Raman rotational

In many cases, the infrared and Raman rotation-vibration spectra contribute complementary structure data, particularly for highly symmetric molecules. Due to the significantly different selection rules a greater line density is observed for Raman due to a larger selection of allowed changes in the rotational energy compared to infrared gas spectra. Raman spectroscopy is, on these grounds, also a valuable supplement to infrared studies. 

We consider below an example in diatomic molecules where the field frequency is low with respect to the electronic excitation, but high with respect to the vibrations and consequently also with respect to the rotations. This will lead to Raman rotational excitations in the ground vibrational state of the ground electronic state (i.e. the ground vibronic state). The laser frequency is such that the ground electronic state is not one- or two-photon resonant with other electronic states. 

R branch, 364, 365 Radiation, by accelerated charge, 6 by electric quadrupolc, 39 by magnetic dipole, 39 by oscillating dipole, 43 Raman activity (see Scilection rules) Raman band types,, 366 Raman lines (see Raman spectrum) Raman rotation-vibration spectra, 365 Raman scattering, 48, 49 (See also Raman spoctnim)... 

Fig. 11, Comparison of initial state population distributions for AJ = 2 transitions of a Raman rotational/vibrational band.

This spectrum is called a Raman spectrum and corresponds to the vibrational or rotational changes in the molecule. The selection rules for Raman activity are different from those for i.r. activity and the two types of spectroscopy are complementary in the study of molecular structure. Modern Raman spectrometers use lasers for excitation. In the resonance Raman effect excitation at a frequency corresponding to electronic absorption causes great enhancement of the Raman spectrum. 

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].)... 

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... 

The out-of-plane vibrations of thiazole correspond to C-type vibration-rotation bands and the in-plane vibrations to A, B, or (A + B) hybrid-type bands (Fig, 1-9). The Raman diffusion lines of weak intensity were assigned to A"-type oscillations and the more intense and polarized lines to A vibration modes (Fig. I-IO and Table 1-23). 

Suites 1 to VIII contain infrared frequencies corresponding to vibration-rotation bands of A, B, or (A-l-B) hybrid types and can thus be assigned to vibrations of A symmetry the corresponding Raman lines are generally polarized. 

The frequencies classified in suites IX and X belong to depolarized Raman lines and correspond to vibrations-rotation bands of the C type. They can be assigned to oscillations of A" symmetry. 

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. 

Electronic, vibrational and rotational transitions may be involved in Raman scattering but, in this chapter, we consider only rotational transitions. 

Rotational Raman spectra of diatomic and linear polyatomic molecules... 

In a diatomic or linear polyatomic molecule rotational Raman scattering obeys the selection rule... 

Figure 5.17 Rotational Raman spectrum of N2 (the lines marked with a cross are grating ghosts and not part of the spectrum)...

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. 

For a symmetric rotor molecule the selection rules for the rotational Raman spectmm are... 

The first three Stokes lines in the rotational Raman spectrum of 02 are separated by 14.4 cm, 25.8 cm and 37.4 cm from the exciting radiation. Using the rigid rotor approximation obtain a value for tq. 

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