Fluorescence scattering spontaneous

Laser-based spectroscopic probes promise a wealth of detailed data--concentrations and temperatures of specific individual molecules under high spatial resolution--necessary to understand the chemistry of combustion. Of the probe techniques, the methods of spontaneous and coherent Raman scattering for major species, and laser-induced fluorescence for minor species, form attractive complements. Computational developments now permit realistic and detailed simulation models of combustion systems advances in combustion will result from a combination of these laser probes and computer models. Finally, the close coupling between current research in other areas of physical chemistry and the development of laser diagnostics is illustrated by recent LIF experiments on OH in flames. 

The fluorescence technique, like other methods based on scatter (elastic or inelastic), has been shown by us - and others to be a reliable unperturbing method of measuring spatial/ temporal flame temperatures and species concentrations. To avoid the dependency of the fluorescence signal on the environment of the emitting species, it has been shown by several workers that optical saturation of the fluorescence process (i.e., the condition occurring when the photoinduced rates of absorption and emission dominate over the spontaneous emission and colli sional quenching rates) is necessary. Pulsed dye lasers have sufficient spectral irradiances to saturate many transitions. Our work has so far been concerned with atomic transitions of probes (such as In, Pb, or T1) asoirated into combustion flames and plasmas. 

The inelastic processes - spontaneous Raman scattering (usually simply called Raman scattering), nonlinear Raman processes, and fluorescence - permit determination of species densities as well as temperature, and also allow one, in principle, to determine the temperature for particular species whether or not in thermal equilibrium. In Table II, we categorize these inelastic processes by the type of the information that they yield, and indicate the types of combustion sources that can be probed as well as an estimate of the status of the method. The work that we concentrate upon here is that indicated in these first two categories, viz., temperature and major species densities determined from vibrational Raman scattering data. The other methods - fluorescence and nonlinear processes such as coherent anti-Stokes Raman spectroscopy - are discussed in detail elsewhere (5). 

We will detect the 2S-3S transition by observing fluorescence from the 3S-2P-1S decay cascade, see Fig. 7. The 30 nm radiation may be detected using a channel electron multiplier. The dominant source of background is expected to be from the spontaneous two-photon decay of the metastable state, giving the characteristic broad, symmetrical frequency spectrum centred on 60.8 nm. A thin aluminium filter will be used to suppress this background and ehminate the scattered 328 nm light. 

Le Ru, E. C., and Etchegoin, P. G. (2005). Surface-Enhanced Raman Scattering (SERS) and Surface-Enhanced Fluorescence (SEF) in the context of modified spontaneous emission. arXiv physics/0509154vl pp.1-14. 

State. This state is broadened by its interaction with the radiative continua A, and the nonradiative manifold L. Spontaneous emission (i.e. fluorescence) is the process in which the state s) decays into the radiative continua R[, that is, to vibronic levels of the ground electronic states plus a photon. A competitive decay channel is the nonradiative decay of s> into the nonradiative manifold(s) L. Light scattering is a process that starts with a 1-photon level of the ground electronic state and ends in another such state, that is, 11, vi, k) 11, vj, k ). The elastic process vi = vj and I k I = I k I is called Rayleigh scattering. The inelastic process where these equalities are not satisfied is Raman scattering. 

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