Two-photon laser-induced fluorescence (TP-LIF) line imaging of atomic hydrogen was investigated in a series of premixed , , and flames using excitation with either picosecond or nanosecond pulsed lasers operating at . Radial TP-LIF profiles were measured for a range of pulse fluences to determine the maximum interference-free signal levels and the corresponding picosecond and nanosecond laser fluences in each of 12 flames. For an interference-free measurement, the shape of the TP-LIF profile is independent of laser fluence. For larger fluences, distortions in the profile are attributed to photodissociation of , , and/or other combustion intermediates, and stimulated emission. In comparison with the nanosecond laser, excitation with the picosecond laser can effectively reduce the photolytic interference and produces approximately an order of magnitude larger interference-free signal in flames with equivalence ratios in the range of , and in flames with . Although photolytic interference limits the nanosecond laser fluence in all flames, stimulated emission, occurring between the laser-excited level, , and , is the limiting factor for picosecond excitation in the flames with the highest H atom concentration. Nanosecond excitation is advantageous in the richest () flame and in flames. The optimal excitation pulse width for interference-free H atom detection depends on the relative concentrations of hydrogen atoms and photolytic precursors, the flame temperature, and the laser path length within the flame.
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