Effective lifetime

Figure Al.6.15. Schematic diagram, showing the time-energy uncertainty principle operative in resonance Raman scattering. If the incident light is detuned from resonance by an amount Aco, the effective lifetime on the excited-state is i 1/Aco (adapted from [15]).

Pulleys should be inspected prior to replacing belts to ensure they are in good operating condition. Damaged, worn, or dirty pulleys will reduce the effective lifetime of belts significantly. Wipe clean any oil or grease that has accumulated on the pulley and use a stiff brush with bristles that are softer than the pulley material to clean off rust and dirt. 

This relation shows that the lifetime of PMC transients indeed follows the potential dependence of the stationary PMC signal as found in the experiment shown in Fig. 22. However, the lifetime decreases with increasingly positive electrode potential. This decrease with increasing positive potentials may be understood intuitively the higher the minority carrier extraction (via the photocurrent), the shorter the effective lifetime... 

The layer of pyrolytic graphite which Is thus generated Is extremely dense, non-porous and resistant to oxidation. According to Clyburn et at (9) this treatment extends the effective lifetime of the graphite elements and Improves analytical precision. 

The chemical and biological implications of compounds that support reversible but covalent processes are often lost in a common assumption that all covalent reactions of DNA are irreversible. Difficulty with reversible reactions is often encountered while attempting to isolate labile products as mentioned above and described more fully in Section 9.2. Yet, reversibility also has the potential to extend the effective lifetime of transient intermediates (Section 9.3.2) and support selective, target-promoted QM... 

Repetitive Capture and Release of a Quinone Methide Extends Its Effective Lifetime... 

Standard alkylating and cross-linking agents such as dimethylsulfate or TV-mustards, respectively, have only one opportunity to partition between various nucleophiles since their reactions are irreversible. In contrast, QMs have the potential to partition between nucleophiles multiple times as long as the resulting adducts are formed reversibly. Continual capture and release of QMs consequently can extend their effective lifetime almost indefinitely and is ultimately limited by only the competitiveness of possible irreversible reactions. For DNA, the strongest nucleophiles act reversibly so terminal quenching remains an infrequent event. 

SCHEME 9.19 Reversible capture and release of quinone methides by dA extends their effective lifetime. 

In the case of radiative transfer between identical molecules, the fluorescence decays more slowly as a result of successive re-absorptions and re-emissions. A simple kinetic model has been proposed by Birks (1970). It is based on the assumption of a unique value for the average probability a that an emitted photon is absorbed, i.e. without distinction between the generations of photons (a photon of generation n is emitted after n successive re-absorptions). This model leads to the following expressions for the effective lifetime and the macroscopic fluorescence quantum yield ... 

From a practical point of view the consequences of TOF dispersion are important only for short intrinsic fluorescence decay times of to < 1 nsec. Figure 8.15 shows an example with to = 50 psec and realistic optical constants of the substrate. The intensity maximum in Fb(t) is formed at At 30 psec after (5-excitation. After this maximum, the fluorescence decays with an effective lifetime of r ff = 100 psec that increases after long times to t > > 500 psec. The long-lived tail disappears as soon as there is some fluorescence reabsorption, and for Ke = K there is practically no difference to the intrinsic decay curve (curve 3 in Figure 8.15). 

The effective lifetimes of all these excited states are determined by radiative as well as collisional deactivation, and which contribution is the more significant depends on pressure and transition probability. The simultaneous recording of the absorption and fluorescence spectra yields information about the ratio of radiative to collisioninduced nonradiative decays. This ratio is proportional to the quotient of total fluorescence from the excited level to total absorbed laser light. Such experiments have been started by Ronn oif... 

To pare the list of VOC oxidations down to the most important processes, we can calculate the effective lifetimes of organics with respect to reactions with each of the oxidants listed in the previous section. Since these natural lifetimes are defined as r = 1 / [X], we also need to assume an average concentration for the oxidant, [X]. We can therefore take a typical organic from each of the major classes (alkane, alkene, aromatic, etc.) and compare the individual lifetimes for reaction with OH, 03, N03, etc. Those reactions having very long lifetimes are insignificant with respect to their contribution to tropospheric chemistry and hence can be ignored for the purposes of this discussion. 

A rather lengthy expression for W can be obtained in terms of the individual rate constants in the reaction scheme (Fig. 26). The conditions for W = 1 are at once obvious. The effective lifetime of the excited dye shoud be ldetermined by the electron transfer reaction e1d and the effective lifetime of the reduced dye hole j)air in the second line should be controlled by the dissociation reactions d and kn- We have shown already that Pes = 1 can be realized for some of the systems in the actual experiment. 

Adequate control of the chemistry in the front end furnace can significantly effect the lifetime and efficiency of the downstream catalyst beds in a sulfur plant. Inadequate removal of Ce+ hydrocarbons from the acid gas feed can result in catalyst fouling by polymeric materials formed under furnace conditions. Toluenes, ethylbenzenes and xylenes have been shown to be particularly troublesome in this regard. Oxygen breakthrough into the catalyst beds can also shorten the effective lifetime of the Alumina catalyst by sulfation i.e. 

The authors are concerned about the indiscriminate use of the term lifetime in discussion of excited states. It is obvious that the lifetime of any excited species will depend upon its environment. An effective lifetime for any species can be defined as long as it decays by a first order process however, if any bimolecular process is involved the term lifetime is undefinable unless the partner in the decay process is present in constant, relatively large concentration, or the higher order Contribution is separated out. 

In the conventional measuring mode the sample stays in the NMR tube, and thus in the radiofrequency Helmholtz coil all of the time. In the continuous-flow mode it resides within the NMR detection coil only for a distinct time of some few seconds (Figure 1.2). This residence time t is dependent upon the volume of the detection cell and the employed flow rate (Table 1.1). For example, a detection volume of 120 jjlI, together with a flow rate of 0.5ml/min, results in a residence time of 14.4 s, while with a detection volume of 8 jjlI the residence time is only 0.96 s. A shorter residence time t within the NMR measuring coil results in a reduction of the effective lifetime of... 

It is necessary to select a value for the effective lifetime of a project, so that the undiscounted NPV is positive (i.e. a lifetime longer than the payback time). A value of 10 years is conventionally chosen... 

Earth Science and Applications from Space Urgent Needs and Opportunities to Serve the Nation (2005) Extending the Effective Lifetimes of Earth Observing Research Missions (2005)... 

When UV irradiated, an octane solution of 1 (R = H, Aik) changes its color from yellowish to deep blue. The initial spectrum is slowly restored at room temperature (the effective lifetime of the colored form at room temperature is about 104s). No concentration dependence of the rate of the dark reaction was observed, which agrees with the intramolecular nature of the reaction. Figure 8.1 portrays the evolution of the absorption spectrum of a perimidinespirocyclohex-adienone 1 (R = Me) during UV irradiation of its hexane solution (Scheme 1). 

Figure 8.4. Photochromic behavior of perimidinespironaphthalenone 4 (R, R R" = H) in ethanol solution, c=-6 x 10 4 (298 K). Irradiation by filtered light (405 nm) of high-pressure mercury lamp.9 Effective lifetime, x, of the colored photoisomer after the light is extinguished is equal to 11.5 h.

A peculiar feature of the photochromic behavior of perimidinespirocyclohex-adienones and their structural analogs is an extremely slow thermal bleaching of the colored quinoneimine photoisomer lb caused by its conversion to the ring-closed form la. The effective lifetimes of the colored isomers formed upon UV irradiation of solutions of perimidinespirocyclohexadienones in nonpolar solvents at room... 

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