Strategy for improved NH 2 detection in combustion environments using an Alexandrite laser

A new scheme for NH 2 detection by means of laser-induced fluorescence (LIF) with excitation around wavelength 385 nm, accessible using the second harmonic of a solid-state Alexandrite laser, is presented. Detection of NH 2 was confirmed by identification of corresponding lines in fluorescence excitation spectra measured in premixed NH 3 -air flames and on NH 2 radicals generated through NH 3 photolysis in a nonreactive flow at ambient conditions. Moreover, spectral simulations allow for tentative NH 2 line identification. Dispersed fluorescence emission spectra measured in flames and photolysis experiments showed lines attributed to vibrational bands of the NH 2 A 2 A 1 ← X 2 B 1 transition but also a continuous structure, which in flame was observed to be dependent on nitrogen added to the fuel, apparently also generated by NH 2 . A general conclusion was that fluorescence interferences need to be carefully considered for NH 2 diagnostics in this spectral region. Excitation for laser irradiances up to 0.2 GW/cm 2 did not result in NH 2 fluorescence saturation and allowed for efficient utilization of the available laser power without indication of laser-induced photochemistry. Compared with a previously employed excitation/detection scheme for NH 2 at around 630 nm, excitation at 385.7 nm showed a factor of ~ 15 higher NH 2 signal. The improved signal allowed for single-shot NH 2 LIF imaging on centimeter scale in flame with signal-to-noise ratio of 3 for concentrations around 1000 ppm, suggesting a detection limit around 700 ppm. Thus, the presented approach for NH 2 detection provides enhanced possibilities for characterization of fuel-nitrogen combustion chemistry.