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Show PMT M,F Nd:YAG- Dye- \) \ Laser - Laser ( .J Burner PMT: Photomultiplier M, F: Monochromator or Filter Figure 1. Schematic setup of a L1F experiment. The most common laser used for LIP is a nmable dye laser. Operation on different dye solutions, together with frequency shifting devices, allows coverage of a large range of wavelength, 200 to 800 om and beyond. As seen from Table 1, this is necessary for the detection of a large number of flame intermediates. The laser is pulsed, providing light only for about 10 os, at typical repetition rates of 10 to 100 Hz. The use of gated electronic signal processors which are "on" only during or shortly after the laser pulse provides a high degree of discrimination for the short-lived signal against continuous background light, such as ordinary flame emission or blackbody surface glow. Excitation and Fluorescence Steps Laser-induced fluorescence can be seen as two successive steps. In the first, the laser excites the molecule from some particu1arrotational-vibrationallevel in the ground electronic state to some level in the excited state. See Fig. 2, which illustrates excitation of a series of levels, each having a different absorption wavelength, as the laser is scanned. The absorption rate depends on the laser intensity IL, the molecular absorption coefficient B, and the number of molecules N present in the flame within the absorbing rotational-vibrational level. 6 |