Lifetime spike

Figure Cl.5.2. Fluorescence excitation spectra (cps = counts per second) of pentacene in /i-teriDhenyl at 1.5 K. (A) Broad scan of the inhomogeneously broadened electronic origin. The spikes are repeatable features each due to a different single molecule. The laser detuning is relative to the line centre at 592.321 nm. (B) Expansion of a 2 GHz region of this scan showing several single molecules. (C) Low-power scan of a single molecule at 592.407 nm showing the lifetime-limited width of 7.8 MHz and a Lorentzian fit. Reprinted with pennission from Moemer [198]. Copyright 1994 American Association for the Advancement of Science.

Another technique for measuring the lifetime of excited activator atoms in solid-state lasers has been published by Gilrs h If the pulsed laser is operated close above threshold, only a single spike (i.e. a short pulse of induced emission) appears, whereas many spikes are emitted when the laser ist running well above threshold. This... 

Indeed, the laser oscillation itself frequently is a transient emission that falls into the broad definition of fluorescent lifetimes. In particular, spiking phenomena (5) and Q Switching (4) clearly bear a close relationship to relaxation times and processes. One could say that laser phenomena are associated with the study of fluorescent decays of inverted populations. 

There is a technique (called Tast) that consists of measuring the current only for a very short period of time, late in the lifetime of the mercury droplet. This method eliminates the saw-toothed spikes from the current curve. However, it leads to a lower sensitivity because the diffusion current decreases as the analyte flux goes from the bulk of the solution to the surface of the droplet (Fig. 19.3). 

Selenomethionine forms optical enantiomers and a chiral column is needed to separate the two forms [160-162], As the presence of salts and organic matter can cause retention times to vary, it is essential that spiking is used to verify them. Also, it should be noted that retention time alone does not allow peaks to be unequivocally assigned. It has been found that the removal of lipid material prior to protein extraction dramatically improves the reproducibility of the chromatographic system and the lifetime of the columns. 

Figure 21A shows the time evolution of the intensity of peak A at 290 K at several pressures. The PL decay rate increases with hydrostatic pressure. In Figure 21B, apart from the fast-decay components appearing as sharp spikes at 2.984 fi s, the PL at 1 bar has almost a single exponential decay with a lifetime of 102.6 /is. This long-time component is still a single exponential at... 

Fig. 1.6. Lifetime function l ip) for box C. The spikes in I indicate the presence of scattering singularities.

While the double exponential form characterizes (approximately) the decays of the 205 cm-1 bands in the high-energy spectra, decays in the vibrationally relaxed region of the spectra are different These other decays, an example of which is shown in Fig. 29, are devoid of any early time spike, but have finite rise times and long-component lifetimes matching those of the long components of Fig. 28. This behavior is consistent with the decay of vibrationally relaxed fluorescence in the dissipative regime of IVR. 

The fluorescence decay curves for initial rotational states 17 < J < 28 were intermediate in behavior between those for stable and unstable states already described. As rotational energy was increased, the initial decay spike became more and more intense in relation to the subsequent slower rate. For the most part, the decay could be approximated by a double exponential. The question arose as to where the sharp onset of predissociation actually occurred, i.e. which was the first rotational level that had a radiative lifetime shorter than that of the stable levels. To answer this question, it was necessary to formulate a kinetic model for the excited state. 

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