Measurements of ultraviolet absorption spectra in lean and stochiometric premixed C<sub>2</sub>H<sub>4</sub>/O<sub>2</sub> flames have been performed with different flame temperatures (1400-1700 K). The spectra of the light transmitted through the flame were measured in the 200-270 nm range by using as an incoherent broadband light source the plasma kernel of optical breakdown generated by a focused pulsed laser beam. Measurements were performed far downstream of the main flame zone in a region where the concentrations of flame products reach equilibrium values. CO<sub>2</sub> and O<sub>2</sub> are the major absorbing species, and their absorption cross sections at flame temperature have been evaluated by changing the flame conditions and thus obtaining different concentration of these species at the flame exit. At temperatures above 1500 K there is strong absorption (σ<sub>a</sub> > 10<sup>-20</sup> cm<sup>2</sup>) due to both CO<sub>2</sub> and O<sub>2</sub> in the 200-230 nm range. A clear increase can be seen in absorption of both CO<sub>2</sub> and O<sub>2</sub> with increasing temperature. CO<sub>2</sub> absorption cross sections also show a different dependence with respect to temperature for the different wavelengths.
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