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Measurement of Molecular Excitation Spectra by Laser Raman Effect ABSTRACT The phenomenon known as the Raman effect allows us to probe the energy levels intrinsic to a given molecule, giving us knowledge about its rotational and vibrational energies. By illuminating a transparent sample, either liquid, gas, or crystal, with an intense source of light, (preferably monochromatic or at least with sufficiently well- known and distinct spectral lines), inelastic scattering of electromagnetic radiation can be measured perpendicular to the beam line to obtain a spectrum which yields information about the energy levels of the molecule being studied. I. INTRODUCTION In 1921, with the foundations of quantum mechanics just being laid, Prof. C. V. Raman began a series of experiments to observe the scattering of light by transparent media. Although such studies were certainly not new as scientists and laymen alike had been studying transparent scattering for centuries (most notable among these researchers were perhaps Sir Isaac Newton and Christiaan Huygens), the depth and breadth of his research led Prof. Raman to observe a very weak secondary radiation scattered from transparent liquids, where the wavelengths were different from those already known. What is perhaps most remarkable about this observation is that it was made with sunlight as the illuminating source. At a meeting of the South Indian Science Association at Bangalore on March 16, 1928, Raman made the first public announcement of the phenomenon, showing his spectrum of carbon tetrachloride [1]. He showed that the frequency shifts, the relative intensities, and the state of polarization, among other features of the new spectral lines were independent of the exciting radiation. Thus, this new scattering method of investigation, which in many ways complemented infrared spectroscopy, revealed an amazingly easy and convenient way of mapping the vibrational and rotational spectra of chemical compounds. In the same year, P. Pringsheim [2] labeled this new scattering phenomenon the Raman effect and therefore the spectrum of new lines, the Raman spectrum. A more detailed account of the historical background and subsequent applications of Raman spectroscopy can be found in the text by Anderson [3]. In the results to follow, it will be our goal to successfully use the technique of Raman spectroscopy to measure the excitation spectra of several chemicals, most notably carbon tetrachloride CCl4, dichloromethane CH2Cl2, and chloroform CHCl3.
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