The OH-radical is one of the most important reaction intermediate in combustion. Detection of planar laser induced fluorescence (PLIF) of OH is a frequently used technique to visualize the location of the reaction zone or the structure of various types of flames. Qualitative two- dimensional mapping of OH-concentration-fields is commonly used in laboratory- and technical applications, quantitative concentration values can be determined for many selected cases. Employing LIF-techniques in microgravity combustion research is currently very limited due to the lack of appropriate laser sources with respect of size, efficiency and spectral performance. Within the electronic transition, OH exhibits the strongest favorable experimental features for LIF exciting the X (v"=O) state to the A state at (v'= 0, 1, 2 and 3) at wavelengths centered at 308nm, 283nm, 262nna and 246nm respectively, with drastically declining absorption efficiencies to the UV. Thus, in particular for planar LIF application, diagnostics using the v'= 2 or 3 excited state, the only appropriate excitation sources have been excimer based systems or, in few cases, specially modified Nd:YAG-lasers with low pulse repetition rates. Consequently, the choice of the rotational transition used to excite LIF was very limited by the spectral properties of the laser source. Exciting the lower vibrational bands in the excited state of OH, diverse configurations with dye-lasers provide sufficient energy for PLIF-applications. However, these setups are not suitable for microgravity experiments.
The "Advanced Disc Laser" (ADL), especially designed for laser diagnostic applications on microgravity platforms, will provide narrow-bandwidth pulsed UV-emission covering an wide tuning range of 254nm to 263nm. Figure la displays the spectral overlap of the ADL-tuning range with the (2,O) vibrational band within the OH (A-X) electronic transition in addition to the spectral emission ranges of the tunable Nd:YAG- and a Raman (D2)- converted KrF-excimer laser. Both latter sources are the only practical means known to the author actually available to be applied for OH-LIF in the (2,O) band for combustion diagnostics [e.g. 1,2].
Konig, J., Grebner, D., Triebel, W., Johannsen, I, Giense, A., The Potential Applying the "Advanced Disc Laser" (ADL) in Microgravity-Combustion for OH-LIF Diagnostics using the (A-X;2,0) -Electronic Excitation Scheme, Sixth International Microgravity Combustion Workshop, NASA Glenn Research Center, Cleveland, OH, CP-2001-210826, pp. 53-56, May, 2001.