Saturated fluorescence

Comparison of concentration measurements made by saturation fluorescence and absorption measurements. See the text for details. 

In summary the recently developed fields of CARS and laser induced saturation fluorescence spectroscopy offer considerable potential as diagnostic techniques for combustion systems. The techniques are complimentary. CARS has its best application for relatively high concentration flame gases and for tenperature measurement. The fluorescence technique is well suited for low concentration measurements of atoms and radicals and flame transients. 

The recent availability of tunable dye lasers has markedly enhanced our ability to inquire into the chemistry and physics of combustion systems. The high sensitivity, spectral and spatial resolution, and non-perturbing nature of laser induced fluorescence makes this technique well suited to the study of trace chemistry in complex combustion media. A barrier to the quantitative application of fluorescence to species analysis in flames has been the need to take into account or bypass the effects of quenching. The use of saturated fluorescence eliminates quenching as a problem and has the further advantage that fluorescence intensity is insensitive to variations in laser power (1, 2 ). However, the generation of high concentrations of excited states under saturated excitation in an active flame environment opens up the possibilities for laser induced chemistry effects that also must be taken into account or avoided (3,4,5). 

Spatial density profiles of atomic (and molecular) species can also be made via saturation fluorescence approaches. For a "2-level" atom, like Sr, a plot of 1/Bp vs 1/E (Bp is the fluorescence radiance, in J s 1m-2sn l, and Ex is the excitation spectral irradiance, in J s nT nm ) allows estimation of the quantum efficiency, Y of the fluorescence process (and thsu estimation of "radiationless" rate constants) and the total number density nj, of the species of interest by means of... 

Saturated fluorescence is not without its complications and difficulties. High laser intensities are required to achieve saturation, but are difficult to obtain at certain wavelengths, e.g. NO at 2265 8. This is one reason OH has received much study, since its absorptions reside at the frequency-doubled wavelengths of powerful Rhodamine dye lasers. For many molecules... 

CH, CN Investigations. In the previous studies of CH and CN at our laboratory (41), the saturated fluorescence values were a factor of two and five respectively below those determined by absorption. If anything, the absorption measurements may have been in error to the low side. The discrepancy, it was believed, was due to the focussed laser beam diameter exceeding the radical production region. This caused an overestimate of the fluorescence sample volume, i.e. assumed to be the laser volume,and an underestimate of the species number density. Recently these experiments have been repeated using laser-pumped dye lasers which have a pulse width of 5-10 (10-9) sec and a 10 Hz repetition rate. 

Lucht, R. P. Laurendeau, N. M. Sweeney, D. W. "Saturated Fluorescence Measurements on Diatomic Flame Radicals, in this volume. 

Laser induced fluorescence is particularly well suited to combustion chemistry, as a sensitive "in-situ" probe for free radicals in flames or under more controlled conditions in laboratory flash photolysis, discharge flow tube, or shock tube experiments. Using laser-saturation fluorescence previous studies from this laboratory (J ) have shown that C2(a3n ) is present in high concentrations in the hot region of an oxy-acetylene flame. C2(a-,n and X1 ) reacts with 0 .(2,3 4) One of the products of this reaction (and/or the reaction of C2H+02) is CC0.(2) In the present study, we report C20()rn.-f, i 7 fluorescence excitation spectra, A"3 , lifetimes and quenching rate constants, and... 

We detected the saturated fluorescence emitted by a beam of 23S metastable atoms as they cross at right angle the slave laser light. A 1015 atoms/s.sterad flux of metastable helium atoms was produced by electronic collisions in a DC discharge of a helium atomic beam, similar to that described in [15]. To improve the precision of the linecenter determination, we increased the signal-to-noise ratio S/N by means of standard frequency modulation the third harmonic demodulated lineshape is shown in Fig. 4. The function expected for a Lorentzian spectrum was fit and linecenters were calculated with an uncertainty ranging between 10 kHz and 20 kHz, that is consistent with the observed S/N, mainly limited by the stability of the reference frequency and of the metastable helium beam. The reproducibility was two or three times worse than the uncertainty,... 

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... 

What is meant by saturated fluorescence Why is saturated fluorescence used for analytical AFS ... 

Figure 2 illustrates the 3rd order spectrometer. CO2 lasers I and II are stabilized to CO2 sub-Doppler saturated fluorescence features using separate (not shown) low pressure cells (13). Their radiation is focused on the diode along with that of a frequency-synthesized, microwave (2-20 GHz) source used to add the tunable sidebands to the... 

In each spectrometer both CO2 reference lasers are frequency modulated at a 1 kHz rate using piezoelectric drivers on the end mirrors and are then servoed to the line center of the saturated fluorescence signals obtained from the external low-pressure CO2 cells. The FIR detectors and lock-in amplifier detect at the modulation rate hence, the derivatives of the absorptions are recorded. The depth of the frequency modulation of CO2 laser I can be increased up to 7 MHz to enhance the FIR frequency modulation, thereby increasing the sensitivity for the broader lines. 

The solid curves in Figs. 7(a) and (b) are fits of Eqs. 6 and 7, respectively, to the data. They show that the functional form of the saturation behaviour is well reproduced by the theory. The fitting parameters in Eq. 7 are the saturation intensity and the fully saturated fluorescence photocount rate (Coo). The latter is given by the product of the fully saturated fluorescence emission rate 7 oo and the detection efli-ciency D Coa = Raa x D). For the case that i oo can be reliably calculated by Eq. 9 -i.e. all photophysical parameters are known from independent measurements - the detection efficiency of the optical setup can be determined. Conversely, if D is known independently, the measured value of Coo can be compared to the theoretical prediction from / Qo. 

J.E.M. Goldsmith Two-step saturated fluorescence detection of atomic hydrogen in flames. Opt. Lett. 10, 116 (1985)... 

The detector, subject to availability, should match the solute characteristics. UV-visible absorbance detectors are suitable for many solutes except those that are fully saturated. Fluorescence and electrochemical detectors should be considered where high sensitivity is required. 

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