Emission decay

Direct evidence for the competition of two counteracting contributions to the transient absorption changes stems from the temporal evolution of the transmission change at 560 nm. From Figure 10-3 it can be seen that the positive transmission change due to the stimulated emission decays very fast, on a time scale of picoseconds. On the other hand the typical lifetime of excitations in the 5, slate is in the order of several hundred picoseconds. Therefore, one has to conclude that the stimulated emission decay is not due to the decay of the. Sj-population (as is typically the case in dye solutions). The decay is instead attributed to the transiei.i build up of spatially separated charged excitations that absorb at this wavelength. 

In principle, pulsed excitation measurements can provide direct observation of time-resolved polarization decays and permit the single-exponential or multiexponential nature of the decay curves to be measured. In practice, however, accurate quantification of a multiexponential curve often requires that the emission decay be measured down to low intensity values, where obtaining a satisfactory signal -to-noise ratio can be a time-consuming process. In addition, the accuracy of rotational rate measurements close to a nanosecond or less are severely limited by tbe pulse width of the flash lamps. As a result, pulsed-excitation polarization measurements are not commonly used for short rotational periods or for careful measurements of rotational anisotropy. 

Luminescence lifetimes are measured by analyzing the rate of emission decay after pulsed excitation or by analyzing the phase shift and demodulation of emission from chromophores excited by an amplitude-modulated light source. Improvements in this type of instrumentation now allow luminescence lifetimes to be routinely measured accurately to nanosecond resolution, and there are increasing reports of picosecond resolution. In addition, several individual lifetimes can be resolved from a mixture of chromophores, allowing identification of different components that might have almost identical absorption and emission features. 

Figure 4. Luminescence decay profile of an oxygen indicator dye excited by a short flash of light, in (a) solution and (b) embedded into a gas-permeable film used to fabricate fiber-optic sensors for such species. The logarithmic scale of the Y-axis allows to compare the exponential emission decay in homogeneous solution and the strongly non-exponential profile of the photoexcited dye after immobilization in a polymer matrix.

L Determined in this work by measurement of emission decay. 

Ultrafast emission measurements are possible with the dendrimer metal nanocomposites. The gold and silver internal dendrimer nanocomposites showed a fast emission decay of approximately 0.5 ps, which was followed by a slower decay process. The fast decay emission is attributed to decay processes of the gold (or silver) metal nanoparticles. Ultrafast emission anisotropy measure-... 

Below is the function Data Em iss ion. m to generate the data. Note that experimental emission decays are exponentials and that the lifetime x is used instead of the more customary rate constant in kinetics. Also, we use the notation C representing the concentration of the exited states, not the normal concentration. 

Figure 4-51. Noise free (-) and highly noisy ( ) emission decay at one particular wavelength.

In addition, the quenching of the fluorescence of fluorophore groups in protein molecules by neighboring groups(35) and its temperature dependence, t36) energy transfer of electronic excitation and its dependence on excitation wavelength,(1) the type of emission decay kinetics,(1,2) and changes... 

Decay and Quenching of Fluorescence 2.3.1. Emission Decay Kinetics... 

Figure 2.7. The fluorescence spectra from unrelaxed (/ = 0) and relaxed ( - oo) states and the emission decay curves at the short-wavelength edge (a), the maximum (b), and the long-wavelength edge (c) of the spectrum.

Photoluminescence involves three types of information (i) emission versus wavelength (spectral-domain), (ii) intensity across a specific wavelength bandwidth (intensity-domain) and (iii) emission decay over time (time-domain or lifetime) where each fluorophore has a unique lifetime. Photoluminesence detection modes in general are classed as either steady state or time-domain where the former involves either spectral- or intensity-domain detection modes. 

Electron emission occurs when plastic deformation, abrasion, or fatigue cracking disturbs a material surface. Triboelectrons are emitted from freshly formed surface. The emission reaches a maximum immediately after mechanical initiation. When mechanical initiation is stopped, the emission decays with time. Strong emission has been observed for both metals and metal oxides. There is a strong evidence that the existence of oxides is necessary. The exoelectron emission occurs from a clean, stain-free metallic surface upon adsorption of oxygen (Ferrante 1977). 

The radioisotope cobalt-60, with a half-life of 5.27 years (1925.3 days) through beta ((3) emission, decays to form the stable element nickel-60. It is used to test welds and metal casts for flaws, to irradiate food crops to prolong freshness, as a portable source of ionizing gamma (Y) radiation, for radiation research, and for a medical source of radiation to treat cancers and other diseases. 

The recording of sequential photon pulses for measurements of low levels of electromagnetic radiation as well as the recording of emission decays. The pulses are recorded from electron emission events from some photosensitive layer in conjunction with a photomultiplier system. See also Time-Correlated Single Photon Counting Fluorescence... 

The importance of the carboxylate donors is underlined by a study of the lanthanide coordination chemistry of the similar terdentate ligand 2,6 -bis( 1 -pyrazol-3 -yl)pyridine, L24 (63). The complex structure of [Tb(L24)3][PF6]3, shown in Fig. 11, appears to be fairly robust in methanolic solution, with Horrocks analysis (q = 0.6) suggesting the 9-coordinate structure is retained the small quenching effect of outer sphere coordination explains the q-value. However, in aqueous solution, the lability of the ligands dramatically changes the luminescence. Whilst the emission decays are not exactly single exponential, approximate lifetimes in H20 and DoO suggest a solvation value of 4-5. 

The electron- and hole-trapping dynamics in the case of WS2 are elucidated by electron-quenching studies, specifically by the comparison of polarized emission kinetics in the presence and absence of an adsorbed electron acceptor, 2,2 -bipyridine [68]. In the absence of an electron acceptor, WS exhibits emission decay kinetics similar to those observed in the M0S2 case. The polarized emission decays with 28-ps, 330-ps, and about 3-ns components. For carrier-quenching studies to resolve the dynamics of electron trapping, it is necessary that the electron acceptor quenches only conduction-band (not trapped) electrons. It is therefore first necessary to determine that electron transfer occurs only from the conduction band. The decay of the unpolarized emission (when both the electron and the hole are trapped) is unaffected by the presence of the 2,2 -bipyridine, indicating that electron transfer docs not take place from trap states in the WS2 case. Comparison of the polarized emission kinetics in the presence and absence of the electron acceptor indicates that electron transfer does occur from the conduction band. Specifically, this comparison reveals that the presence of 2,2 -bipyridine significantly shortens the slower decay component of the polarized... 

Emission Decay Time of Rare-Earth-Activated Lithium Magnesium Aluminum Silicate Glasses... 

Directly Excited Emission Decay Times of Glass Powders at 300°K... 

The emission can be classified as fluorescence, which has a very rapid decrease in intensity, or phosphorescence, where emission decay is much slower. A compound can both fluoresce and phosphoresce. 

The three forms of Pr differing in molecular weight exhibit very similar photophysical and photochemical properties [7,113,114], regarding the shape of the stationary red fluorescence and excitation spectra, the triexponential emission decay function and its component composition, the emission parameters (cf. tJ, 3>f(expti.corr) cf- also Aussenegg et al. [139]), the heat release (a) by the P and I700 species, and the total photochemical quantum yield ( r-/r) (Table 1). 

Electron transfer kinetics from the triplet excited state of TMPD to PA in polystyrene has been monitored by phosphorescence emission decay in ref. 85. The rate constant has been found to be invariant over the temperature interval 77-143 K. Parameters ae and ve calculated from the phosphorescence decay using eqn. (12) were found to be ae = 3.46 A and vc = 104 s 1. 

Another technique that uses the fluorescence properties of trivalent lanthanides is that of the detection of fluorescence emission decay induced by pulsed dye laser excitation. Horrocks and Sudnick (17) have applied this technique to the study of water molecules bound to metal ions in small complexes and proteins. In one study they found that the exponential decay of Tb3+ fluorescence is altered when H20 is replaced by D20 and that this change can be used to determine the number of coordinated water molecules on the metal ion. With thermolysin, bound Tb3+ had 1-2 water molecules in the first coordination shell. This number is consistent with the x-ray structure. 

The first application of the chemiluminescent (CL) probe to the photocatalytic reactions was implemented by the authors.32, 33) When a CL probe, luminol, is present in Ti02 suspension at pH 11, a very weak light emission was observed after photoirradiation at 387 nm. The intensity of the emission decays as shown in Fig. 5.9(A), where the two components, that is, fast and slow decays, can be distinguished. This figure shows that only the slow decay component of the luminescence increases in the presence of H202. The emission spectra for both components were measured and shown in Fig. 5.9(B), both of which are attributable to the chemiluminescence of luminol. 

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