Development and Evaluation of Coherent Raman Spectroscopy in the Condensed Phase.

Abstract

This work has been concerned with the development and evaluation of two coherent Raman techniques: coherent anti-Stokes Raman spectroscopy and ac-coupled inverse Raman spectroscopy. Initial effort was directed toward the construction of a suitable optical apparatus and data acquisition and processing system for coherent Raman experiments. A novel beam angle adjustment system for coherent anti-Stokes Raman spectroscopy is described. Also, a detailed description of a microprocessor-controlled data acquisition system for pulsed laser spectroscopy is given. A summary of the basic theory behind the nonlinear response of matter to intense electric fields is presented. From this theory, the generation of coherent anti-Stokes Raman spectroscopy (CARS) is derived in order to describe its drawbacks and unusual effects. Particular attention is paid to the aqueous solution detection limits of CARS, which are about 100 times higher than those obtained by spontaneous Raman. Also described is the development of a new coherent Raman technique called ac-coupled inverse Raman spectroscopy. This technique is shown to give spectra which are exactly analogous to spontaneous Raman spectra. Suitable experimental conditions for this technique are discussed and aqueous solution detection limits only 5 times higher than those for spontaneous Raman spectroscopy are demonstrated. Theoretical calculations indicate that detection limits 50 times lower than spontaneous Raman limits may be achieved using refined procedures and apparatus. At present, interference from argon ion laser power supply noise, particulate matter in the sample, and thermal blooming effects prevent attainment of theoretical detection limits. Competition from stimulated electronic effects intereferes with the resonance-enhanced ac-coupled inverse Raman process. This problem limits experimental conditions to those which keep both lasers from simultaneously being resonant with the electronic transition.