Intensity decay

The "add-to-memory" signal averaging method currently available to us distorts fluorescence intensity versus time plots when the fluorescence intensity is a non-linear function of incident laser energy and the laser energy varies from shot to shot. For this reason we have not attempted detailed kinetic modelling of the observed fluorescence intensity decay curves recorded at high 532 nm laser fluence. 

No. of standard reflections Frequency of standard reflections (min) Intensity decay (%)... 

In the work described below, the dissociation rates were obtained from the product growth with respect to the delay time if the dissociation rate is fast enough before the parent molecules and fragments fly out of the probe region. This is demonstrated in the photodissociation study of toluene. Alternately, dissociation rates were obtained from the disk-like image intensity decay for slow dissociation rate, as illustrated in the rate... 

Fig. 22. The product buildup times. The solid line is the simulation from reactions (27)-(36). The non-zero intensity at I, - 0 and the intensity decay of mje = 94 is due to the dissociative ionization (reaction (36)).

Similar results were obtained for the photodissociation of d Q-ethylbenzene and n-propylbenzene, as illustrated in Figs. 27 and 28. For n-propylbenzene, the intensity decay rate of the disk-like image was found to be 4.9 x 104 s 1, and the product growth rates obtained from simulation are 5 x 104 s 1 and larger than 5 x 105 s 1 for the slow velocity component and the fast recoil velocity component, respectively. 

Fig. 26. Ion images of phenyl radical obtained from the photodissociation of ethylbenzene at 248 nm at two different delay times (a) 15 ps, (b) 32 ps. (c) Intensity decay of the disk-like image as a function of delay times. A decay rate of 10B s 1 was obtained, (d) The fragment translational energy distribution for the reaction C6HbC2Hb -> C6HbCH2 + CH3.

B) Corresponding sum intensity decay curves plotted on a semilogarithmic scale. From Ref. 86 with permission. 

If one does not use the short gradient pulse (SGP) approximation, the term A has to be substituted with (A 8/3). In the case of a mono-disperse system, the plot of ln(E) versus y2g282A is a straight line having the absolute value of the slope equal to the self-diffusion coefficient. For polydisperse sample, the signal intensity decay can be interpreted in terms of a distribution of diffusing species ... 

Guinier s law states that for any scattering pattern, /(s), of a diluted system the initial intensity decay is approximated by a Gaussian... 

Anisotropic Particle Scattering Varying Intensity Decay in Different Directions. In case of anisotropy the decay of the scattering intensity 7 (s) is a function of the direction chosen. The intensity extending from s = 0 outward in a deliberately chosen direction i is mathematically the deAnition of a slice (cf. Sect. 2.7.1, p. 22). Thus, the Fourier-Slice theorem, Eq. (2.38), turns the particle density function Ap (r) into a projection Ap (r) j (r,) and the scattering intensity is related to structure by... 

Because the Rg are constants, the intensity decay of the unoriented material con-... 

Intensity Decay Between SAXS and WAXS Porod s Law... 

At present, two main streams of techniques exist for the measurement of fluorescence lifetimes, time domain based methods, and frequency domain methods. In the frequency domain, the fluorescence lifetime is derived from the phase shift and demodulation of the fluorescent light with respect to the phase and the modulation depth of a modulated excitation source. Measurements in the time domain are generally performed by recording the fluorescence intensity decay after exciting the specimen with a short excitation pulse. 

During evacuation at elevated temperature the XRD pattern of the CdCl2/NaY mixture did not remain unchanged. The observed intensity decay of the CdCl2 reflection at 20=15.1° can be interpreted as the loss of the long-range order of the salt crystals due their distribution in the zeolite micropores... 

In Fig. 4 the intensity decay of the 20=15.1° (d().59 nm) reflection of the CdCl2 phase is followed in interaction with porous supports, having different specific surface areas. Over magnesia the CdCl2... 

Figure 5. Intensity decay of the XRPD reflection 20=15.1° of CdCl2 phase and the intensity gain of the 20=14.6° reflection of zeolite Cd,H-Y as a function of evacuation time of a CdCl2/NH4-Y mixture at various temperatures.

In this paper, to determine the steady state SCP across the transition layer, we analyze the fluorescence intensity decay of dye molecules covalently bound to the polymer chains. The decay is due to the permeation of... 

Poly(methyl methacrylate) [PMMA] is an excellent polymer for studying photoresist dissolution because of its minimal swelling characteristic. For this work, PMMA molecules were labelled with phenanthrene (Phe) dye since its fluorescence is quenched by MEK. In addition, this dye has the advantage of forming few excimers (23-241 which results in self-quenching. Thus, the reduction in fluorescence intensity of PMMA-Phe is virtually solely due to MEK quenching. Consequently, the permeation of MEK into a PMMA film can be monitored from fluorescence intensity decay. 

A typical time profile of the excited PMMA-Phe fluorescence intensity decay is shown in Figure 2. The MEK permeation commences at 24 sec. The SPR increases during the plasticization period until it becomes constant, the onset of the steady state. It is characterized by a linear relationship between the amount of solvent absorbed and time. It was determined from a linear regression analysis that the PMMA-Phe fluorescence intensity starts to deviate from linearity at 197 sec. This indicates a decrease in the SPR and/or the unquenched PMMA-Phe. The decrease in SPR is unexpected at this film thickness since the SPR in thicker PMMA-Phe films show no anomaly at 1 /tm. A more plausible explanation is the reduction in available PMMA-Phe, which is expected when the front end of the SCP reaches the substrate. 

We determine the SCP as it is terminated at the substrate in the following manner. First, we calculate I = dl/dt for the linear portion of the intensity decay. A°linear least squares fit to the data in the time interval 170 - 190 sec produces a value of dl/dt = 1150 counts/sec. This represents the steady state quenching rate which is intimately related to the steady state SPR. Secondly, we calculate I. = dl/dt at one-second intervals as the steady state endsrwith the arrival of the SCP at the substrate. For example, for the data in Figure 2,... 

It is important to notice that a change in lifetime is not a necessary result of a change in fluorescence intensity. For instance, the Ca2+ probe Fluo-3 displays a large increase in intensity on binding Ca2+, but there is no change in lifetime. This is because the Ca-free form of the probe is effectively nonfluorescent, and its emission does not contribute to the lifetime measurement. In order to obtain a change in lifetime, the probe must display detectable emission from both the free and cation-bound forms. Then the lifetime reflects the fraction of the probe complexed with cations. Of course, this consideration does not apply to collisional quenching, when the intensity decay of the entire ensemble of fluorophores is decreased by diffusive encounters with the quencher. 

Lifetime-based sensing can be mostly insensitive to these real-world effects. This is because these factors are not expected to alter the rate at which the intensity decays (Figure 1.2, middle). In our opinion, phase-modulation sensing provides additional advantages (Figure 1.2, bottom). The instruments take advantage of radio-frequency... 

Th fluorescence lifetime of a sample is the mean duration of time the fluorophore remains in the excited state. Following pulsed excitation, the intensity decays of many fluorophores are single exponential 2 23 ... 

There are two widely used methods for measuring fluorescence lifetimes, the time-domain and frequency-domain or phase-modulation methods. The basic principles of time-domain fluorometry are described in Chapter 1, Vol.l of this series(34) and those of frequency-domain in Chapter 5, Vol. 1 of this series.<35) Good accounts of time-resolved measurements using these methods are also given elsewhere/36,37) It is common to represent intensity decays of varying complexity in terms of the multiexponential model... 

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