Multiple quantum efficiency

Peroxyoxalate chemiluminescence is the most efficient nonenzymatic chemiluminescent reaction known. Quantum efficiencies as high as 22—27% have been reported for oxalate esters prepared from 2,4,6-trichlorophenol, 2,4-dinitrophenol, and 3-trif1uoromethy1-4-nitropheno1 (6,76,77) with the duorescers mbrene [517-51-1] (78,79) or 5,12-bis(phenylethynyl)naphthacene [18826-29-4] (79). For most reactions, however, a quantum efficiency of 4% or less is more common with many in the range of lO " to 10 ein/mol (80). The inefficiency in the chemiexcitation process undoubtedly arises from the transfer of energy of the activated peroxyoxalate to the duorescer. The inefficiency in the CIEEL sequence derives from multiple side reactions available to the reactive intermediates in competition with the excited state producing back-electron transfer process. 

SPC techniques are hardly affected by additive noise and multiplicative noise is absent. However, subtractive noise due to the collection efficiency and transmission of optics and the quantum efficiency of the detector do play a role. In addition, at high count rates, the efficiency goes down due to pileup effects. 

Under anodic conditions hole transfer to HF electrolytes is accompanied by electron injection that may lead to quantum efficiencies greater than 1. This effect is known as current multiplication and is discussed in Section 4.4. 

The heavy atom effect affects also the non-radiative transitions, as shown above in Figure 4.76. Transitions between higher excited states of different nominal multiplicities are efficient, so that the luminescence quantum yields depend on the excitation wavelength. (Vavilov s rule breaks down in such cases.)... 

However, further experimentation embracing a variety of ternary systems will be required to determine the degree of interaction between such multiple centers. Preliminary results for Fe W0, confirm the superposition of two characteristic sets of interband transitions. The optical band gap and flat-band potential are essentially the same as in FeNbO, but the quantum efficiency is considerably greater. This suggests that there may be some enhancement of the photoresponse due to interaction between the iron and tungsten centers. 

In practice, HNCO is now carried out by a somewhat more complex pulse sequence than that given in Fig. 12.16 in order to improve its efficiency. Pulsed field gradients are added to aid coherence pathway selection an INEPT transfer from N to K replaces the multiple quantum coherence step and the N evolution is carried out with a constant time experiment. 

SateUite Transition MAS (SATRAS or STMAS), developed by Gan in 2000 [25], is an alternative approach to MQMAS for the acquisihon of high-resolution NMR spectra of quadrupolar nuclei. The principal advantage of SATRAS over MQMAS is that it is not dependent upon an efficient transfer of multiple-quantum coherences. like MQMAS, SATRAS is a 2D experiment performed under MAS conditions. The technique involves exciting the sateUite transitions in the spin manifold of quadmpolar nuclei using short radio frequency (rf) pulses. The second-order... 

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