Oriented gas

Many readers will have some familiarity with the standard expressions for the angular distribution of photofragments ejected from a randomly oriented (gas-phase) molecule by perfectly polarized light ... 

Nonlinear processes being basically of Intramolecular nature, corresponding terms In the macroscopic and microscopic dipoles expansions can be related by the following tensorlal summation (given here for SHG coefficients), following an oriented gas description ... 

In solution, although solute contributions can generally be singled out, difficulties arise sometimes solvent-solute interactions may induce a shift of the solute absorption and consequently of its susceptibility or hydrogen bonded molecular complexes may modify the liquid structure. This situation has been studied both theoretically and experimentally by Zyss and Berthier (10) and by Ledoux and Zyss (13) in the case of urea derivatives in various solvents and in crystal showing the importance of environment considerations and thus the limitations of an oriented gas model for crystals. 

Interpretable Polarization. Since C02 is a linear molecule, the IR polarization of its asymmetric stretching mode is particularly easy to interpret. In the absence of resonant interactions with modes of neighboring molecules (see below), it is safe to assume that the oriented gas model holds, and that the absorption of polarized light is maximum in the direction of the long axis [52]. The polarization of the bending mode can also be interpreted, although we have not used it as much. 

In the EFISH method, the molecule of interest is dissolved in an appropriate solvent and put into a cell of the type shown in Figure 9. Electrodes above and below the cell provide the means for a D.C. electric field, which orients the solute (and solvent) molecules through its interaction with the molecular dipoles. Similar to the poled polymer approach, the average molecular orientation is increased along the field direction and an oriented gas model used to extract p. 

In the limit of the oriented gas model with a one-dimensional dipolar molecule and a two state model for the polarizability (30). the second order susceptibility X33(2) of a polymer film poled with field E is given by Equation 4 where N/V is the number density of dye molecules, the fs are the appropriate local field factors, i is the dipole moment, p is the molecular second order hyperpolarizability, and L3 is the third-order Langevin function describing the electric field induced polar order at poling temperature Tp - Tg. 

Since the dipoles of chromophore molecules are randomly distributed in an inert organic matrix in amorphous PR materials, the material is centrosymmet-ric and no second-order optical nonlinearity can be observed. However, in the presence of a dc external field, the dipole molecules tend to be aligned along the direction of the field and the bulk properties become asymmetric. Under the assumption that the interaction between the molecular dipoles is negligible compared to the interaction between the dipoles and the external poling field (oriented gas model), the linear anisotropy induced by the external field along Z axis at weak poling field limit (pE/ksT <[Pg.276]

The usual way of treating the optical properties of systems containing organic molecules is in terms of the oriented gas model, that is, the macroscopic property is treated as the sum of molecular contributions, allowing... 

An assembly of molecules, weakly interacting in a condensed phase, has the general features of an oriented gas system, showing spectral properties similar to those of the constitutive molecules, modulated by new collective and cooperative intrinsic phenomena due to the coherent dynamics of the molecular excitations. These phenomena emerge mainly from the resonant interactions of the molecular excitations, which have to obey the lattice symmetry (with edge boundary, dimensionality, internal radiation, and relativistic conditions), with couplings to the phonon field and to the free radiation field. 

Rigid molecules are oriented definitely the oriented gas approach... 

When free molecules are incorporated in a lattice, their axes are first oriented, i.e., v h the angle between the u and the a axis is 6 = 63.6 °. This is one aspect of the oriented gas model . However, as Fig. 2.7-9a shows, the molecules are not exactly planar the hydrogen atoms are directed toward the neighboring S atoms. The symmetry plane (j uv) of the free molecule is thus lost, the new point group of the molecule is determined by the symmetry of the lattice site, Q. According to the site symmetry model, the a and b species are therefore combined to afford a species a, while the U2 and hj species afford a , see Table 2.7-3 and Fig. 2.7-8. 

The existence of two polymorphic structures of the dichloro derivative of 6-XXXV (R = Cl) (Bernstein and Izak 1976) provided an opportunity for the direct examination of the relationship between the molecular structure and the electronic spectrum. The two structures are conformational polymorphs, with the metastable very pale yellow triclinic needle form exhibiting a planar molecular conformation a = fi = 0°) (Bernstein and Schmidt 1972) and the stable yellow orthorhombic form (with chunky rhombic crystals) exhibiting a non-planar conformation (a = 25° fi = —25°) (Bernstein and Izak 1976). Assuming that the two crystal structures merely serve to hold the molecule in the two different conformations (i.e. the oriented gas model), the absorption spectra should reflect the difference in conformation that measured on the triclinic structure, with a planar conformation, should closely resemble the spectra of 6-XXXVI and 6-XXXVII, while that for the orthorhombic structure, with the nonplanar molecular conformation, should retain the characteristics of 6-XXXV (R = H) in solution. 

At the opposite extreme from the oriented gas model for molecular crystals, the neighbouring molecules do interact with each other resulting in spectral properties of the bulk that differ considerably from those of the individual molecule. Interacting molecules of this type often tend to form aggregates even in solution, a phenomenon that has been exploited by the photographic industry for the tuning of the spectral response of silver halide emulsions (Herz 1974 Smith 1974 Nassau 1983). Aggregate formation can lead to the development of new, and often quite intense absorption bands... 

Schnepp. J. Chem. Phys. 23, 234-7 (1955). IR dichroism oriented gas model tested, naphthalene. 

The negative sign applies to attraction. For random orientation (gas or ideal solution) the average that is relevant is ... 

Table 6.3. Polar Point Croups and Maximum Fractional Value of the Rigid Oriented Gas for a Phase-Matched Interaction for Optimum Molecular Orientation in the Unit Cell...

Previous studies of aligned and oriented gas-phase reactions had relied on the use of laboratory-based fields (lasers, hexapoles, etc.) to fix reactants relative to the lab system, thereby introducing geometric bias into the reaction [11-20]. However, referring to Figure 1, when a weakly bonded complex is used as a precursor, one of the reactants is encapsulated within one... 

The well-known NLO molecular crystal POM (3-methyl-4-nitropyridine-1-oxide) is simulated through cluster calculations by Guillaume et al.216 Semi-empirical and MP2 ab initio results are considered and comparisons of the NLO response with those obtained from the usual oriented gas model are made. POM is also selected by Hamzaoui et al 11 as an example of an NLO molecular crystal on which to test their procedure for relating the polarizabilities to the multipolar components of the ground state charge distribution determined by X-ray diffraction methods. 

The applications were extended to mixed molecular crystals such as tetracene contained as a dilute impurity in anthracene. Choudhury and Ganguly [213] had measured the absorption spectmm of tetracene at 480 nm and interpreted it with the assumption, later confirmed by crystal packing calculations, that the molecules are held in the anthracene lattice in almost the same position and orientation as the molecules they replace. They made the remarkable discovery that the intensity ratio for absorption along the b and a crystal axes was not 7.7 1 expected on the oriented gas model, but around 2 1. We later refined the measurements [84] to give values close to 3 1. 

By this time Thuraiappah Thirunamachandran had come to me as a PhD student, with study leave from his staff position in the University of Ceylon. He showed [74] that the experimental results in this and some other cases were very well accounted for by the transfer of intensity from strong to weaker transitions under the dipole-dipole coupling (3.3). It had become clear that the oriented-gas model was not a satisfactory approximation. 

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