Luminescence excited flames

Experimental determination of the excitation factor shows that with chemical luminescence excitation (chemiluminescence) in flames the f value is, as a rule, much higher than the thermal emission factor. This stems from the non-equilibrium nature of this kind of emission, directly related to the energy liberated by some or other elementary chemical process. This shows the high importance of chemiluminescence both for the identification of labile intermediates and for the elucidation of certain fine details of the chemical reaction mechanism. 

The blue luminescence observed during cool flames is said to arise from electronically excited formaldehyde (60,69). The high energy required indicates radical— radical reactions are producing hot molecules. Quantum yields appear to be very low (10 to 10 ) (81). Cool flames never deposit carbon, in contrast to hot flames which emit much more intense, yellowish light and may deposit carbon (82). 

Excited states may be formed by (1) light absorption (photolysis) (2) direct excitation by the impact of charged particles (3) ion neutralization (4) dissociation from ionized or superexcited states and (5) energy transfer. Some of these have been alluded to in Sect. 3.2. Other mechanisms include thermal processes (flames) and chemical reaction (chemiluminescence). It is instructive to consider some of the processes generating excited states and their inverses. Figure 4.3 illustrates this following Brocklehurst (1970) luminescence (l— 2)... 

Thus, although the colour of sparks is dependent upon flame temperature and may be similar to that of black body radiation, the overall colour effect can include contributions from atomic line emissions, from metals (seen in the UV and visible regions of the electromagnetic spectrum), from band emissions from excited oxide molecules (seen in the UV, visible and IR regions) and from continuum hot body radiation and other luminescence effects. So far as black body radiation is concerned, the colour is known to change from red (500 °C glowing cooker... 

The emission spectra are similar but often not identical to those excited by UV (18). The energy source is the recombination of free radicals that occurs in the flame and thus flame-excited luminescence is the same as the radical recombination luminescence observed when free neutral radicals from plasmas are used as an excitation source. A simple hydrogen diffusion flame is the simplest source for demonstrating the phenomenon. 

The popularity of the BCD can be attributed to the high sensitivity to organohalogen compounds, which include many compounds of environmental interest, including polychlorinated biphenyls and pesticides. It is the least selective of the so-called selective detectors but has the highest sensitivity of any contemporary detector. The NPD or thermionic ionization or emission detector is a modified FID in which a constant supply of an alkali metal salt, such as rubidium chloride, is introduced into the flame. It is a detector of choice for analysis of organophosphorus pesticides and pharmaceuticals. The FPD detects specific luminescent emission originating from various excited state species produced in a flame by sulfur- and phosphorus-containing compounds. 

In addition to these induced effects, even undisturbed excited states will not live forever. The general deactivation is a radiationless process. Relatively few molecules exhibit spontaneous emission, called luminescence in the visible, or emission. This deactivation process of the excited state is a statistical effect and does not directly correlate with an act of excitation. Except induced absorption, plasma coupling, hot flames, or sparks can yield a relatively high population in the excited state which will depopulate by emission. This emission is used in analytics, especially in atomic emission spectroscopy. Since atoms in the gases are not influenced by the surrounding and their energies are not smeared by vibrational interactions, they will exhibit sharp characteristic lines for different metals. The advantages are discussed in more detail in Chap. 6 of this book. 

Besides the excited OH (H2 flame), CO2 (CO flame), present in the luminescent flame zone in concentrations much higher than their equilibrium concentrations at the flame temperature, unexcited active species, atoms and radicals have also been found in concentrations by several orders higher than the equilibrium values. Such are, for instance, the concentrations of atomic hydrogen and oxygen and of OH radicals measured by the EPR technique [18] in rarefied hydrogen flames for various H2 and O2 contents (Fig. 59). The maximum concentration of hydrogen atoms is here of the order 10 cm , i.e. more than 30% of the whole species content. 

The optical luminescence exhibited by some of the rare earth complexes and ions in solids has been utilized for the detection and determination of the rare earths at the trace and ultratrace levels for many years (El Yashevich, 1953 Dieke, 1968 Sinha, 1966). To date, optical luminescence of the rare earth ions in solids has been excited by flames (candoluminescence) (Neunhoeffer, 1951 Sweet et al., 1970) by ultraviolet or visible radiation (photoluminescence or UVEOL) (Ankina Ozawa/snrf ]trp 1968 Poluektov et al., 1971 ) by... 

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