Rotational Raman spectroscopy

The incident radiation should be highly monochromatic for the Raman effect to be observed clearly and, because Raman scattering is so weak, it should be very intense. This is particularly important when, as in rotational Raman spectroscopy, the sample is in the gas phase. 

Figure 5.17 shows the rotational Raman spectrum of N2 obtained with 476.5 nm radiation from an argon ion laser. From this spectrum a very accurate value for Bq of 1.857 672 0.000 027 cm has been obtained from which a value for the bond length tq of 1.099 985 0.000 010 A results. Such accuracy is typical of high-resolution rotational Raman spectroscopy. 

Raman scattering is normally of such very low intensity that gas phase Raman spectroscopy is one of the more difficult techniques. This is particularly the case for vibration-rotation Raman spectroscopy since scattering involving vibrational transitions is much weaker than that involving rotational transitions, which were described in Sections 5.3.3 and 5.3.5. For this reason we shall consider here only the more easily studied infrared vibration-rotation spectroscopy which must also be investigated in the gas phase (or in a supersonic jet, see Section 9.3.8). 

Selected physical properties of chloroprene are Hsted in Table 1. When pure, the monomer is a colorless, mobile Hquid with slight odor, but the presence of small traces of dimer usually give a much stronger, distinctive odor similar to terpenes and inhibited monomer may be colored from the stabilizers used. Ir and Raman spectroscopy of chloroprene (4) have been used to estimate vibrational characteristics and rotational isomerization. 

Raman Spectroscopy. Raman spectroscopy is an excellent method for the analysis of deuterium containing mixtures, particularly for any of the diatomic H—D—T molecules. For these, it is possible to predict absolute light scattering intensities for the rotational Raman lines. Hence, absolute analyses are possible, at least in principle. The scattering intensities for the diatomic hydrogen isotope species is comparable to that of dinitrogen, N2, and thus easily observed. 

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... 

At higher pressures only Raman spectroscopy data are available. Because the rotational structure is smoothed, either quantum theory or classical theory may be used. At a mixture pressure above 10 atm the spectra of CO and N2 obtained in [230] were well described classically (Fig. 5.11). For the lowest densities (10-15 amagat) the band contours have a characteristic asymmetric shape. The asymmetry disappears at higher pressures when the contour is sufficiently narrowed. The decrease of width with 1/tj measured in [230] by NMR is closer to the strong collision model in the case of CO and to the weak collision model in the case of N2. This conclusion was confirmed in [215] by presenting the results in universal coordinates of Fig. 5.12. It is also seen that both systems are still far away from the fast modulation (perturbation theory) limit where the upper and lower borders established by alternative models merge into a universal curve independent of collision strength. 

Temkin S. I., Thuet J. M., Bonamy L., Bonamy J., Robert D. Angular momentum and rotational energy relaxation in N2-N2 collisions calculated from coherent and stimulated Raman spectroscopy data,... 

Vibrational spectroscopy and in particular Raman spectroscopy is by far the most useful spectroscopic technique to qualitatively characterize polysulfide samples. The fundamental vibrations of the polysulfide dianions with between 4 and 8 atoms have been calculated by Steudel and Schuster [96] using force constants derived partly from the vibrational spectra of NayS4 and (NH4)2Ss and partly from cydo-Sg. It turned out that not only species of differing molecular size but also rotational isomers like Ss of either Cy or Cs symmetry can be recognized from pronounced differences in their spectra. The latter two anions are present, for instance, in NaySg (Cs) and KySg (Cy), respectively (see Table 2). 

Stoicheff, B. P. High resolution Raman spectroscopy of gases. X. Rotational... 

Vibrational spectroscopy measures and evaluates the characteristic energy transitions between vibrational or vibrational-rotational states of molecules and crystals. The measurements provide information about nature, amount and interactions of the molecules present in the probed substances. Different methods and measurement principles have been developed to record this vibrational information, amongst which IR and Raman spectroscopy are the most prominent. The following focuses on these two techniques, the corresponding instrumentation and selected applications. 

Raman spectroscopy comprises a family of spectral measurements based on inelastic optical scattering of photons at molecules or crystals. It involves vibrational measurements as well as rotational or electronic studies and nonlinear effects. Following, Raman will be used in the established but slightly inaccurate way as a synonym for the most important and most common technique of the family, linear vibrational Raman scattering. 

The dipole and polarization selection rules of microwave and infrared spectroscopy place a restriction on the utility of these techniques in the study of molecular structure. However, there are complementary techniques that can be used to obtain rotational and vibrational spectrum for many other molecules as well. The most useful is Raman spectroscopy. 

In general, though, Raman spectroscopy is concerned with vibrational transitions (in a manner akin to infrared spectroscopy), since shifts of these Raman bands can be related to molecular structure and geometry. Because the energies of Raman frequency shifts are associated with transitions between different rotational and vibrational quantum states, Raman frequencies are equivalent to infrared frequencies within the molecule causing the scattering. 

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