Resonance Raman spectroscopy excitation sources

Fig. 3. Diagram of continuous wave (cw) laser sources suitable for metalloprotein resonance Raman spectroscopy. The best quality spectra are provided by Ar, Kr, He-Ne, and He-Cd lasers operating at fixed frequencies (the lengths of the lines indicate the relative output for a given laser) throughout the visible and near-UV region. An intracavity frequency-doubled (ICFD) Ar laser has been developed with five useful cw excitation wavelengths in the far-UV region (257, 248, 244, 238, and 228.9 nm). The high-powered Ar and Kr lasers can also be used to pump dye lasers which are tunable between the near-UV and near-IR region. The cw Nd YAG laser with a fundamental at 1064 nm is the primary excitation source in FT Raman spectrometers.

Resonance Raman spectroscopy combines both vibrational and electronic spectroscopies. The vibrational spectrum at a particular excitation wavelength provides the first dimension. The excitation spectrum, the intensity of each vibrational band as a function of the excitation wavelength, provides the second dimension. Since most molecules have resonance enhancement in the UV, this approach is quite general, but not universal. The availability of a range of excitation frequencies from the laser source makes exploitation of this form of Raman scattering possible. 

Figure 14.24 Schematic illustration of the concept of Raman excitation, (a) Conventional Raman spectroscopy excitation to a virtual state leads to reemission of a photon of the same frequency as the excitation source, Rayleigh scattering, or inelastic scattering where the reemitted photon has greater (Anti-Stokes) or less energy (Stokes) than the incident photon, (h) Resonance Raman conditions the frequency of the incident photon matches or is close to the energy of an optical absorbance, resulting in scatter which originates from an excited electronic state.

Figure 1 (A) Afibre optic spectroscopy system with separate illumination and collection path is based on an excitation source, which is a laser or a white light source (reflectometry) or a monochromator filtered arc lamp (fluorescence). Optics couple the excitation light into the flexible probe. A probe collects the emitted light. Coupling optics adapt the numerical aperture of the probe to the spectrograph or filter system. An optical detector (charge coupled device (CCD), photodiode array, photomultiplier tube) is read out and digitized. (B) A fibre optic spectroscopy system with a probe that incorporates one optical fibre needs a dichroic beam splitter and well aligned optics to separate excitation and fluorescence light. Reproduced with permission of Optical Society of America Inc. from Greek LS, Schulze HG, Blades MW, Haynes CA, Klein K-F and Turner RFB (1998) Fiber-optic probes with improved excitation and collection efficiency for deep-UV Raman and resonance Raman spectroscopy. Applied Optics Z7 ).

The first laser Raman spectra were inherently time-resolved (although no dynamical processes were actually studied) by virtue of the pulsed excitation source (ruby laser) and the simultaneous detection of all Raman frequencies by photographic spectroscopy. The advent of the scanning double monochromator, while a great advance for c.w. spectroscopy, spelled the temporary end of time resolution in Raman spectroscopy. The time-resolved techniques began to be revitalized in 1968 when Bridoux and Delhaye (16) adapted television detectors (analogous to, but faster, more convenient, and more sensitive than, photographic film) to Raman spectroscopy. The advent of the resonance Raman effect provided the sensitivity required to detect the Raman spectra of intrinsically dilute, short-lived chemical species. The development of time-resolved resonance Raman (TR ) techniques (17) in our laboratories and by others (18) has led to the routine TR observation of nanosecond-lived transients (19) and isolated observations of picosecond-timescale events by TR (20-22). A specific example of a TR study will be discussed in a later section. 

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. 

The most conventional excitation source for Raman spectroscopy, a 514-nm Ar-ion laser, is known to cause a strong fluorescence during the analysis of ND samples. Compared to visible Raman, UV-Raman analysis offers a stronger diamond signal due to the resonance enhancement effect [85]. It is therefore preferred to use UV (244 and 325 nm) excitations for the analysis of ND powders. 

In this contribution we present two laser spectroscopic methods that use coherent resonance Raman scattering to detect rf-or laser -induced Hertzian coherence phenomena in the gas phase these novel coherent double resonance techniques for optical heterodyne detection of sublevel coherence clearly extend the above mentioned previous methods using incoherent light sources. In the case of Doppler broadened optical transitions new signal features appear as a result of velocity-selective optical excitation caused by the narrow-bandwidth laser. We especially analyze the potential and the limitations of the new detection schemes for the study of collision effects in double resonance spectroscopy. In particular, the effect of collisional velocity changes on the Hertzian resonances will be investigated. 

Raman spectroscopy has an advantage over IR spectroscopy since H2O is transparent in Raman spectroscopy and most of solvents leave little trace on compensation. In-situ Raman spectroscopy closely reveals changes in chemical structures during electrochemical reaction in PPy. The resonant Raman effect makes it possible that the specific segments of a polymer molecule is identified when the segments are excited by the Raman energy source(ll, 12). 

Unlike the typical laser source, the zero-point blackbody field is spectrally white , providing all colours, CO2, that seek out all co - CO2 = coj resonances available in a given sample. Thus all possible Raman lines can be seen with a single incident source at tOp Such multiplex capability is now found in the Class II spectroscopies where broadband excitation is obtained either by using modeless lasers, or a femtosecond pulse, which on first principles must be spectrally broad [32]. Another distinction between a coherent laser source and the blackbody radiation is that the zero-point field is spatially isotropic. By perfonuing the simple wavevector algebra for SR, we find that the scattered radiation is isotropic as well. This concept of spatial incoherence will be used to explain a certain stimulated Raman scattering event in a subsequent section. 

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