Three-colour CARS spectroscopy of the OH radical at triple resonance

Abstract

A coherent anti-Stokes Raman-scattering (CARS) study of the OH radical is conducted using resonance enhancement from the A²σ⁺-X²Π transition. A review of previous work on resonance-enhanced CARS is first given. The OH radical has specific one-photon spectral properties that lead to original CARS features; main and satellite CARS processes are thus defined at resonance. The use of resonance-enhanced CARS for the detection of OH is discussed. Optimum detectivity is obtained at triple resonance. Triple resonance can be nearly achieved using only two lasers on satellite CARS lines, but the best results are obtained on the main CARS lines using three lasers. This is verified experimentally in a discharge and in a one-atmosphere flat flame. Extra resonances from the excited electronic-state vibration are clearly seen. The results indicate that detection of OH in flames at high pressures should be easily feasible.