Emission intensity, measurement techniques

Despite the fact that emission intensity measurements require supplementary lifetime determinations, they have the advantage over laser flash absorption techniques that they by-pass complications due to absorption by secondary products, such as radicals or solvated electrons (see e.g. Klaning et al., 1973). 

Experimentally the technique is simple since the sample may be irradiated from time to time and the emission intensity measured by a photomultiplier over a period of tens of seconds (Lange et al, 1998). However, these studies confirmed the limited usefulness of CRL alone for real-time monitoring since the emission is significant only in the glassy state, so this technique cannot detect the gel point for the majority of networks. When combined with other luminescence methods it has potential use because it is independent of the atmosphere and is readily adaptable to fibre-optics. 

In the direct insertion technique, the sample (liquid or powder) is inserted into the plasma in a graphite, tantalum, or tungsten probe. If the sample is a liquid, the probe is raised to a location just below the bottom of the plasma, until it is dry. Then the probe is moved upward into the plasma. Emission intensities must be measured with time resolution because the signal is transient and its time dependence is element dependent, due to selective volatilization of the sample. The intensity-time behavior depends on the sample, probe material, and the shape and location of the probe. The main limitations of this technique are a time-dependent background and sample heterogeneity-limited precision. Currently, no commercial instruments using direct sample insertion are available, although both manual and h ly automated systems have been described. ... 

Controlled double-potential techniques have also allowed measurement of relative emission intensity as a function of the time in which each of the potential steps is allowed to proceed. Comparisons of the emission intensities obtained with those predicted by mathematical models68,69 have been used as mechanistic evidence,85,67 see Sect. IV-B. 

One of the main goals of the crossed-beam experiment is to measure the internal energy AEvlh rol transferred to the molecule. In principle, this is possible in either of two ways. First, the scattered molecules could be detected and their product-state population analyzed. Infrared emission or absorption techniques may be considered, similar to those used in cell experiments.13 21 Although such studies would lead to the most detailed results (at least for polar molecules), under crossed-beam conditions they are impossible for intensity reasons, even if the possibility of measuring differential cross sections is renounced and the molecules in the scattering volume itself are detected. Detection via electronic molecular transitions may be invisaged. Unfortunately, the availability of tunable lasers limits this possibility to some exotic molecules such as alkali dimers. The future development of UV lasers could improve the situation. Hyper-Raman... 

Absolute H-atom measurements also were made using the Na/Li method (1(3) in sulfur free flames. An aerosol of an equimolar solution of NaCl and LiCl was added to the central core flow through the nebulizer. Relative intensity measurements were made of the Na 589.0 nm and Li 670.8 nm emission from which the H-atom concentrations were calculated. The H-atom measurements could only be made in the sulfur free flames. Reaction of Na or Li with sulfur species would render the technique inoperative. 

In the strobe or pulse sampling technique, the sample is excited with a pulsed light source. The intensity of the fluorescence emission is measured in a very narrow time window on each pulse and saved on the computer. The time window is moved after each pulse. When data have been sampled over the appropriate range of time, a decay curve of emission intensity vs. time can be constructed. 

The name strobe technique comes about because the photomultiplier PMT is gated -or strobed - by a voltage pulse that is synchronized with the pulsed light source. The strobe has the effect of turning on the PMT and measuring the emission intensity over a very short time window (Figure 7.6) (Bennett 1960 James et al. 1992). 

The rates of diffusion of solutes and surfactants in and out of micelles have been measured using photophysical techniques. The most commonly used method is to measure the deactivation of excited states of the probe by added quenchers, which are only soluble in the aqueous phase. The measurement of either the decrease in emission intensity or a shortening of the emission lifetime of the probe can be employed to determine exit and entrance rates out of and into micelles 7d). The ability of an added quencher to deactivate an excited state is determined by the relative locations and rates of diffusion of the quenchers and excited states. Incorporation of either the quencher or excited state into a surfactant allows one to determine the rates of diffusion of surfactants. Because of the large dynamic range available with fluorescent and phosphorescent probes (Fig. 3), rates as fast as... 

Radicals.—The measurement of emission intensities from electronically excited small free radicals has become an important means of determining radical concentrations in hostile environments such as flames. When combined with laser excitation, the technique is very powerful, offering temporal, spectral, and spatial resolution. Just has reviewed laser techniques for the measurement of both radical concentrations and local temperatures in flames, and has demonstrated the use of laser-induced saturated fluorescence to measure the concentrations of CH and OH radicals in low-pressure acetylene-oxygen flames. Vanderhoff ei al. used a novel Kr " and Ar laser intracavity technique to... 

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