Light sources, Raman

The light source must be highly monocln-omatic so that the Raman scattering occurs at a well-defined... 

Raman spectroscopy requires an intense, monocliromatic light source. The field thus developed rapidly when lasers... 

For the visible and near-ultraviolet portions of the spectmm, tunable dye lasers have commonly been used as the light source, although they are being replaced in many appHcation by tunable soHd-state lasers, eg, titanium-doped sapphire. Optical parametric oscillators are also developing as useful spectroscopic sources. In the infrared, tunable laser semiconductor diodes have been employed. The tunable diode lasers which contain lead salts have been employed for remote monitoring of poUutant species. Needs for infrared spectroscopy provide an impetus for continued development of tunable infrared lasers (see Infrared technology and RAMAN spectroscopy). 

Raman spectroscopy, long used for quaHtative analysis, has been revitalized by the availabiHty of laser sources. Raman spectroscopy is based on scattering of light with an accompanying shift in frequency. The amount by which the frequency is shifted is characteristic of the molecules that cause the scattering. Hence, measurement of the frequency shift can lead to identification of the material. 

Because the Raman cross-section of molecules is usually low, intense light sources and low-noise detectors must be used, and high sensitivities - as required for surface analysis - are difficult to achieve. Different approaches, singly and in combination, enable the detection of Raman spectroscopy bands from surfaces. 

The first Raman and infrared studies on orthorhombic sulfur date back to the 1930s. The older literature has been reviewed before [78, 92-94]. Only after the normal coordinate treatment of the Sg molecule by Scott et al. [78] was it possible to improve the earlier assignments, especially of the lattice vibrations and crystal components of the intramolecular vibrations. In addition, two technical achievements stimulated the efforts in vibrational spectroscopy since late 1960s the invention of the laser as an intense monochromatic light source for Raman spectroscopy and the development of Fourier transform interferometry in infrared spectroscopy. Both techniques allowed to record vibrational spectra of higher resolution and to detect bands of lower intensity. 

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. 

In 1994, we proposed that a metallic needle having a nano-tip at its apex be employed as a nano-light-source for microscopy attaining nanometric spatial resolution [2]. Later, we expanded the technique to Raman spectroscopy for molecular nano-identification, nano-analysis and nano-imaging. In this chapter, we give a brief introduction to local plasmons and microscopy using a metallic nano-needle to produce the local plasmons. Then, we describe the microscope that we built and... 

A nano-light-source generated on the metallic nano-tip induces a variety of optical phenomena in a nano-volume. Hence, nano-analysis, nano-identification and nanoimaging are achieved by combining the near-field technique with many kinds of spectroscopy. The use of a metallic nano-tip applied to nanoscale spectroscopy, for example, Raman spectroscopy [9], two-photon fluorescence spectroscopy [13] and infrared absorption spectroscopy [14], was reported in 1999. We have incorporated Raman spectroscopy with tip-enhanced near-field microscopy for the direct observation of molecules. In this section, we will give a brief introduction to Raman spectroscopy and demonstrate our experimental nano-Raman spectroscopy and imaging results. Furthermore, we will describe the improvement of spatial resolution... 

Similar to IR sensors, Raman sensors have profited from miniaturisation and improvement of light sources and optics. Essentially, a Raman sensor consists of (i) a monochromatic source, a (ii) sensor head, a (iii) filter separating the Raman lines from the excitation radiation and Rayleigh scattering and a (iv) spectral analyser. 

Raman (R) and resonance Raman (RR) spectroscopy detects vibrational modes involving a change in polarizability. For RR, enhancement of modes is coupled with electronic transition excited by a laser light source. This technique is complementary to IR and is used for detection of v(O-O) and v(M-0), especially in metalloproteins. In porphyrins, one may identify oxidation and spin states. 

Raman and resonance Raman (RR) measurements of fullerene-like particles of MoS2 recently have been carried out (93). By using 488-nm excitation from an Ar ion laser light source, the two strongest Raman features in the Raman spectrum of the crystalline particles, at 383 and 408 cm-1, which correspond to the E g and Aig modes, respectively (see Table I), were found to be dominant also in IF-MoS2... 

Raman scattering are generally different so information from Raman experiments in general supplements information that is obtainable from infrared absorption. Using lasers as light sources can greatly reduce experimental difficulties in this technique. At present it has become one of the standard tools for polymer analysis. 

J. P. Dakin, D. J. Pratt, G. W. Bibby, and J. N. Ross, Distributed optical fibre Raman temperature sensor using a semiconductor light source and detector, Electron. Lett. 21, 569 (1985). 

The lower absorptivities and stronger light sources allow for deeper penetration into the process broth centimeters vs. millimeters. NIR was also the first spectroscopic technique used for process control by large numbers of chemical manufacturers. As a consequence, more equipment and expertise are available for NIR applications than for MIR or Raman. 

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