Final Report: Investigation of Saturated Degenerate Four-Wave Mixing Spectroscopy For Quantitative Concentration Measurements

Abstract

Principal Investigator: Robert P. Lucht (rlucht @ tamu.edu) Degenerate four-wave mixing (DFWM) and polarization spectroscopy (PS) are techniques that shows great promise for sensitive measurements of transient gas-phase species, and diagnostic applications of DFWM are being pursued actively at laboratories throughout the world. However, significant questions remain regarding strategies for quantitative concentration measurements using these techniques. The primary objective of this research program is to develop and test strategies for quantitative concentration measurements in flames and plasmas. Theoretically, we are investigating the physics of the PS and DFWM processes by direct numerical integration (DNI) of the time-dependent density matrix equations that describe the four-wave mixing interaction. Significantly fewer restrictive assumptions are required when the density matrix equations are solved using this DNI approach compared with the assumptions required to obtain analytical solutions. For example, the Zeeman structure of the resonance and effects such as Doppler broadening can be included, both the pump and probe beams can be saturating in the DNI calculations. As computer processing speeds have increased, we have incorporated more complicated physical models into our DNI codes.The incorporation of the Zeeman state structure of the laser-coupled energy levels has played a very significFnt role in our theoretical efforts over the last three years. For example, it has enabled us to develop theoretical models of the PS process. During the last three years of the grant we (1) developed a computer code for single-mode PS and investigated the effects of collisions and Doppler broadening , (2) investigated the effects of multi-axial-mode laser radiation on PS signal generation, (3) performed an extensive investigation of saturation effects in DFWM for different polarization configurations, (4) began a numerical study of picosecond PS. The incorporation of the Zeeman state structure in our DNI codes has given us a very powerful tool for the investigation of molecular spectroscopy, and much of our proposed future work is based upon this capability. Our diagnostic experiments are performed in wellcharacterized flames to assess the potential of the techniques for quantitative concentration measurements, and for meaningful comparison with our theoretical results.