Cars Diagnostic on a Photochemical Reactor for IR Laser Induced Production of Si and Si3N4 Powders

In a flow reactor a low power (up to 50 W) CW CO 2 laser tuned at 944,19 cm −1 has been focussed in orderto produce Si and Si 3 N 4 ultrafine powders from SiH 4 and SiH 4 /NH 3 mixtures. Among possible on-line optical diagnostics, two different CARS techniques have been used to monitorthe excitation process and to measure average reaction temperatures in collinear geometry. In broad-bandCARS at low resolution (≈6.0 cm −1 ) the reactant temperature is measured from the attenuation of thecorresponding integrated peak intensity below and at the dissociation threshold. In narrow-bandexperiments the temperature reached by the dissociating reactants below and above the threshold isinferred from the spectral shape (measured with 0.2 cm −1 resolution) of the envelope of rovibrationalCARS transitions involved. Results obtained at the threshold for SiH 4 dissociation are in agreement with previous data on gas-phasepyrolysis in a thermal process. For the SiH 4 /NH 3 reaction the difficulty in obtaining stoichiometric Si 3 N 4 has been related to the cooling effect of large NH 3 addition to the SiH 4 warmed up in the laser absorption.