Raman spectroscopy Stokes lines

The best resolution of Q-branch rotational structure in a N2-Ar mixture was achieved by means of coherent anti-Stokes/Stokes Raman spectroscopy (CARS/CSRS) at very low pressures and temperatures (Fig. 0.4). A few components of such spectra obtained in [227] are shown in Fig. 5.9. A composition of well-resolved Lorentzian lines was compared in [227] with theoretical description of the spectrum based on the secular simplification. The line widths (5.55) are presented as... 

Anh-Stokes lines - [INFRAREDTECHNOLOGYANDRAMANSPECTROSCOPY- RAMAN SPECTROSCOPY] (Vol 14) - [SPECTROSCOPY, OPTICAL] (Vol 22)... 

XN.R., the non-resonant susceptibility, gives rise to the background interference in Coherent Anti-Stokes Raman Spectroscopy (CARS) (J5). This interference which arises from solvents or closely spaced lines is responsible for the CARS band shape distortion observed under certain conditions. 

Section 2.3). The scattered light consists of two types one, called Rayleigh scattering, is strong and has the same frequency as the incident beam (vo), and the other, called Raman scattering, is very weak ( 10 5 of the incident beam) and has frequencies vo vm, where vm is a vibrational frequency of a molecule. The vo — vm and vo + vm lines are called the Stokes and anti-Stokes lines, respectively. Thus, in Raman spectroscopy, we measure the vibrational frequency (vm) as a shift from the incident beam frequency (vo). In contrast to IR spectra, Raman spectra are measured in the UV-visible region where the excitation as well as Raman lines appear. 

Inverse Raman spectroscopy The Inverse Raman effect is a form of Raman scattering, first noted by W.J. Jones and B.P. Stoicheff, wherein stokes scattering can exceed anti-Stokes scattering resulting in an absorption line (a dip in intensity) at the sum of irradiated monochromatic light and Raman frequency of the material. This phenomenon is referred to as the inverse Raman Effect, application of the phenomenon is referred to as inverse Raman spectroscopy, and a record of the continuum is referred to as an inverse Raman spectrum. 

Raman spectroscopy is a technique in which monochromatic radiation in the visible range (400-800 nm) is scattered by a sample (Woodward, 1967 Olsen, 1975 Long, 1977). Most of the scattered radiation is due to Rayleigh scattering (scattered radiation of same frequency as incident radiation) however, a small fraction of the scattered radiation is shifted to lower frequencies (Stokes lines) and an even smaller fraction to higher frequencies... 

Figure B2.3.8. Energy-level schemes describing various optical methods for state-selectively detecting chemical reaction products left-hand side, laser-induced fluorescence (LIF) centre, resonance-enhanced multiphoton ionization (REMPI) and right-hand side, coherent anti-Stokes Raman spectroscopy (CARS). The ionization continuum is denoted by a shaded area. The dashed lines indicate virtual electronic states. Straight arrows indicate coherent radiation, while a wavy arrow denotes spontaneous emission.

The vibrational selection rules are the same for Raman spectroscopy as for infrared spectroscopy. In the Stokes process, the intense, monochromatic radiation t es a molecule from the v = 0 state to a virtual state, VO, from which it falls back to the v = 1 state. Similarly, in the anti-Stokes process, the virtual state VI is involved in the overall transfer of the molecule from the v = 1 to the v = 0 state. The Stokes and anti-Stokes transitions lie on the low and high wavenumber sides, respectively, of the exciting radiation. The intensity of the anti-Stokes line, relative to the Stokes transition is very low because of the lower population of the v = 1 state, compared to that of the v = 0 state. Consequently, Raman spectroscopy uses only the Stokes transitions. 

Raman spectroscopy is useful for studying aqueous solutions, e.g., of polymers, because the spectra are hardly affected by the presence of water. The two major problems of the technique are (1) the low intensities of the Stokes lines (hence data acquisition is slow), and (2) laser-induced fluorescence effects, which may be so intense that they can completely wipe out all Raman scattering of interest. 

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