Cavity ring down spectroscopy measurements of absolute CN concentrations during flame deposition of diamond

Cavity ring down spectroscopy (CRDS) was used for measuring absolute concentration profiles of the CN (cyano) radical during oxyacetylene flame deposition of diamond. Profiles were measured for three different nitrogen additions to the flame. Novel ways for the determination of the lateral position of the laser beam and its height above the deposition substrate are presented. Measured column density profiles show that the influence of added nitrogen is limited to a central area within a radius of 2.7 mm. Outside this area the profiles are hardly influenced by the added nitrogen flow. Comparison with work of S. J. Firchow and K. L. Menningen [J. Phys. D: Appl. Phys. 32, 937 (1999)] shows a good correspondence. Absolute concentration profiles were obtained by applying Abel inversion processing and using a temperature distribution resulting from numerical simulations. The resulting profiles show an off-axis maximum for all three nitrogen additions. With an increasing nitrogen addition the maximum and central concentrations also increased. Upon inspection of the profiles it follows that the gas phase above the deposition area can be divided into three sections: a central one in which the added nitrogen is the dominant nitrogen source, an outer one where the nitrogen from the ambient plays a dominant role, and an intermediate zone where both nitrogen sources have a significant influence. The latter is the area in which the maximum of the CN concentration is situated. Previous laser-induced fluorescence (LIF) profiles of CN, measured under very similar experimental conditions, show similar distributions. Differences between the CRDS and LIF results are discussed. (C) 2002 American Institute of Physics.