Raman spectroscopy IDSRS

5 Ionization detected stimulated Raman spectroscopy (IDSRS) 

IDSRS spectra of the benzene dimers C6H6 - (which are denoted do - d(,) and CfeDf, - CftD6 (dfi - de), respectively. From measurements of a number of isotopomers of the species in several spectral regions of the benzene monomer, several significant results relating to structure have been obtained (Felker et al., 1992). 

From the sophisticated measurements performed in Felker s laboratory it seems that the various versions of ionization-detected. stimulated Raman techniques (with mass analysis) have great capabilities in the high resolution vibrational spectroscopy of weakly bound complexes and clusters. One expects that IDSRS will become increasingly productive in the studies of cluster ground-state structure and dynamics. 

The application of NIR spectroscopy has been further stimulated by the development of NIR diffuse reflectance techniques which are widely used in the analysis of agricultural, pharmaceutical, biochemical and synthetic polymer materials (Siesler, 1991). The rapidly increasing use of NIR spectroscopy is illustrated in the book Making Light Work Advances in Near infrared spectroscopy , edited by Murray and Cowe (1992) as well as in the Handbook of Near-Infrared Analysis by Bums and Ciurcak (1992). 

Near-infrared absorption is therefore essentially due to combination and overtone modes of higher energy fundamentals, such as C-H, N-H, and O-H stretches, which appear as lower overtones and lower order combination modes. Since the NIR absorption of polyatomic molecules thus mainly reflects vibrational contributions from very few functional groups, NIR spectroscopy is less suitable for detailed qualitative analysis than IR, which shows all (active) fundamentals and the overtones and combination modes of low-energy vibrations. On the other hand, since the vibrational intensities of near-infrared bands are considerably lower than those of corresponding infrared bands, optical layers of reasonable size (millimeters, centimeters) may be transmitted in the NIR, even in the case of liquid samples, compared to the layers of pm size which are detected in the infrared. This has important consequences for the direct quantitative study of chemical reactions, chemical equilibria, and phase equilibria via NIR spectroscopy. 

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Figure 3.6-5 Energy-level diagram illustrating the two excitation steps of Ionization Detected Stimulated Raman Spectroscopy (IDSRS).

Fig. 7 Raman gain = stimulated Raman gain spectroscopy (SRGS), inverse Raman = inverse Raman spectroscopy (IRS) or stimulated Raman loss spectroscopy (SRLS), coherent anti-Stokes Raman spectroscopy (CARS), photoacoustic Raman spectroscopy (PARS), or ionization-detected stimulated Raman spectroscopy (IDSRS). In the following sections, the various methods are briefly described. More detailed information can be found in books [59-61], reviews [45,46,57,58,62,63] and conference reports [64-73].

A technique which combines the high sensitivity of resonant laser ionization methods with the advantages of nonlinear coherent Raman spectroscopy is called IDSRS (ionization detected stimulated Raman spectroscopy). The excitation process, illustrated in Figure 5, can be briefly described as a two-step photoexcitation process followed by ion/electron detection. In the first step two intense narrow-band lasers (ct L, 0) ) are used to vibrationally excite the molecule via the stimulated Raman process. The excited molecules are then selectively ionized in a second step via a two- or multiphoton process. If there are intermediate resonant states involved (as state c in Figure 5), the method is called REMPI (resonance enhanced multi-photon ionization)-detected stimulated Raman spectroscopy. The technique allows an increase in sensitivity of over three orders of magnitude because ions can be detected with much higher sensitivity than photons. 

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