Detection Raman

Plenary 9. J W Nibler et al, e-mail address niblerj chem.orst.edu (CARS and SRS). High resolution studies of high lymg vibration-rotational transitions in molecules excited in electrical discharges and low density monomers and clusters in free jet expansions. Ionization detected (REMPI) SRS or IDSRS. Detect Raman... 

However, as Raman scattering is a two-photon process, the probability of the Raman scattering process is lower than that of fluorescence and IR absorption processes. The cross section of Raman scattering is 10 cm, which is much smaller than that of fluorescence ( 10 cm ) and IR absorption ( 10 °cm ). When we detect Raman scattering at the nanoscale, the number of photons obtained is less than with the usual micro-Raman spectroscopy due to reduction in the detection area or the number of molecules. To overcome this problem, we need to devise a method for amplification of Raman scattering. 

The implementation of notch filters makes the application of compact single-stage spectrographs, as used for chemical sensors, possible in the first place. Additionally, it permits to detect Raman lines much closer to the Rayleigh wavelength than previously possible. 

An experimental setup with a notch Alter enables us to detect Raman spectra of many minerals from 5-10 m, starting from a Raman shift of approximately... 

Before melting and for some time after only the band at 625 cm of the AA [C4CiIm]+ cation was observed in the 600-630 cm i region. Gradually 603 cm i band due to the GA conformer became stronger. After about 10 min the AA/GA intensity ratio became constant. The interpretation [50] is that the rotational isomers do not interconvert momentarily at the molecular level. Most probably it involves a conversion of a larger local structure as a whole. The existence of such local structures of different rotamers has been found by optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES) [82], Coherent anti-Stokes Raman scattering (CARS) [83],... 

The size of the enhancement factor or the effective SERS cross section is a key question for the application of SERS as a tool for ultrasensitive detection. The effective cross section must be high enough to provide a detectable Raman signal from a few molecules. In the early SERS experiments, Van... 

Since the results obtained by matrix isolation Raman spectroscopy have been reviewed extensively (88-90), only two typical examples are discussed here to show the utility of this technique. It should be noted that both works took advantage of resonance Raman spectroscopy to detect Raman signals using low laser power. 

The following section contains a more detailed treatment of the theory behind the nonresonant spectroscopy of liquids. This will be followed by a description of the experimental implementation and data analysis techniques for a common OKE scheme, optical-heterodyne-detected Raman-induced Kerr-effect spectroscopy (22). We will then discuss the application of this technique to the study of the temperature-dependent dynamics of simple liquids composed of symmetric-top molecules. 

Cong P, Deuel HP, Simon JD. Structure and dynamics of molecular liquids investigated by optical-heterodyne detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). Chem Phys Eett 1995 240 72-78. 

The use of laser Raman spectrometry in order to quantitatively investigate the urea synthesis under process conditions has been reported by Van Eck et al. (1983). Only Raman spectroscopy seems to suit the problem, since the visible radiation which is used to excite and detect Raman transitions can easily be directed to a measuring cell. Furthermore, water, which is an acceptable solvent, and all compounds involved in the synthesis show characteristic Raman bands. In order to compensate for many of the instrumental factors relative intensities were used instead of absolute intensities. Reproducible window mountings are a necessity. The effect of pressure and temperature on the Raman intensity have to be taken into account if measurements are to be carried out in situ (Sec. 6.8). The effect of the temperature is moderated by using an internal standard. 

At this point, it is important to clarify what is meant by the enhancement factor before discussing hot spots further. Typically, the enhancement factor is calculated as the ratio of the detected Raman signal under SERS conditions compared to the signal obtained under normal conditions, for equal numbers of active molecules and surface area exposed to the laser beam. This value is proportional to the intensity of the local electromagnetic field ( ) to the fourth power, i.e., jEj which results from the enhancement of both the incident and emitted photons [15]. Throughout the rest of the chapter, the enhancement factor that is quoted refers to this the enhancement of SERS intensity, rather than that associated with the local electromagnetic field unless otherwise stated. 

OHD-RIKES(Optical Heterodyne Detected Raman Induced Kerr Effect Spectroscopy)... 

Similar to any bench-scale optical detections, there are multiple measurement modes commonly used with optical fibers, including fluorescence (excite at a certain wavelength and detect the emission at a longer wavelength), colorimetry (color detection), Raman (inelastic scattering... 

---