Sensing combustion intermediates by femtosecond filament excitation.

Simultaneous monitoring of multiple combustion intermediates using femtosecond filament-induced nonlinear spectroscopy is demonstrated. Clean fluorescence emissions from free radicals CH, CN, NH, OH, and C(2), as well as atomic C and H, are observed when a femtosecond filament is formed in the laminar ethanol/air flame on an alcohol burner. The fluorescence signals of these species are found to vary as functions of the position of interaction of the filament with the flame along the vertical axis of the central combusting flow, opening up a possibility for remote combustion diagnostic in engines by the excitation of femtosecond laser filament.