Spontaneous Raman scattering vibrational

Conventional spontaneous Raman scattering is the oldest and most widely used of the Raman based spectroscopic methods. It has served as a standard teclmique for the study of molecular vibrational and rotational levels in gases, and for both intra- and inter-molecular excitations in liquids and solids. (For example, a high resolution study of the vibrons and phonons at low temperatures in crystalline benzene has just appeared [38].)... 

The inelastic processes - spontaneous Raman scattering (usually simply called Raman scattering), nonlinear Raman processes, and fluorescence - permit determination of species densities as well as temperature, and also allow one, in principle, to determine the temperature for particular species whether or not in thermal equilibrium. In Table II, we categorize these inelastic processes by the type of the information that they yield, and indicate the types of combustion sources that can be probed as well as an estimate of the status of the method. The work that we concentrate upon here is that indicated in these first two categories, viz., temperature and major species densities determined from vibrational Raman scattering data. The other methods - fluorescence and nonlinear processes such as coherent anti-Stokes Raman spectroscopy - are discussed in detail elsewhere (5). 

Several complementary techniques exist for the experimental study of dephasing, and here we will underline only the differences between them. The simplest experimental access to dephasing is by spontaneous Raman scattering. A laser of frequency scattered light at to, around cuj = frequency resolved. The isotropic Raman scattering cross section is directly related to the vibrational correlation function... 

Spontaneous Raman scattering always occurs when the laser excitation frequency is less than the frequency associated with an allowed electronic transition of the molecule. As the probe laser frequency approaches that of an electronic transition in the molecule, certain vibrational modes that couple strongly to the transition increase in intensity (pre-resonance) with respect to other Raman allowed modes of the molecule. When the excitation frequency coincides with the electronic transition frequency (resonance), a dramatic increase in vibrational band intensities is observed. This effect has been observed in many molecules and especially in polymer films, such as polydiacetylene, that consist of extended regions of electron delocalization owing to the presence of conjugated double and triple carbon-carbon bonds in the linear network (40)(41). 

From Eq. (1) it is apparent that in molecular media, totally S3nn-metric modes of vibration would lead to the highest Raman gain. This follows from our knowledge that in spontaneous Raman scattering these vibrational modes exhibit the largest rates of change of polarizability and the sharpest Raman lines. Also, since usually > 0, an in-... 

While the intensity of anti-Stokes radiation is very small in spontaneous Raman scattering due to the low thermal population density in excited molecular levels (Sect. 8.1), this is not necessarily true in stimulated Raman scattering. Because of the strong incident pump wave, a large fraction of all interacting molecules is excited into higher vibrational levels, and strong anti-Stokes radiation at frequencies col + has been found. 

Very short vibrational relaxation times of molecules in liquids or in gases at high pressures can be studied with picosecond techniques (see Chap. 11). The excitation of vibrational levels may be performed by stimulated Raman scattering (see Sect.9.3) and the time evolution of the excited state can be monitored by spontaneous Raman scattering. This method allows direct measurements of vibrational relaxation times of large molecules, such as long-chain alcohols [12.21]. 

Raman scattering and (b) anti-Stokes Raman scattering. In Stokes scattering, tlie cluomophore is initially in the ground vibrational state, g, and oi > CO2. hr spontaneous anti-Stokes scattering, the cluomophore must be initially m an excited vibrational state,/ Also note that in (b), M2 is (arbitrarily) defined as being greater than... 

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. 

Hydrogen is the most abundant chemical element in the universe, and in its various atomic and molecular forms furnishes a sensitive test of all of experimental, theoretical and computational methods. Vibration-rotational spectra of dihydrogen in six isotopic variants constituting all binary combinations of H, D and T have nevertheless been recorded in Raman scattering, in either spontaneous or coherent processes, and spectra of HD have been recorded in absorption. Despite the widely variable precision of these measurements, the quality of some data for small values of vibrational quantum number is still superior to that of data from electronic spectra [106], almost necessarily measured in the ultraviolet region with its concomitant large widths of spectral lines. After collecting 420... 

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