6 Laser-Induced Fluorescence Spectroscopy
Laser Probes for Combustion Chemistry Downloaded from pubs.acs.org by IMPERIAL COLLEGE LONDON on 11/26/18. For personal use only.
Applied to the Hydroxyl Radical in Flames M. J. COTTEREAU and D. STEPOWSKI Laboratoire de Thermodynamique, L.A. C.N.R.S. N° 230, Facultédes Sciences et des Techniques de Rouen, B.P. 67 76130 Mont-Saint-Aignan, France
One of the main problems met in Laser Induced Fluorescence measurements is the excited population dependence on the quenching due to collisional deexcitation. The saturation mode proposed to avoid this dependence is very difficult to achieve (1) (2) particularly with molecular species and moreover the very strong laser pulses required may cancel the non-perturbing characteristic of the method. Therefore precise knowledge of the quenching is necessary in some experimental circumstances. This paper is devoted to the work we are pursuing at the University of Rouen to study this problem. It consists essentially in using low pressure flames and short duration laser pulses to obtain : 1) direct measurements of the quenching rate in various experimental conditions, 2) direct local concentration measurements in single pulse mode. Until now the work has been performed on OH in flames but it can easily be used for any other species. Theory For molecular species measurement the exciting laser pulse (with a spectral energy density Uυ) is tuned on a rotational line of an electronic transition. If N is the population of the lower vibrational level, the population N of the excited vibrational level increases according to 1
2
dN
