LIF technique

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

Kanazawa, S., Sumi, T., Sato, N. et al. (2005) Wide-range two-dimensional imaging of NO density profiles by LIF technique in a corona radical shower reactor, IEEE Trans. Ind. Appl. 41, 200-5. 

There are claimed advantages and disadvantages for both techniques. Both have sensitivities of about 10 cc per quantum state for moderate power laser sources. Of course increased laser power provides increased sensitivies in either case. One advantage (we feel) to the LIF technique for NO detection is that a single calibration of the apparatus constant (photon collection and detection efRciencies) is valid for pressure from 10" torr to one atmosphere. 

The OH intercomparison results were not of sufficient quantity to allow one to conclude that OH measurements in the clean, remote troposphere can be made with sufficient accuracy or reliability. The near-zero nighttime results from the two LIF techniques, however, do indicate that there are no major problems that can be attributed to artifacts or interference effects for clean, remote tropospheric measurements. Finally, it is of some significance that within the accuracy of the ensemble of OH measurements reported during... 

Using this laser-induced fluorescence (LIF) technique, they have been able to determine branching ratios in the photolysis of various compounds. 

Previous workers had used the molecular beam TOF technique (134) and the VUV flash photolysis LIF technique (135). Ling and Wilson (136) had suggested that either the A(2n) state of CN is produced in the original photolysis process or that I atoms were produced in the Pi/2 and 3/2 states. It had been previously shown (135), by collisional quenching studies, that the A state of CN was not produced. This earlier work has been reviewed by Baronvaski (137) but recently both he and others have done further work on this molecule using excimer laser sources in both static gases and pulsed molecular beams. 

In recent years, several workers have studied the photodissociation dynamics of this molecule in its various absorption bands. Two principal techniques are used, namely emission spectroscopy at wavelengths below the threshold for the production of CN(b2e+) and the LIF technique for and A n state CN... 

Recent work (161) with a tunable VUV flash lamp has shown that the CN(A2II) can be detected directly using the LIF technique. Thus one is able, in principle, to determine the vibrational and rotational population of each of the fragments (CN(X2E), (A2n)). The tunable UV flash lamp allows one to measure these quantum state distributions as a function of the vibrational frequency of the upper electronic state. The results from these studies thus far are summarized in Table 8. 

The photochemical dynamics of H2S has been studied in its first absorption band between 180 and 260 nm (2) using LIF measurements to determine the quantum state distribution of the SH fragment (169-171), as well as TOF measurements of the velocity distribution of H atom fragment (172). In the former case, the vibrational and rotational distribution of the SH fragment was only measured in the v" = 0 level because fewer radicals with v" > 0 are produced and the LIF technique does not efficiently detect these excited radicals. 

The TOF results suggest that the geometry of the excited state is very similar to the geometry of the ground state, in agreement with the observation of Hawkins and Houston, who found very little rotational excitation in the SH fragment. The LIF technique was used to map out the rotational, and nominally the vibrational distribution, of the SH fragment. Very little vibrational excitation was observed, presumably because the detection sensitivity is much less for v" > 0. 

In conventional RE processes, the diffusive resistance is concentrated mainly inside the droplet, whereas the aqueous-side resistance can be neglected. This has been proven in Ref. 203 using the laser-induced-fluorescence (LIF) technique. Usually the organic phase is more viscous and the diffusion coefficients of the organic complexes are larger than those at the aqueous side, which supports this finding. 

In the gas-liquid two-phase flows illuminated by a laser sheet, for example, the intensity of light reflected from the gas-liquid interface (mostly the gas bubble s surface) not only saturate the CCD camera, but also overwhelm the intensity of light from the seeded tracer particles in its vicinity. Fluorescent particles are often used to realize the laser-induced fluorescence (LIF) technique together with PIV (e.g., Broder and Sommerfeld, 2002 Fujiwara et al., 2004a, b Kitagawa et al., 2005 Liu et al., 2005 Tokuhiro et al., 1998,1999), so that both images of gas-liquid interface (e.g., bubble s geometry) and velocity distribution in the liquid phase around the gas bubbles can be obtained. Issues on PIV measurement of gas-liquid two-phase flows will be further illustrated in the latter sections. 

A representative time sequence of four PIV/LIF-derived velocity vector distributions together with the SIT-derived bubble shadows is plotted in Figure 16 as a typical result obtained by the PIV/LIF/SIT system. Note that even with LIF technique used, there are also "white-out" regions (intensity saturation), and the laser sheet entering from the... 

Attempts to detect HGeF in the reaction of Gel [4 with fluorine atoms or in a pulsed discharge of HjGeF using the LIF technique were unsuccessful78 and it has been concluded that HGeF either cannot be obtained by these methods or does not fluoresce in the excited state78. 

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