Electronically resonant CARS detection of OH

Abstract

CARS (coherent anti-Stokes Raman spectroscopy) is a nonlinear spectroscopic technique capable of making remote, highly accurate measurements of temperature and concentration which are both temporally and spatially precise, in extremely difficult environments such as internal combustion engines and gas turbine exhausts. A major disadvantage of CARS methods is that application is limited to those species whose concentration is 1 % or greater. In order to extend CARS detectivity, electronically resonantly enhanced CARS can be employed, whereby one of the CARS laser frequencies is selected to be resonant with an electronic transition. This results in a large signal enhancement (possibly several orders of magnitude). This paper reports the results of applying resonant CARS to the detection of the hydroxyl radical in a methane/oxygen flame. Both the theory of electronically resonant CARS, predicted spectra, and the experimental procedure are described in detail. The variation of the resonant CARS spectra with different choices of electronic resonance is shown. A preliminary demonstration of saturation is also illustrated, as is the correlation of the resonant CARS spectrum with dependence of OH concentration on flame height.