Knowledge Bank

The high relative intensity of Rayleigh scatter historically has hindered the observation of transition with small Raman shifts. Recently, there has been great interest in pure rotational Raman as a diagnostic technique. This interest was primarily the result of the realization that such an experiment would permit one to (theoretically) obtain temperature and number densities for all species of interest with a single shot of a single laser dye. Unfortunately, pure rotational Raman suffers from the classic difficulty of scattered exciting radiation. By judiciously choosing either polarizations or phase matching arrangement, or both, CARS researchers have overcome this problem to large measure. The current state-of-the-art CARS spectrometer can easily observe to below $5cm-1$. Although developed for pure rotational measurements, these instruments are also well suited to low frequency vibrational studies. This paper will focus on the instruments requirement of low frequency Raman spectroscopys and results will be presented illuminate the merits of this approach.