Polarisability interaction-induced

Epoi - the polarisation interaction i.e. the effect of the distortion of the electron distribution of A by B, and vice versa, and the higher-order coupling resulting from such distortions. This component includes the interactions between all permanent charges and induced multipoles and is always an attractive interaction. 

Our discussion of elecfronic effects has concentrated so far on permanent features of the cliarge distribution. Electrostatic interactions also arise from changes in the charge distribution of a molecule or atom caused by an external field, a process called polarisation. The primary effect of the external electric field (which in our case will be caused by neighbouring molecules) is to induce a dipole in the molecule. The magnitude of the induced dipole moment ginj is proportional to the electric field E, with the constant of proportionahty being the polarisability a ... 

When the oscillating electric held of an incident light ray interacts with a molecule, a small oscillating dipole moment is induced in the molecule as a consequence of its polarisability, a. Polarisability itself is a measure of the change in the dipole moment of a molecule induced by an electric held, and in the simplest case, where the electric held E and induced dipole moment p are in the same direction ... 

Induced dipolar interactions. The electron clouds in many (especially large) organic molecules are readily polarised resulting in the formation of induced dipoles that can interact, resulting in complex stabilisation. Both cations and anions can induce dipoles in aromatic molecules, for example. 

Debye interaction occurs when a permanent dipole induces a dipole in a polarisable atom or molecule. The induced dipole is oriented in such a way that attraction occurs. 

In Sect. 7.3, Eqs. (18) and (19) describe the Maxwell stress forces acting on a conductive tip when a combined d.c./a.c. voltage is applied. For the PFM set-up we have to complete the total interaction force by the additional effects of piezoelectricity, electrostriction and the spontaneous polarisation. Both electromechanical effects cause an electric field-induced thickness variation and modulate the tip position. The spontaneous polarisation causes surface charges and changes the Maxwell stress force. If the voltage U(t)=U[)c+UAc sin((Ot) is applied, the resulting total force Ftotai(z) consists of three components (see also Eq. 19) Fstatic, F(0 and F2m. Fstatic is the static cantilever deflection which is kept constant by the feedback loop. F2a contains additional information on electrostriction and Maxwell stress and will not be considered in detail here (for details see, e.g. [476]). The relevant component for PFM is F(0 [476, 477] ... 

As soon as we consider the molecular nature of a material, we realise that the internal electric field will vary from point to point as a consequence of the interaction of fields from the dipoles which are induced on each molecule by the applied field, although the space-average electric field over a volume large in comparison with molecular size (this is equivalent to the classical electric field based on a continuum model) may still be uniform. The field acting on an individual polarisable entity like an atom or molecule is called the local field Eh, and it is an important concept in linking observable bulk behaviour of a material with the properties of its constituent atoms or molecules. 

The mean electron moment given by Eq. 2.69 will only be parallel to Bq if g is isotropic. If the nucleus being observed is not the ion with the initial unpaired electron, spin s-orbital polarisation can still be induced through effects transferred via the chemical bond, in which case it is called the transferred hyperfine interaction. This effect can still be strong although it will decrease rapidly as the number of intermediate bonds... 

For most purposes the induced polarisation of the H-C bond can be ignored, while in H-O, H-F and H-Cl bonds it is significant. One consequence of this polarisation is that lone pairs can interact with the 8+ hydrogens by forming a hydrogen bond, which is a weak Coulombic interaction. This bond can be either intermolecular or intramolecular. 

The principles of nonlinear optics and the main techniques used to evaluate the second-order NLO properties are briefly presented here. Major details can be found in more specialised reviews and books. At the molecular level, the interaction between polarisable electron density and the alternating electric field of the laser light beam (E) induces a polarisation response (Afi) that can be expressed following Equation 1.1 ... 

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