Raman effect, fundamentals

A similar calculation will show that the stimulated Raman effect applied to frequency tripled radiation from a Nd YAG laser, with a fundamental wavelength of 1064.8 nm, produces wavelengths of 299 nm, with H2, and 289 nm, with H2. 

When an incident beam of radiation of frequency y falls on a molecule, some radiation is scattered and in this scattered radiation we get, as well as v, frequencies y where vp is a fundamental frequency. This is called the Raman effect and when a fundamental frequency appears in the Raman spectrum it is said to be Raman active. 

Polymers, with their highly stereoregular structures, are frequently of sufficiently high symmetry for infrared spectroscopy to give only an incomplete picture of the vibrational characteristics of the compounds. In some, as many as half of the fundamental modes are infrared inactive. These non-absorbing modes can frequently be observed in the Raman effect (e.g. polyethylene where mutual exclusion" applies and at least eight modes are Raman active and infrared silent ). 

The normal Raman spectrum obtained with 647.1 nm excitation serves as a comparison for the Raman spectra obtained with excitation frequencies of 488.0 and 514.5 nm, which lie within the 5- 5 absorption band. The tremendous enhancement of the i>,(Mo-Mo) alg mode, the high overtone progression in v, the increase in overtone bandwidth with increasing vibrational quantum number, and the increased intensity of the overtones relative to the fundamental as the excitation frequency approaches the electronic absorption maximum are all attributable to the resonance Raman effect. Polarization... 

Progress in the Raman spectroscopic study of carbohydrates became possible during the past few years owing to the introduction of laser sources. Before discussing the results of laser-Raman spectroscopy applied to carbohydrates, we shall give a brief recapitulation of the physical principles of the Raman effect. Experimental techniques of infrared spectroscopy have been described in previous reviews,116,17 but no such description has been given for the Raman method. That is why the Description Section, which follows, will include the physical fundamentals of the method, as well as the sampling techniques. 

Not long after the discovery of the stimulated Raman effect in liquids 63> it was also detected in single crystals 64), namely diamond, calcite, and a-sulfur. Only much later could it be shown that the effect can also be observed in crystal powders 651. The stimulated Raman effect 99 > is excited by giant-pulse lasers with a power of several MW. The strongest Raman lines of a substance are amplified until their intensity is of the same order of magnitude as that of the exciting line furthermore second, third, etc. Stokes lines of the fundamentals in question are observed with twice, thrice, etc. the frequency shift. 

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. 

The fundamentals of the Raman effect can be understood by consideration of a classical model, in which an incident beam of radiation (i.e., laser beam, for all practical purposes, in flame diagnostics) passes through an ensemble of molecules. The resultant laser beam electric field distorts the electronic cloud distribution of each molecule, causing oscillating dipoles these induced dipoles are related to the incident laser beam electric field by the molecular polarizability. The dipoles, in turn, produce a secondary radiating field at essentially the same frequency as that for the incident beam. This radiation is termed Rayleigh scattering. 

One of these methods was the vibrational spectroscopy which had its roots in the late 1920 s and early 1930 s. One of them was the fundamental understanding of molecular vibrations on the basis of quantum mechanics it was first put in evidence by the absorption of infrared radiation and later also found in the modulations of scattered visible light in the Raman effect. The two... 

For example, in Ds , it was found in Sec. 7-6 that the a s have the species 2A[ E E". Consequently, for molecules of this symmetry, such as cyclopropane, only those vibration frequencies with species A, E, or E" should occur as fundamentals in the Raman effect. Since the tables in Appendix X indicate the irreducible representations for and for a, it is possible to tell very quickly for any molecule how many fundamental freciuencies should be allowed in the infrared and how many in the Raman spectrum. 

One of these methods is the vibrational spectroscopy which has its roots in the late 1920 s and early 1930 s. One of them was the fundamental understanding of molecular vibrations on the basis of quantum mechanics it was first put in evidence by the absorption of infrared radiation and later also found in the modulations of scattered visible light in the Raman effect. The two methods complement each other dramatically because of their different response to the selection rules which control the transition probabilities between different vibrational states of a molecular framework. The other root was a gradual and substantial improvement of the experimental techniques, such as stronger and more uniform sources of the primary radiations, higher resolution in the spectroscopic part of the equipment, and, perhaps most of all, more sensitive and reliable receivers. 

---