Vibrational spectra, electrical interaction

Inversion splitting of the vibrational spectrum of ammonia has been used to create the first molecular microwave amplifier (maser) [86, 87]. The inversion population in the ammonia maser is achieved by transmission of the molecular beam through a non-homogeneous electric field. Ammonia molecules in symmetric and antisymmetric states interact with the electric field in different ways and they are therefore separated in this field. They are then directed to the resonator. 

The vibrational spectrum of benzene around 1000 cnf has also been measured. IQ. Benzene was physisorbed on a cooled copper substrate in the vacuum chamber. Figure 19 shows the transmission for several thicknesses of benzene and a prism separation of 3 cm. The thickness was determined from the measured transmission in transparent regions using Eg. (7). The solid curves were calculated from Eqs. (5) and (6) using optical constants for benzene obtained from an ordinary transmission experiment.il The benzene film was assumed to be isotropic. Of the two absorption lines seen, one belongs to an in-plane vibrational mode, and one to an out-of-plane vibration. Since the electric field of the SEW is primarily perpendicular to the surface, the benzene molecules are clearly not all parallel or all perpendicular to the copper surface. Also it should be noted that the frequencies are the same (within the experimental resolution) as those of solid benzene22 and of nearly the same width. These features indicate that the benzene interacts only weakly with the copper surface, as would be expected for physisorbed molecules. 

The objective of this first part of the book is to explain in a chemically intelligible fashion the physical origin of microwave-matter interactions. After consideration of the history of microwaves, and their position in the electromagnetic spectrum, we will examine the notions of polarization and dielectric loss. The orienting effects of the electric field, and the physical origin of dielectric loss will be analyzed, as will transfers between rotational states and vibrational states within condensed phases. A brief overview of thermodynamic and athermal effects will also be given. 

In Eq. (10), E nt s(u) and Es(in) are the s=x,y,z components of the internal electric field and the field in the dielectric, respectively, and p u is the Boltzmann density matrix for the set of initial states m. The parameter tmn is a measure of the line-width. While small molecules, N<pure solid show well-defined lattice-vibrational spectra, arising from intermolecular vibrations in the crystal, overlap among the vastly larger number of normal modes for large, polymeric systems, produces broad bands, even in the crystalline state. When the polymeric molecule experiences the molecular interactions operative in aqueous solution, a second feature further broadens the vibrational bands, since the line-width parameters, xmn, Eq. (10), reflect the increased molecular collisional effects in solution, as compared to those in the solid. These general considerations are borne out by experiment. The low-frequency Raman spectrum of the amino acid cystine (94) shows a line at 8.7 cm- -, in the crystalline solid, with a half-width of several cm-- -. In contrast, a careful study of the low frequency Raman spectra of lysozyme (92) shows a broad band (half-width 10 cm- -) at 25 cm- -,... 

Figure 8.29. Electric resonance spectrum of CsF in strong fields, showing resonance from molecules in five different vibrational levels (v = 0 to 4). The hyperfine structure resulting from nuclear-molecular interactions is not resolved [50].

The absorption of IR light by a vibrating molecule follows dipolar selection rules. This means that, to interact with the electric field of an incident photon so as to excite a normal mode of a molecule, absorb the photon, and be observed in the IR spectrum, a given normal mode must distort the molecule such that it alters the molecule s dipole moment. On the other hand Raman scattering, which is a two-photon process (the two being the incident and scattered photons), follows quadrupolar selection rules. This means that, to interact with an incident photon so as to result in the excitation of the normal mode of a molecule and the scattering of a Raman-shifted photon, the mode must distort the molecule such that it alters the molecule s polarizability. 

In the field of catalysts characterization the use of small unreactive probe molecules to identify coordinatively unsaturated sites is well established [89]. Not always, however, a direct correlation between the CO vibrational frequency, the strength of the interaction, and the surface electric field exists. Recent DPT cluster calculations [90] have shown that CO adsorbed on step sites gives rise to a relatively strong interaction but to a negligible CO vibrational shift this is due to the inhomogeneity in the electric field above a MgO(lOO) step. This study [90] has permitted the complete attribution of the IR spectrum of CO adsorbed on MgO [81,83,91], Table 2. 

Infrared and Raman spectroscopy are important analytical tools used to investigate a wide variety of molecules in the solid, liquid, and gas states, and yielding complementary information about molecular structure and molecular bonds. Both methods supply information about resonances caused by vibration, vibration-rotation, or rotation of the molecular framework, but because the interaction mechanism between radiation and the molecule differs in the two types, the quantum-mechanical selection rules differ as well. Therefore, not all of the molecular motions recorded by one type of spectroscopy will necessarily be recorded by the other. The geometrical configuration of the molecule and the distribution of electrical charge within that configuration determine which molecular motions may appear in each type of spectrum. 

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