excited state formation (f>ts (data from [197]) as a function of the reaction exothermicity AGe for the ECL system Mo6Cl]7/D+ ( ), MosClJi/A- (O) and MosClJ /R (O) in acetonitrile solutions. AGe values are calculated from the difference in the standard redox potentials corrected for the Coulombic interactions between reactants and products.

Apart from the gradient itself, the degree of suppression is related to the efficiency of excitation by the selective pulse and thus the choice of selective pulse is important with, for example, hyperbolic secant pulses perform better than sine pulses." If the solvent Ti is very short and on the order of the time for the dephasing procedure, then some (unwanted) z-magnetization will be re-established prior to the excitation pulse. One method of circumventing this... 

At temperatures of 40 C and above the chemiluminescence from a paper In a moist atmosphere was always observed to be smaller than In a dry atmosphere. Possibly the moisture allows the macro-molecular segments In the paper to slide past each other more easily, so that less bond scission occurs. Other explanations, however, are possible In terms of an effect of moisture on the efficiencies of excited state production, or an Increased mobility of excited state quenchers. Below 40 C, on the other hand, the maximum emission from samples In moist air was larger In many cases than In dry air, and we may be dealing with an effect Involving an Increased emission due to the change of atmosphere from the laboratory air to that In the humidified oven (cf. 

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 primary experimental goals of ECL experiments are to define the nature of the emitting state, the mechanism by which it is produced, and the efficiency of excited-state production. ECL is also useful for chemical analysis. 

The efficiencies of excitation energy transfer from tyrosine to tryptophan residues in globular proteins in native and denatured states has been measured by studying the wavelength dependence of the fluorescence quantum yield.The results are summarized in Table 25. Unlike findings from earlier work, energy transfer is almost completely absent in the denatured state. 

Photochemical isomerisation of [6](l,4)naphthalenophane (33) and [6](l,4)an-thracenophane (34) produces the corresponding Dewar valence isomers (35) and (36) respectively, and the efficiency of excited product formation in the adiabatic photocycloreversion of the bridged biplanemer (37) has been determined by the size of its side-chain substituents which influence the interchromo-phore distance in photoproduct (38). ... 

In this relation, E is the Arrhenius activation energy of an elementary chemical reaction the coefficient a is the efficiency of excitation energy in overcoming the aetiva-tion barrier kRo is the pre-exponential factor and 9 x — xo) is the Heaviside function... 

Spectral interference is much more common in plasmas than in flames due to the great efficiency of excitation in plasmas. Elements such as Fe, Mn, Ta, Mo, W, and U emit thousands of lines in a plasma source. Ideally, the analyte wavelength chosen should have no interference from other emission lines, but this is often not possible. 

Photo DSC experiments conducted in a ciear coating at L 400 nm - where only the substituted thioxanthone Z (Ri - Rg R3 H R4 = C00(CH2CH20)8 H) absorbs -demonstrates the increase of the poiymerization efficiency [25]. Under exposure, the mixture of Z + fi leads to a considerabie polymerization enthalpy whereas, in the presence of Z or S alone, the exothermic signal remains very small. The same is true when using a laser light at X, - 440 nm [26] for the excitation of a mixture S+fi- No polymerization occurs in the presence of or . The relative reactivity of fi at 363 nm and -t- at 440 nm shows a 35 1 ratio, thus defining a low quantum efficiency of excitation transfer. 

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