Time-Resolved Luminescence Techniques

The emission spectra can also be recorded at different times after the excitation pulse has been absorbed. This experimental procedure is called time-resolved luminescence and may prove to be of great utility in the understanding of complicated emitting systems. The basic idea of this technique is to record the emission spectrum at a certain delay time, t, in respect to the excitation pulse and within a temporal gate. At, as schematically shown in Figure 1.12. Thus, for different delay times different spectral shapes are obtained. 

Time-resolved luminescence spectroscopy may be extremely effective in minerals, many of which contain a large amount of emission centers simultaneously. With the steady state technique only the mostly intensive centers are detected, while the weaker ones remain unnoticed. Fluorescence in minerals is observed over time range of nanoseconds to milliseconds (Table 1.3) and this property was used in our research. Thus our main improvement is laser-induced time-resolved spectroscopy in the wide spectral range from 270 to 1,500 nm, which enables us to reveal new luminescence centers in minerals previously hidden by more intensive centers. 

The same equipment, which is used for time-resolved Ivuninescence application is suitable for other laser-based spectroscopies. Thus several spectroscopic methods may be applied simultaneously. The most important techniques, which may be used together with time-resolved luminescence, are laser-induced breakdown spectroscopy, Raman spectroscopy and Second Harmonics Generation spectroscopy. 

The Applications of Laser-induced Time-resolved Spectroscopic Techniques chapter starts with a short description of laser-induced spectroscopies, which may be used in combination with laser-induced luminescence, namely Breakdown, Raman and Second Harmonic Generation. The chapter contains several examples of the application of laser-based spectroscopies in remote sensing and radiometric sorting of minerals. The proljlem of minerals as geomaterials for radioactive waste storage is also considered. 

An example of the utility of the time-resolved technique in eliminating the interference from background fluorescence in bioimaging is shown in Figure 13.17b. Nagano and coworkers compared time-resolved luminescence microscopy with conventional microscopy using live cultured HeLa cells injected with a Eu + complex Eu-36 (or Eu-37). In the prompt fluorescence images, both the luminescence of Eu-36 (or Eu-37) and weak autofluorescence from... 

This time-resolved fluorescence technique allows a measure of the time dependence of fluorescence intensity after a short excitation pulse. It consists of obtaining a spectrum measured within a narrow time window during the decay of the fluorescence of interest. The usefulness of this technique is now well proven for biochemical assays and immunoassays. Lanthanide chelates have luminescence decay times over 600 ps, which allows time-gated fluorecence detection, with a complete rejection of other fluorecence signals. For these quantitative applications, the primary source is generally a quartz lamp associated with a splitter. 

In our opinion the paper by Moses et al. [36] on the scintillation mechanisms in CeF.3 is a fine example of how scintillators should be studied from a fundamental point of view. A combination of techniques was u.sed, viz. (time-resolved) luminescence spectroscopy, ultraviolet photoelectron spectroscopy, transmission spectroscopy, and the excitation region was extended up to tens of eV by using synchrotron radiation. Further, powders as well as crystals with composition Lai-xCexFy were investigated,... 

Basically, instalments for measuring fluorescence and phosphorescence spectra have similar construction and should be called luminescence spectrometers. However the group of molecules that exhibit fluorescence is by far larger than that exhibiting phosphorescence, hence the term fluorescence spectrometer is used. The main spectral features of luminescence are spectral distribution, polarization and radiation lifetime. For analytical purposes spectral distribution and polarization are mainly used. Measuring the lifetimes requires a rather sophisticated time-resolved spectroscopic technique. It is very seldom used for analytical purposes and will not be discussed in this chapter. 

The main technique used to look at exchange processes in equilibrium systems employs labeled surfactants, particularly with ESR spectroscopy. Fox s ESR study [92] of a paramagnetic surfactant in micellar solution was the first of its kind, and yielded a solution-micelle monomer exchange rate of 10 s" at room tanperature for 2,2,6,6-tetramethylpiperidine-oxidedodecyldimethylammonium bromide. These techniques, along with time-resolved luminescence quenching, have shown that the entry of surfactant molecules into micelles is near-diffusion controlled, whereas loss from micelles is rate limiting, and hence kinetically controlled [93]. A decade later (1981), Bolt and Turro [94] were able to find the separate exit and reentry rate constants for 10-(4-bromo-l-naphthoyl)decyltrimethylammonium bromide as 3.2 x 10 s and 5.7 x 10 mok s", respectively. 

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