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

Abstract

Two-photon laser-induced fluorescence for detection of carbon monoxide (CO) frequently shows interferences by emission from photolytically produced C2 radicals encountered under fuel-rich combustion conditions. Reduced C2 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 8ns duration and 4mJ pulse energy, picosecond pulses of 80ps duration and around 0.5mJ pulse energy gave ∼10 times higher peak power, which allowed for efficient CO excitation and resulted in stronger signal with lower C2 interference. CO fluorescence with picosecond excitation showed a linear to quadratic power dependence, indicating photoionization, whereas a more quadratic dependence was found for the C2 interference. A sub-nanosecond effective lifetime of CO resulted in a rapid fluorescence decay compared with C2 and allowed for efficient reduction in C2 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 C2 interference.