Single-shot imaging of formaldehyde in hydrocarbon flames by XeF excimer laser-induced fluorescence

A novel way to excite laser-induced fluorescence (LIF) of formaldehyde was applied to measure LIF of hydrocarbon flames (diethyl either, ethene, n-heptane, and kerosene). LIF excitation spectra of formaldehyde in a reference cell and of flames were obtained by tuning a spectrally narrowed XeF excimer laser (Delta lambda = 4 pm, 30 mJ pulse energy) between 353.1 and 353.6 nm and integrating the emission between 360 and 550 nm. The LIF excitation spectra of the flames showed a contribution from formaldehyde and a broadband background from, for example, polycyclic aromatic hydrocarbons (PAHs). Exploiting the high excitation efficiency and pulse energy of the narrowband XeF excimer laser, single laser shots were sufficient to measure two-dimensional distributions of formaldehyde LIF (light sheet technique [3 X 3 cm(2)], intensified charge-coupled device [CCD] camera). Fast wavelength switching between on- and off-resonant excitation of formaldehyde was successfully applied to subtract background emission in the case of diethyl ether, n-heptane, and ethene flames. Furthermore, it was shown that the images of formaldehyde LIF complement the measurements of OH LIF. OH is only found in higher-temperature flame regions.