Quenching factor

Y, the Ham quenching factor, corresponds to the vibrational overlap of the displaced Jahn-Teller potentials. Assuming Harmonic potentials, Y is given by... 

Table 1.2. Quenching factors Q of the OPVn and MP-Ceo fluorescence of the OPVn C6o dyads in toluene and ODCB. Singlet excited state lifetime r of the OPVns in toluene solution. Rate constants et for energy and k cs, Uqs for charge separation...

The shorter decay (instrument-limited) was attributed to the fluorescence from adsorbred molecules strongly coupled to the surface, indicating that the electron transfer rate is much faster than 40 ps. In order to get a more accurate value for the injection rate, the integrated fluorescence intensities of the fast component of the sensitized semiconductor decay curve were compared with the intensity of the oxazine on the tape reference sample [48]. This analysis yielded a quenching factor of about 10 . From this an electron injection rate of 3 x lO s corresponding to an electron transfer time of around 40 fs, was obtained. 

Figure 5. Dependence of quenching factor Q = (Iq-I)/I on the number of jumps during exciton lifetime (n) and ratio mol quencher /mol basic unit (c) in a lD-random walk (48)...

Usually the defects reduce the lifetime of non-equilibrium carriers and, consequently, their diffusion length in semiconductors. However, because of the presence of the closely spaced AlGaAs barriers, the carrier capture by the QDs in our samples is not diffusion-limited. That is why a difference in the quenching factor of the PL intensity at a given irradiation dose for the above- and below-bandgap excitation for all energies above the m = 2 QD excited state (Fig. lb) is not observed. Thus, the loss of carriers occurs mainly in the dots due to tunneling of caniers from the dots to adjacent non-radiative recombination centers. 

An ideal atomizer should have good atomization features and low background emission. In addition, the atomizer should not contain fluorescence quenching factors and the distance for atoms passing through the excitation zone should be as long as possible. It is quite difficult to unite all these requirements in one atomizer. Commonly used atomizers in AFS are flames, plasmas, electrothermal atomizers, and various special techniques. 

During many commercial procedures, a concern exists for heterogeneous nucleation and precipitation during continuous cooling. Quench factor analysis has been developed to use the entire time-temperature-sensitization curve [43]. The quench factor, r, is given by... 

Quench factor analysis has successfully predicted the IGC characteristics of AA 2024-T4 [43]. 

Fig. 15 Graphical illustration of method for determination of r in quench factor analysis. Cn is the critical time to achieve a fraction transformed at a certain temperature indicated by the time-temperature-transformation curve. (Ref [43], reprinted with permission from ASM International.)...

The effects of quench rate on IGC for Al-Gu, Al-Gu-Mg, and Al-Cu-Mg-Mn alloys as well as for austenitic stainless steels is considered to be well-understood [43, 74, 75, 106]. Integration of the effects of precipitation and solute depletion at each temperature during a quench (i.e., quench factor analysis) can be compared to isothermal time-temperature-sensitization diagrams in order to predict the quench rate required to avoid IGC [43, 74]. Alloys... 

Equation (8) introduces notation, in which a labels the specific matrix element of rank R, SR(jij ) is a single-particle matrix element for the transition j-, - jf in the impulse approximation, and the quenching factor qJUiJf) corrects SrO jV) for the finite size of the model space and some effects of the nuclear medium so that the effective value of Spijijf) is qa jijf)SR jijf) = SKy ijf, eff). The DR jij[) are the one-body-transition densities that are the result of the shell-model calculations. For the special case of the axial-charge matrix element Mj the defining equation has the emedjijf) of Table 1 incorporated into the sum, i.e.. 

FIGURE 3 (left side) Relative amount of oxidized plastoquinone molecules after an increase in light intensity from 10 to 20 Wm"2 calculated from f, f, fQ and oxygen yield r data (7). The calculation is based on (a) a statistical matrix model of PS 2, (b) a connected unit model, (c) a separate unit model. The dotted line gives the qQ quenching factor of fluorescence ( = (f n"f)/(f n W ) which is quite often assumed to be closely related to the PQ-pool redox-state. All curves - no matter what ps 2 structure is assumed - indicate clearly a more reduced state of the PQ-pool after the increase in light intensity. 

From these calculated rate constants and the quenching factors (Q in equation 12), values of ratio of intensities of nitrogen emissions (using equation 11) are calculated for various EVDFs. By comparing the calculated values of ratio of emission intensities with measured one, the actual EVDF and its corresponding reduced electric field are determined. 

If no oil is eliminated, the sample must be considerably diluted. Even if the oil PCB content is minimal, the sample should be diluted with a solvent such as heptane at a factor of 100 to 200, or else the quenching factor will modify the detector response. A dilution of this magnitude performed on samples with a low PCB content (e.g., < 20 ppm) results in a high sensitivity on the ECD. 

T-T annihilation is a considerable quenching factor(s) under higher concentration. But it can be ignored because of the compartmentation of CD cavity or surfactant micelles if host concentration is much higher than the guest concentration in CD or micellar aqueous solutions. 

---