Molecular vibrations peak intensities

Figure 5. HREELS cross-section curves (peak height normalized to the elastic peak intensity) for the excitation of selected molecular vibrational bands of polyimide. The dielectric theory predicts a E 1 behaviour.

Typically, the UV Raman spectra of various hydrocarbons adsorbed in zeolites have been found to be similar to their spectra in solution, as a pure liquid, or as a pure solid (25). This is an important finding because the UV Raman spectra of free molecules (which are relatively quick and easy to measure) can be used for fingerprint identification of adsorbed species. One minor exception to this rule is the Raman spectrum of naphthalene, which shows some changes in the pattern of peak intensities between solid naphthalene and naphthalene adsorbed in ultrastable Y-zeolite. In this case, the adsorbed naphthalene spectrum more closely resembles that of the molecule in solution with benzene or CCI4, which suggests that interaction with the pore walls of the zeolite was similar to solvent interactions. The smaller pore diameters and pore intersections in zeolite MFI compared to Y-zeolite might be expected to produce more pronounced changes in molecular vibrational spectra as a consequence of steric interactions of the molecules with the pore walls. 

Field ionization involves passing the sample molecules very close to a thin wire that carries a high electrical potential. The strong electric field in the vicinity of this wire ionizes the molecules of sample. Molecular ions formed in this manner do not possess the high degree of vibrational energy found in molecular ions formed by electron impact. Consequently, the ions formed in field ionization are considerably more stable. The molecular ions formed in this manner are much more abundant than those formed by other means of ionization, and the molecular ion peak is usually fairly intense. In some cases, a molecular ion peak may be observed easily through the use of field ionization, whereas no peak would be observed after electron bombardment. 

The radiation scattered from Raman active molecules thus has three components the central line at a frequency Vo due to Rayleigh scattering and two ancillary lines displaced to either side the Stokes (Vo + v) and the anti-Stokes (vg-v) lines (Figure 6.6). The magnitude of the shift and the peak intensity can be related to molecular vibrations using normal coordinate analysis and other mathematical tools discussed in Section 6.3. 

Infrared dichroism is based on the interaction between linearly polarized infrared radiation and the oriented material. The atoms of a polymer molecule vibrate in characteristic normal modes, each of which produces a change in dipole moment (the transition moment) that has a specific direction. Each mode absorbs infrared energy at a characteristic frequency, giving rise to peaks in the infrared spectrum. The peak intensity (i.e. the absorbance) depends on the angle between the transition moment and the electric field vector of the radiation, and it is this that provides information on the molecular orientation. The orientation is defined in terms of the second moment of the orientation function Pjlcos 0), where ... 

An asynchronous 2D IR spectrum (Figure 1-11) provides complementary information cross peaks appear if IR signals are not synchronized to be completely in phase with each other. This feature is particularly useful, since IR bands arising from molecular vibrations of different functional groups, or even of similar groups in different local environments, may exhibit substantially different time-dependent intensity fluctuations. Thus, asynchronous 2D IR spectra can be used to differen-... 

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