Resonance Raman spectroscopy wavelength selection

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. 

Lasers that have wavelengths in the UV and visible regions of the spectrum are used for resonance Raman spectroscopy. Tunable dye lasers are often used these lasers can be set to a selected wavelength over the UV/VIS range of 200-800 nm. This permits maximum flexibility in the choice of excitation wavelength. 

Resonance Raman spectroscopy (RRS) leads to increased selectivity in Raman spectral measurements. The Raman spectrum of individual components in a complex mixture can be selectively enhanced by a judicious choice of laser wavelength. Only the Raman bands of the chromophore which is in resonance at the wavelength of excitation are significantly enhanced. Raman bands of non-absorbing species are not enhanced and do not interfere with those of the chromophore. Clearly, resonance Raman is a very sensitive analytical tool capable of providing detailed molecular vibrational information. 

The choice of excitation laser is essential for DUV resonance Raman spectroscopy. With a wavelength-changeable UV laser, molecularly selective resonance Raman microscopy could be realized. The second harmonics of an argon ion laser offers several emission lines in the DUV range (257, 244, 238, 229 nm) that are suitable for wavelength-selective resonance Raman spectroscopy. Solid-state DUV lasers are also available based on harmonic generation with infrared lasers, typically... 

The selection rules for CARS and CSRS are the same as for standard, spontaneous Raman spectroscopy however, it has the advantage of a vastly increased intensity. Experimentally, CARS is realized by using one fixed-wavelength laser and then mning the second one into resonance with the ro-vibrational levels of the target molecule. 

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