Development of hybrid fs/ps CARS for quantitative detection of molecular nonequilibrium and concentration imaging of Cl and O radicals*

Hybrid fs/ps CARS has been recently developed for plasma applications as a highly accurate and time-resolved detection method for instantaneous vibrational and rotational temperatures of plasma constituents, particularly N₂.  Here, we report on recent progress in our group for improving detection of molecular nonequilibrium and for extending the hybrid CARS approach for imaging of the time-resolved concentration of Cl and O radicals.  Virtually imaged phased array (VIPA) resolved hybrid CARS measurements are demonstrated as a highly sensitive and widely applicable method for detecting molecular nonequilibrium among rotational and vibrational degrees of freedom.  One of the clear advantages of this approach is the multiplex nature of probing the low-frequency transition region of the Raman spectrum.  Plasma-generated radicals, such as Cl, F, and O, are important for plasma-enhanced chemical vapor deposition and etching processes in the semi-conductor fabrication industry.  We probe the possibility of using such multiplex CARS measurements to drive transitions in electronic spin-orbit split ground states of Cl, F, and O for coherent detection.  Further, we take advantage of CARS imaging approaches developed in our lab to acquire the spatial distribution of target molecules normal to a surface.  Time-resolved data allows determination of spectroscopic constants important to the modeling and quantification of measured species.