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The advent of the laser has unlocked several new Raman processes which have providednew approaches to many important diagnostic problems. These applications include biochem-istry, high resolution spectroscopy, combustion engineering, structural chemical analysis,chemical kinetics, surface chemistry, etc. Included in these techniques are: CoherentAnti - Stokes Raman Spectroscopy (CARS), Raman Induced Kerr Effect (RIKES), and StimulatedRaman Gain. A brief review of these methods and their utility are described with the mainemphasis on CARS. Variations of the techniques, e.g., BOXCARS, are also discussed andtheir use in making temperature and concentration measurements in hostile environments suchas turbulent media are presented.IntroductionSir C. V. Raman discovered the effect that bears his name.' For that discovery in1928 Raman received the Nobel Prize in Physics. The Raman effect has been well -known as apowerful analytical technique for the chemist and physicist. With the advent of the laserin 1960 there came a renaissance in Raman spectroscopy, as these new light sources had manyadvantages over conventional, incoherent lamps. However, physicists were then finding newRaman phenomena that could only arise from the enormous electric field strengths and pro-perties previously unavailable from conventional sources. The first of these new Ramanphenomena to be observed was the stimulated Raman effect, where electric field strengthsbecome so great that strongly allowed Raman transitions are driven by high gains3to emitstimulated coherent, laser -like emission, Stokes- shifted by the Raman frequency. Almostsimultaneously Terhune and Maker and Terhune discovered that by three- or four -wave mix-ing, stimulated Raman emissions coupled with the original laser light to produce coherentanti - Stokes output as well as many higher -order effects. It was this discovery that led tothe early foundations of a new kind of Raman spectroscopy, now called CARS or coherentanti - Stokes Raman spectroscopy.6 Of all the nonconventional Raman spectroscopies, CARShas probably received the most attention over the past few years and consequently thispaper treats this topic extensively.Soon after three- or four -wave mixing phenomena were discovered, Jones and Stoicheff7introduced another kind of Raman spectroscopy, called the inverse Raman effect, whichformed thR basis for the stimulated Raman gain process° and for photoacoustic Raman spec-troscopy.The list of new Raman spectroscopies that hâve resulted from the interaction of laerlight and matter seems enormous. Hyper- Raman1° and Raman - induced Kerr effect or RIKES'' |
