Picosecond excitation for reduction of photolytic effects in two-photon laser-induced fluorescence of CO

Two-photon laser-induced fluorescence for detection of carbon monoxide (CO) frequently shows interferences by emission from photolytically produced C-2 radicals encountered under fuel-rich combustion conditions. Reduced C-2 interference for excitation with laser pulses in the picosecond regime is here demonstrated by comparison with excitation using nanosecond pulses for measurements in laminar premixed ethene-air flames. Compared with nanosecond pulses of 8 ns duration and 4 mJ pulse energy, picosecond pulses of 80 ps duration and around 0.5 mJ pulse energy gave similar to 10 times higher peak power, which allowed for efficient CO excitation and resulted in stronger signal with lower C-2 interference. CO fluorescence with picosecond excitation showed a linear to quadratic power dependence, indicating photoionization, whereas a more quadratic dependence was found for the C-2 interference. A sub-nanosecond effective lifetime of CO resulted in a rapid fluorescence decay compared with C-2 and allowed for efficient reduction in C-2 interference by minimizing the detection gate. In addition, interference compensation using time-resolved detection could be demonstrated. Altogether, picosecond pulses provide efficient two-photon excitation of CO in terms of signal strength as well as reduced C-2 interference. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved