LIF with Tunable Excimer Lasers as a Possible Method for Instantaneous Temperature Field Measurements at High Pressures

Abstract

A new method for instantaneous temperature field measurements at high pressures is presented. This method is based on spectrally dispersed LIF studies of OH, O2 and H2O in an open atmospheric flame with a tunable KrF-excimer laser. The crucial problem of quenching of the fluorescence at high pressures is almost completely eliminated by excitation to a fast predisso-ciating state. In this case the fluorescence is emitted only during the short predissociation lifetime t of the excited electronic state. For a sufficiently short predissociation lifetime there will be no quenching within this lifetime, even at high pressures. The fact that the excited state molecules radiate only in the short time τ means on the other hand that only a very small part of the molecules does fluoresce. Although this causes a tremendous loss in LIF signal, this is partly compensated because the densities are much higher than in normal LIF experiments. The large power of the KrF-laser is therefore also of particular importance. This laser delivers 450 mJ per pulse in a bandwidth of 0.5 cm-1. For two dimensional measurements a large sheet can be illuminated with this laser, still having efficient excitation of even weak transitions.