Molecular vibrations Raman scattering

Complementary to other methods that constimte a basis for the investigation of molecular dynamics (Raman scattering, infrared absorption, and neutron scattering), NIS is a site- and isotope-selective technique. It yields the partial density of vibrational states (PDOS). The word partial refers to the selection of molecular vibrations in which the Mossbauer isotope takes part. The first NIS measurements were performed in 1995 to constitute the method and to investigate the PDOS of... 

For a limited number of metal surfaces, adsorption of a molecular species in a thin (monomolecular layer) film results in a huge increase in the effective vibrational Raman scattering cross-section (again, as with RR scattering, up to ca. 106 times) of the adsorbate species. The SERS effect was discovered more than ten years ago for pyridine adsorbed at a silver electrode surface in contact with an aqueous electrolyte [1, 2]. In the intervening period, many hundreds of papers devoted to SERS phenomena have been published, extending the studies to other metals than silver, to non-aqueous as well as aqueous electrolytes, to colloidal dispersions of metals as well as metal electrodes, and even to vacuum-deposited thin film systems under UHV conditions. This review will concentrate on studies of metal-electrolyte interfaces. 

Chandrasekhar Venkata Raman (1888-1970), Indian physicist, professor at the University of Calcutta and at the Indian Scientific Institute in Bangalore. In 1928 Raman discovered light scattering that is accompanied by a change of frequency (by frequency of the molecular vibrations). Raman received the Nobel prize in 1930 for his work on the scattering ofBgfit and for the discovery of the effect named after him . 

Principles and Characteristics As already indicated in Chp. 1.2.3, Raman scattering induced by radiation (UV/VIS/NIR lasers) in gas, liquid or solid samples contains information about molecular vibrations. Raman specfioscopy (RS) was restricted for a long time primarily to academic research and was a technique rarely used outside the research laboratory. Within an industrial spectroscopy laboratory, two of the more significant advances in recent years have been the allying of FT-Raman and FTIR capabilities, coupled with the availability of multivariate data analysis software. Raman process control (in-line, on-line, in situ, onsite) is now taking off with various robust commercial instrumental systems equipped with stable laser sources, stable and sensitive CCD detectors, inexpensive fibre optics, etc. With easy interfacing with process streams and easy multiplexing with normal (remote) spectrometers the technique is expected to have impact on product and process quality. 

In particular, if a molecular vibration causes a change in the molecular polarizability, the scattered radiation will contain the sum (and difference) of the incident frequency and the molecular vibrational frequency This is the vibrational Raman scattering, which we will consider in the following. 

Conventional spontaneous Raman scattering is the oldest and most widely used of the Raman based spectroscopic methods. It has served as a standard teclmique for the study of molecular vibrational and rotational levels in gases, and for both intra- and inter-molecular excitations in liquids and solids. (For example, a high resolution study of the vibrons and phonons at low temperatures in crystalline benzene has just appeared [38].)... 

Ulness D J, Stimson M J, Kirkwood J C and Albrecht A C 1997 Interferometric downconversion of high frequency molecular vibrations with time-frequency-resolved coherent Raman scattering using quasi-cw noisy laser light C-H stretching modes of chloroform and benzene J. Rhys. Chem. A 101 4587-91... 

Chang T-C and DIott D D 1988 Picosecond vibrational cooling in mixed molecular crystals studied with a new coherent Raman scattering technique Chem. Phys. Lett. 147 18-24... 

The selection mles for molecular vibrations involved in hyper Raman scattering are summarized by... 

Laser Raman Microprobe. A more sophisticated microscope is the Laser Raman Microprobe, sometimes referred to as MOLE (the molecular orbital laser examiner). This instmment is designed around a light microscope to yield a Raman spectmm (45) on selected areas or particles, often <1 ia volume. The data are related, at least distantly, to iafrared absorption, siace the difference between the frequency of the exciting laser and the observed Raman frequency is the frequency of one of the IR absorption peaks. Both, however, result from rotational and vibrational states. Unfortunately, strong IR absorption bands are weak Raman scatterers and vice versa hence there is no exact correspondence between the two. 

Raman spectroscopy detects the scattering of light, not its absorption. Superposed on the frequency of the scattered light are the frequencies of the molecular vibrations. The detection occurs in the IR spectral region while the excitation happens in the visible region. Since laser light sources have become well developed, Raman spectroscopy has become an important tool for the analysis of biomolecules. 

The interactions of photons with molecules are described by molecular cross-sections. For IR spectroscopy the cross-section is some two orders of magnitude smaller with respect to UV or fluorescence spectroscopy but about 10 orders of magnitude bigger than for Raman scattering. The peaks in IR spectra represent the excitation of vibrational modes of the molecules in the sample and thus are associated with the various chemical bonds and functional groups present in the molecules. The frequencies of the characteristic absorption bands lie within a relatively narrow range, almost independent of the composition of the rest of the molecule. The relative constancy of these group frequencies allows determination of the characteristic... 

In crystalline solids, the Raman effect deals with phonons instead of molecular vibration, and it depends upon the crystal symmetry whether a phonon is Raman active or not. For each class of crystal symmetry it is possible to calculate which phonons are Raman active for a given direction of the incident and scattered light with respect to the crystallographic axes of the specimen. A table has been derived (Loudon, 1964, 1965) which presents the form of the scattering tensor for each of the 32 crystal classes, which is particularly useful in the interpretation of the Raman spectra of crystalline samples. 

Raman spectroscopy is an inelastic light scattering experiment for which the intensity depends on the amplitude of the polarizability variation associated with the molecular vibration under consideration. The polarizability variation is represented by a second-rank tensor, oiXyZ, the Raman tensor. Information about orientation arises because the intensity of the scattered light depends on the orientation of the Raman tensor with respect to the polarization directions of the electric fields of the incident and scattered light. Like IR spectroscopy, Raman... 

Raman spectroscopy is primarily useful as a diagnostic, inasmuch as the vibrational Raman spectrum is directly related to molecular structure and bonding. The major development since 1965 in spontaneous, c.w. Raman spectroscopy has been the observation and exploitation by chemists of the resonance Raman effect. This advance, pioneered in chemical applications by Long and Loehr (15a) and by Spiro and Strekas (15b), overcomes the inherently feeble nature of normal (nonresonant) Raman scattering and allows observation of Raman spectra of dilute chemical systems. Because the observation of the resonance effect requires selection of a laser wavelength at or near an electronic transition of the sample, developments in resonance Raman spectroscopy have closely paralleled the increasing availability of widely tunable and line-selectable lasers. 

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