Atoms fluorescence decay

Homogeneous Time Resolved Fluorescence (HTRF) (Cisbio International) is an assay based on the proximity of a lanthanide cryptate donor and a fluorescent acceptor molecule whose excitation wavelength overlaps that of the cryptate s emission. The utility of this technique is based on the time resolved fluorescence properties of lanthanides. Lanthanides are unique in the increased lifetime of their fluorescence decay relative to other atoms, so a delay in collection of the emission intensity removes the background from other fluorescent molecules. An example of the HTRF assay is a generic protein-protein interaction assay shown in Fig. 2. 

There exists a distribution of microstates associated with the internal dynamics at the level of atomic groups. This may also result in nonexponential fluorescence decay if the transitions between microstates occur more slowly than the decay. 

Geusic et al. (118) made measurements of the fluorescent lifetime of neodymium in yttrium aluminum garnet (Y3A15012). For neodymium concentrations up to 3 atomic per cent, the measured fluorescent-decay time is approximately 200 jtxsec at both IT and 300°K. Above 6 atomic per cent, a marked decrease in the fluorescent lifetime is observed. They suggest that this is due to neodymium interactions. It is to be noted that yttrium aluminum garnet is a laser material of very exceptional quality. 

Metlay (145) studied the fluorescent decay of both EuD3 and EuD4. These compounds were prepared by the method of Crosby et al. (142). As Metlay points out, Whan and Crosby (146) in a later paper describe their preparation in more detail. They indicate that a period of heating in vacuum is necessary to convert the chelate from one containing four molecules of dibenzoylmethane per atom of europium to one containing but three. 

Nemzek and Ware [7] have studied the fluorescence decay of 1,2-benzanthracene (and naphthalene) in 1,2-propanediol or purified mineral oil by the single photon counting technique over the temperature range 10—45°C. The fluorescence lifetimes, t0, were measured. In further experiments, which included a heavy atom fluorescence quencher, carbon tetrabromide in concentration [Q] 0.05—0.29 mol dm-3, no longer could the decay be characterised by an exponential with a constant lifetime. However, the decay of fluorescence was well described by an expression of the form... 

Figure 2.7 Theoretical level widths for K-shell ionization as a function of the atomic number. The total level width T is the sum of two contributions that come from radiative (fluorescence) decay, TR, and non-radiative (Auger) decay, TA. From [Kra79].

The first observations of energy transfer were made in gas phase atomic systems. There have been many publication dealing with similar studies on polyatomic systems in the gas phase, liquid phase and in molecular crystals. Only very little, however, is known about transfer processes and mechanisms when donor and acceptor molecules are adsorbed at the surface of a solid (1-3). In this paper we present some photoacoustic measurements and compare the results with fluorescence decay time measurements. As model substances we have chosen four different donor dyes and two acceptor dyes. The dyes were adsorbed on silica. 

For both la and lb, biexponential fluorescence decays provide evidence for the presence of two forms, even at low temperatures. At the same time, fluorescence remains depolarized. It implies that the rate of excited trans-trans conversion may be faster than that of the cis-trans reaction. In other words, simultaneous transfer of two hydrogen atoms is favored over a single hydrogen transfer process. 

Pulsed laser excitation of evaporated substances and subsequent fluorescence decay detection have been used in the studies reviewed here. In our opinion this is the most convenient method for lifetime measurements in the UVAAJV spectral range. The principal scheme of an experimental set-up is shown in Fig.l. It has been slightly modified in various measurements, but the main parts are the same and correspond to the three tasks atomization, excitation and detection. (The excitation laser pulse has to be of short duration and in the UVA TJV region.) Each of these tasks are discussed more in detail below. 

The equatorially coordinated adduct ligand has little impact on the potential around the central uranium atom and therefor has a negligible effect on the structure of the emission spectra. (30-33) However, as shown in Table II, changing the adduct of a complex did result in small changes in the measured fluorescence decay time. 

Many of the photochemical advances that have occurred during the past two decades have followed from the development of microwave discharge vacuum ultraviolet light sources, emitting either intense monochromatic atomic resonance radiation at fixed wavelengths (determined by nature rather than for utility) or over broad continua produced by the fluorescent decay of rare gas excimers which provide tunable sources after passage through a vacuum monochromator (but at the cost of reduced intensity). 

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