ABSTRACT
A nonlinear optical spectroscopy based on degenerate four-wave mixing has made possible direct measurements of species temperature and concentration profiles through the boundary layer of a reactive plasma at atmospheric pressure. Spectra were obtained for CH and C(2) radicals over a range of conditions including those for the plasma chemical vapor deposition of diamond films. Numerical simulations based on a one-dimensional stagnation-point flow model are in good agreement with the measurements. The CH mole fraction is shown to rise and fall as a function of distance from the substrate, which is compelling experimental evidence for the complex chemistry that is occurring in the plasma boundary layer.
ABSTRACT
An optical system employing a tunable carbon dioxide laser has been used to investigate backscatter signatures of aerosols as a function of wavelength. Submicron sulfuric acid or ammonium sulfate aerosols are produced with a vapor-condensation aerosol generator. The aerosol is contained in a 1-m long windowless aerosol chamber, and laser radiation backscattered from the irradiated aerosol is collected and measured as the laser is tuned from 9.2 to 10.8 microm. The volume backscatter coefficient is calculated from the lidar equation to yield the theoretical IR spectrum of the aerosol. The measured spectral signature is compared with the theoretical signature, which is computed from Mie theory. Backscatter signatures show excellent agreement with calculated signatures. The spectral signature of ammonium sulfate is readily distinguished from that of sulfuric acid for the conditions of the experiment. Because of vapor pressure characteristics of sulfuric acid, it is possible to concentrate the acid in the generator over time and look for a change in the acid concentration in the aerosol. Not only has this concentration process been observed optically, but under these experimental conditions the acid concentration in the aerosol can be determined by observing backscatter at just two wavelengths.
