Laser-Induced Fluorescence and Related Techniques

Since Q is frequently 10 times larger than A, a very high spectral energy density p(u) is needed to obtain satm ation with a laser beam. However, 

An experimental set-up for studying LIF in flames is shown in Fig. 10.4. The output of a Nd YAG pumped dye laser can be frequency doubled and, if needed, the doubled output can be mixed with residual 1.06 jiim radiation to achieve still shorter wavelengths. The beam is directed through the flame and the fluorescence can be spectrally analysed with the spectrometer shown in 

If the two-photon transition is pumped sufliciently hard, strong amplified spontaneous emission is obtained in the forward direction [10.28]. (The 

Laser beam deflection can also be used to detect optical resonance. In the region of optical excitation the index of refraction of the gas changes and a probing laser beam (frequently a He—Ne laser beam) crossing the excited region will be deflected [10.42]. 

Hydrogen atoms can be detected observing H or H emission from the n = 3 or 4 levels following two- [10.23] or three-photon excitation [10.24] or step-wise excitations [10.25]. CO molecules are also best detected using two-photon excitation [10.26]. The excitation of flame species to an upper level can also be detected by other means. Optogalvanic spectroscopy (Sect.9.2.6) [10.27] and photoacoustic spectroscopy (Sect.9.2.8) [10.29] employing pulsed lasers have been used. The former method is not non-intru-sive in nature because of the need for electrodes. The latter technique util- 

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