Fundamentals of Raman spectroscopy

Energy transfer actually occurs as a two-photon event  

one photon interacts with the electron cloud of the molecule, inducing a dipole moment and modifying the vibrational energy levels of electrons and then 

provokes emission of a second photon in response to those changes. 

Accordingly, the selection rules for Raman and IR spectroscopy are different. In Raman spectroscopy, there must be a change in the molecule s polarizability upon excitation, whereas a change in dipole moment is required for IR. A dipole moment is the magnitude of the electronic force vector between the negative and positive charges or partial charges on a molecule. A permanent dipole moment exists in all polar mol- 

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A variety of other excellent texts are available for in-depth review of the fundamentals of Raman spectroscopy, including core technologies and applications [2, 3]. This is intended as a very brief, non-rigorous overview for non-spectroscopists who may be unfamiliar with the principles of Raman, its strengths, and practical limitations. For discussion of the experimental details of variant techniques such as ROA (Raman optical activity) or SERS, the reader is directed to the appropriate chapters in this text. 

J. Popp, W. Kiefer, Fundamentals of Raman spectroscopy, in EtKyclopedia of Analytical Chemistry (WUey, New York, 2001)... 

This chapter discusses the use of Raman spectroscopy for analysis of biofluids, specifically blood and urine. After a brief overview of the clinical motivations for analyzing biofluids, the benefits of optical approaches in general and Raman spectroscopy in particular are presented. The core of the chapter is a survey of equipment, data-processing, and calibration options for extracting concentration values from Raman spectra of biofluids or, in the in vivo cases, volumes that include biofluids. The chapter finishes with a discussion of fundamental limits on how accurately concentrations can be determined from Raman measurements and how closely current experiments approach that limit. 

A significant contribution of Raman spectroscopy to the analytical characterization of biomedical issues has been made in the area of biomaterials, especially in the identification of biodegradation and deterioration [1, 2]. The general impact of Raman spectroscopy on the study of biomaterials has been described by this author in three recent review articles [3-5]. In this chapter, the topic of Raman characterization of biomaterials is revisited with particular emphasis placed on those biomaterials widely employed for load-bearing surfaces in artificial joints. Important recent case studies are presented to illustrate the power of the Raman technique to answer key questions of broad medical, scientific, and technological interest. The analytical and physical science lying behind the Raman effect is shown to contribute to the accumulation of a wealth of fundamental information about the medical and technical achievements of prosthesis makers. 

After a short outline of the early history of infrared and Raman spectroscopy (Section 1), a general survey is given of different aspects of vibrational spectroscopy (Section 2). This survey is sufficient for readers who intend to get an impression of the fundamentals of vibrational spectroscopy. It serves as a common basis for subsequent chapters, which de.scribe special experimental features, the theory, and applicational details Section 3, Tools for infrared and Raman Spectroscopy Section 4, Vibrational spectroscopy of different classes and states of compounds Section 5, Evaluation procedures, and Section 6, Special techniques and applications. 

The post-1986 developments that caused the Raman renaissance are mainly technological, but they largely overcome the fundamental problems of a weak Raman signal and interference from fluorescence. To be sure, there were major technical developments preceding 1986, such as photon counting detection and the introduction of the laser, but the more recent technical innovations have been responsible for the transition of Raman spectroscopy from the research lab to the real world. These advances are listed here and discussed in detail in subsequent chapters. 

For a comprehensive overview of Raman spectroscopy fundamentals, theory, and applications, see (a) McCreery, R. L. in Chemical Analysis Vol 157, Wmefordner J. D, Ed. Wiley New York, 2000. (b) Lewis, I. R. Edwards, H. G. M. Handbook of Raman Spectroscopy, Erom the Research Laboratory to the Process Line Marcel Dekker New York, 2001. (c) Pivonka, D. E. Chalmers, J. M. Griffiths, R R. Eds. Applications of Vibrational Spectroscopy in Pharmaceutical Research and Development WUey New York, 2007. (d) Dollish, F. R. Fateley, W. G. Bentley, F. F. Characteristic Raman Frequencies of Organic Compounds Wiley New York, 1974. 

The overriding drawback of Raman spectroscopy is that Raman scatter is fundamentally a weak phenomenon. Resonance Raman spectroscopy and surface enhanced Raman spectroscopy (SERS) are two methods which can be exploited in a spectroelectrochemical experiment to enhance the signal and increase the selectivity of the signal. 

It is difficult to describe the atomic motion in a molecule during a vibration and interpret the meaning of a fundamental vibration until a model or structure is chosen for the molecule. Many molecular structures have been confirmed by infrared and Raman spectroscopy, and we shall describe the methods used for such confirmations in this chapter. While the methods of Raman spectroscopy do not fall within the scope of this work, we shall refer to Raman spectra from time to time. It is sufficient for the reader to know that Raman spectra are similar to infrared spectra in some respects but differ in that the absorption of radiation occurs as a result of a different process. Further comparisons between Raman and infrared spectroscopy will be developed in subsequent sections. 

Note dimer refers to 2,4-diphenylpentane and trimer refers to 2,4,6-triphenylheptane Data from B. Jasse, R.S. Chao andd J.L. Koenig, Journal of Raman Spectroscopy, 1979, 8, 244. Data obtained by Kellar and co-workers [52] i8> > 4> 2 ll fundamental vibrational modes. Herzberg nomenclature 6b> 4 lob i6b> lu )> )> and Vjo = fundamental vibrational modes, Wilson Nomenclature. vibrational assignment not made. On Wilson Nomenclature Vp v - derived from in-plane vibrations of phenyl ring, Vjj, v, and P (CH2j derived from out of plane modes or backbone motions, t = trans form polymorph, g = gauche form polymorph Reprinted with permission from E.J.C. Kellar, C. Galiotis and E.H. Andrews, Macromolecules, 1996, 29, 10, 3515. 1996, ACS [52] ... 

Application of Raman spectroscopy to obtain crystallinity data, on-line, during fiber spininning of polypropylene polymers is described. These data were obtained to develop validated fundamental fiber spinning models. These validated fiber spinning models will be used to guide fiber spinning for rapid product development. 

The application of Raman spectroscopy in the deformation of aramid - thermoplastic elastomeric composites has made it possible to obtain fundamental information about the micromechanics of the fiber / matrix interface, by monitoring the point - to - point variation in stress or strain. 

The planar structure of thiazole (159) implies for the molecule a Cj-type symmetry (Fig. 1-8) and means that all the 18 fundamental vibrations are active in infrared and in Raman spectroscopy. Table 1-22 lists the predictions made on the basis of this symmetry for thiazole. 

Infrared Spectrophotometry. The isotope effect on the vibrational spectmm of D2O makes infrared spectrophotometry the method of choice for deuterium analysis. It is as rapid as mass spectrometry, does not suffer from memory effects, and requites less expensive laboratory equipment. Measurement at either the O—H fundamental vibration at 2.94 p.m (O—H) or 3.82 p.m (O—D) can be used. This method is equally appticable to low concentrations of D2O in H2O, or the reverse (86,87). Absorption in the near infrared can also be used (88,89) and this procedure is particularly useful (see Infrared and raman spectroscopy Spectroscopy). The D/H ratio in the nonexchangeable positions in organic compounds can be determined by a combination of exchange and spectrophotometric methods (90). 

Band gaps in semiconductors can be investigated by other optical methods, such as photoluminescence, cathodoluminescence, photoluminescence excitation spectroscopy, absorption, spectral ellipsometry, photocurrent spectroscopy, and resonant Raman spectroscopy. Photoluminescence and cathodoluminescence involve an emission process and hence can be used to evaluate only features near the fundamental band gap. The other methods are related to the absorption process or its derivative (resonant Raman scattering). Most of these methods require cryogenic temperatures. 

Raman spectroscopy can in principle be applied to this problem in much the same manner as infrared spectroscopy. The primary difference is that the selection rules are not the same as for the infrared. In a number of molecules, frequencies have been assigned to combinations or overtones of the fundamental frequency of the... 

There are, at present, two overriding reasons an experimentalist would choose to employ laser Raman spectroscopy as a means of studying adsorbed molecules on oxide surfaces. Firstly, the weakness of the typical oxide spectrum permits the adsorbate spectrum to be obtained over the complete fundamental vibrational region (200 to 4000 cm-1). Secondly, the technique of laser Raman spectroscopy is an inherently sensitive method for studying the vibrations of symmetrical molecules. In the following sections, we will discuss spectra of pyridine on silica and other surfaces to illustrate an application of the first type and spectra of various symmetrical adsorbate molecules to illustrate the second. 

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