Continuous excitation

Fig. 20, The optical spectrum of the products of a AgiCgH, (cyclooctane) = 1 10 cocondensation reaction, (A) after deposition at 10-12 K, and (B)-(D) after 15-, 30-, and 60-min, narrow-band (8 nm), 335-nm, continuous excitation (152).

Figure 3. Continued. Excitation-emission spectra for hydrophobic neutral (e) and hydrophilic acid (f) fractions.

Figure 9.3. Illustration of the approach to steady-state conditions for the populations of states So, Si, and Ti under the continuous excitation conditions of Example 9.5...

During the course of these studies, it was found that fluorescence intensity from the polymeric films rapidly decreased on continued excitation in a fluorescence spectrophotometer (ca. 30% loss in 1 min for L). Herein, we (1) elaborate further upon the fluorescence loss studies, (2) provide direct evidence for RET from fluorescence lifetime measurements, and (3) present preliminary findings on the photochemistry of model compounds for polymer 1. The results support the conclusion, from previous studies, that the effectiveness of added stabilizer decreases with time due to formation of a photoproduct(s) from the polymer which competes in RET, and is less able to dissipate the resulting excitation energy.1... 

Shell Chemical Company), exhibits a maximum at 300 nm, corresponding to that of the model chromophore anisole. The fluorescence intensity decreases monotonically with increasing concentration of 2,4-dihydroxybenzophenone (DHB) and, furthermore, decreases with time on continued excitation (274 nm) in the spectrophotometer. The fluorescence loss with time may be resolved into two exponential decays. Initially, a relatively rapid fluorescence loss is observed within 20 sec, followed by a slower loss. Loss constants for the initial (k ) and secondary (kj) exponential decays for 1.5 ym films (on glass slides) containing varying concentrations of DHB are provided in Table I (entries 1-3). The initial loss constants are seen to decrease more markedly with increasing DHB concentration than the secondary constants. 

On continued excitation in the phase fluorimeter, the fluorescence lifetime of polymer 1 films also decreased with time. The lifetime decrease was exponential with an average loss constant of 8.2 1.2 x 10 lf sec-1 (1.5 pm thick film) from measurements at different sites on the film. These findings constitute direct evidence for RET from the polymer to a photoproduct(s) in support of the fluorescence intensity measurements. 

The luminescence lifetime may also be extracted from phase-sensitive detection of the modulated emission that originates from modulated continuous excitation of the indicator dye (equation 9)2 ... 

The large conjugation system required for NIR dyes may render the dyes unstable when continuous excitation is applied. But the best signal-to-noise ratios are obtained when the residence time approaches the average bleaching time of the dye. Therefore, the dye should be able to withstand continuous-wave laser excitations. 

This is the Stern-Volmer relationship with = k /k(j, and is an important basis for determining quenching rate constants after pulsed excitation. The quantum yield of (pro-duct)o can be measured without (0q) and with (0) quencher under continuous excitation (0 = moles of product/einsteins of light absorbed by system). Assuming that a steady state concentration of S exists in both cases. 

The ADF has been measured in deoxygenated solutions with continuous excitation of either argon, krypton or helium-neon lasers. The exciting wavelengths corresponded to the bands of Sq > 1 metalloporphyrin absorption (excitation wavelengths Xex = 514.5 530.9 568.2 nm for ZnTPP and Xex = 568.2 632.8 nm for ZnTBP and CdTBP). 

Firstly we shall consider the various forms of conventionally determined spectra, as distinct from time-resolved spectra, which can be obtained using continuous excitation. The measurement of emission spectra enables parameters such as the spectral position, spectral width, and spectral intensity to be determined. Moreover, the variation of these parameters with temperature is very important in deciding the origin of spectral bands. Sometimes the luminescence from single crystals is polarized and this can also give information as to the nature of the center. 

The emitted light is detected along y through a polarizer oriented either along z (Fz) or along x (Fx). In fluorescence polarization studies with continuous excitation (steady-state experiments), the emission anisotropy r and the emission polarization p are defined in eqs 8a and 8b. 

An interesting phenomenon observed during the decay measurements was the high relative peak intensity of the short-life 5DX emission in comparison to its emissions under continuous excitation. An analysis of the kinetics of a two-step decay process readily convinces one that this is quite normal. 

It is of some importance to note that on spectral plates taken using continuous excitation the emissions from the 5Z)1 level are extremely faint. As Bhaumik points out, the time-resolved spectral technique is of great value in energy-transfer studies and is also useful at times in obtaining spectra of short-lived states whose emissions would normally be obscured by other more intense fluorescences. Clearly, it is of value in ascertaining the particular state from which a group of lines originates. 

Any action that disturbs the de-excitation process integral to the photoreceptor cell will cause the OS to become transparent to continual excitation, i.e., the chromophore molecules will all become excited and the result will be the OS will bleach to transparency (13R). 

Before we can start to develop a model we also have to decide how to interpret the behavior observed in Fig. 2.1. The variations in insulin and glucose concentrations could be generated by a damped oscillatory system that was continuously excited by external perturbations (e.g. through interaction with the pulsatile release of other hormones). However, the variations could also represent a disturbed self-sustained oscillation, or they could be an example of deterministic chaos. Here, it is important to realize that, with a sampling period of 10 min over the considered periods of 20-24 h, the number of data points are insufficient for any statistical analysis to distinguish between the possible modes. We need to make a choice and, in the present case, our choice is to consider the insulin-glucose regulation to operate... 

The only pieces of hardware needed for photo-CIDNP are a light source and an unmodified NMR spectrometer. Pulsed lasers are most convenient for illumination, as they allow both time-resolved experiments (when the laser flash is followed by an acquisition pulse after a variable time delay) and steady-state ones (when the laser is triggered with a high repetition rate, thus providing quasi-continuous excitation). All the examples of this work draw on the second variant. Nevertheless, they yield kinetic information about much faster processes than would be observable by direct... 

In the absence of the SLR processes, the value of this ratio would depend on the mode of preparation of the triplet state. Three extreme cases could be realized, in the first case, the system is being continuously excited and decays continuously, i.e., the system is in a steady state. By equating the rates of pumping of any zf level with its rate of decay, one obtains the following ratios ... 

Fluorescence Polarization under continuous excitation are presently developed. But these experiments need the a priori choice of a model of motion to be interpreted, and such models do not exist so far for the local dynamics in bulk polymers. This limitation is very troublesome, since experiments carried out on polymers in solution have shown that varying the choice of the model used in data treatment could lead to important discrepancies in the derived correlation times or activation energies. In the following, we will show how Fluorescence Anisotropy Decay may help to overcome this difficulty, and we will give some examples of original information that can be obtained using this technique in conjunction with the powerful synchrotron light source. 

The local reorientation processes observed in FAD and the macroscopic relaxation have the same temperature behavior. Similar observations have already been made using NMR or Fluorescence polarization under continuous excitation However,... 

We have analyzed the influence of the annealing temperature, structural disorder, and the frequency of a continuous excitation laser radiation Vl on the first- and the second-order Raman spectra of several nanostructured carbon materials including single-wall carbon nanotubes (SWCNT), SWCNT-polymer composites, and nanostructured single-crystalline graphites. Consideration of the high-order nonlinear effects in Raman spectra and anharmonicity of characteristic Raman bands (such as G, G, and D modes) provides important information on the vibration modes and collective (phonon-like) excitations in such ID or 2D confined systems... 

Over a substantial number of years the phase-shift or frequency-domain method has been employed for the measurement of fluorescence lifetimes. The technique requires the continuous excitation of a fluorescent sample with a source of varying intensity. The fluorescence response would normally be expected to increase and decrease to reflect the changes in excitation intensity. However, in a frequency-domain experiment the excitation beam is modulated at a high frequency, (o = 2nf, to produce a sinusoidally changing intensity given by ... 

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