Knowledge Bank

Saturated fluorescence is used to measure OH radical concentrations in sub-atmospheric Flat flames. Measurements are performed over a wide range of flame pressure, temperature and composition to assess the influence of collisional rates on the fluorescence process at saturation conditions. A high peak power, few nanosecond pulse from a Nd: YAG-pumped dye laser excites an isolated rotational transitional in the (0,0) band of the $A2\Sigma +-x2\pi$ electronic transition. The resulting fluorescence measurements are compared quantitatively with independent absorption measurements. The fluorescence signal from the directly excited upper rotational level is analyzed to obtain OH number densities. Computer modeling of laser excitation of OH, using gas kinetic rotational relaxation rates and literature values for electronic quenching cross sections, shows that the total population of the two rotational levels which are directly connected by the laser radiation is nearly constant during the laser pulse. Thus, a simple two level saturation model can be formulated and used to relate the population of the directly excited upper rotational level to the total molecular population.