Electric dipole moment instantaneous

FIGURE 5.5 The rapid fluctuations in the electron distribution in two neighboring molecules result in two instantaneous electric dipole moments that attract each other. The fluctuations flicker into different positions, but each new arrangement in one molecule induces an arrangement in the other that results in mutual attraction. 

Whenever the molecule is linear, the electric dipole moment will be zero. Whenever it is bent, it will have an instantaneous dipole moment. During one vibrational cycle the electric dipole moment will alternate between positive and negative values, but if we observe the molecule over a longer time than it takes to complete a vibrational cycle, that is for longer than 100 ps, the dipole moment will average out to zero the molecule has no permanent dipole moment. 

Figure 1.6 Orientations of the E and B fields associated with the linearly polarized light wave described by Eqs. 1.99. The E field, directed along the x axis, interacts with the X component of the molecule s instantaneous electric dipole moment the B field, directed along the y axis, interacts with the y component of the molecule s instantaneous magnetic dipole moment.

The instantaneous electric dipole moment of the atom is given by... 

T IE- E j erj IT/2 >. Here E Ej eri is the one-electron operator describing the interaction of an electric field of magnitude and polarization E with the instantaneous dipole moment... 

The Einstein coefficients are related to the most fundamental quantity which describes the transition probability, known as the transition moment. During an electronic transition for instance, an electron jumps from one orbital to another. Its distance from the nucleus changes, so there is a change in the instantaneous dipole moment. The greater this change, the more probable the transition because it is the interaction between this transition dipole and the electric vector of light. 

Weak, short-range forces of attraction, independent of normal bonding forces, are known as van der Waals forces, after the 19th-century Dutch physicist. These forces arise because, at any particular moment, the electron cloud around a molecule is not perfectly symmetrical. In other words, there are more electrons (thus net negative charge) on one side of a molecule than on the other, generating an instantaneous electrical dipole. 

The dipole moment, p, induced in the particle is proportional to the instantaneous electric field vector ... 

---