Picosecond-LIF-Spectroscopy with NO in a High Pressure Cell

Flame radicals are fragments of molecules with high reactivity and control the process of combustion to a high degree. Therefore, the knowledge of accurate number densities of these species is very important, e.g. when modelling flames. Because of the relative large cross sections compared to other optical methods, laser-induced fluorescence (LIF) spectroscopy is one of the most sensitive techniques for accurate determination of concentrations and temperatures [1]. However, when LIF is applied for quantitative diagnostics at high pressures (10 bar >p> 1 bar) and high temperatures, which is typical for industrial combustions, several problems associated with the LIF-method itself appear, and limit the accuracy of the method. The laser excites an upper level population of the molecule or atom under investigation, which decays by spontaneous emission and radiationless by collisional induced processes (quenching). The latter one reduces the fluorescence yield considerably, two or three orders of magnitude are typical for atmospheric pressure. If the measurements are performed with a time resolution better than the quenching rates, the LIF-intensities can be used to extract absolute number densities. However, this requires a laser and a detection system with picosecond time resolution. Since important atomic radicals like O, C, N, H or diatomic molecules like NO, CO and OH can only be excited from the ground state via two- or one photon absorption in the spectral range between 200 and 300 nm [2] a powerful ultraviolet laser system is required in these experiments. However, the quantitative interpretation of the picosecond LIF-intensity measurements still needs accurate quenching rate data for the relevant pressures and temperatures and the species that are present in the combustion process. In the data analysis also systematic influences like photodissociation effects by the strong uv-laser pulses have to be considered. Therefore, in this paper improved quenching rate measurements of NO with NO, N2 and O2 for pressures up to p=10 bar, and photodissociation effects of NO are reported.