Orientation by homogeneous electric field

If polar diatomic molecules are previously aligned in a beam (see Section 6.2) there is another possibility, proposed in [43], of producing angular momenta orientation using alignment-orientation conversion in a homogeneous electric field due to the second-order Stark effect (see Section 5.4). We will consider this method in more detail since it is a nice example of how to make use of handling the different approaches presented in Chapter 5 simultaneously. 

Because of the quantum origin of the Stark effect (see Section 5.2), the evolution of the density matrix elements /mm must be considered for the ground state molecules. In the field region we must choose the 

Now the question is how to connect °/mm in (6.19) with ax coefficients which are defined by (6.18) in the xf,y, z coordinate system. The sequence of necessary transformations follows from the treatment presented in Chapter 5 and Appendix D. First, as is clear from (D.54), we can easily pass to the classical polarization moments (multipoles) p defined by (2.16) in the x, y, z system. In particular, po = 5a2 supposing po = °o = 1- Second, we have to turn to the classical multipoles pq in the x,y,z frame with z see Fig. 6.6(a). This can be achieved by means of Wigner D-matrices, and according to (A. 12), we obtain 

Consequently, as a result of transformation 02 — po Pq °/q - °/mm i we obtain °/mm and substitute them into (6.19), thus obtaining the desired density matrix elements /mm - 

In order to analyze the electric field induced level splitting effect we will again expand /mm over quantum polarization moments fq. As may be seen from (5.40), which is the reverse of (6.21), we obtain in the x, y, z frame 

---

For an axial molecule with the dominant dipole and hyperpolarizibility directed along the molecular z axis, i.e. ju and oi ly/ dispersed in a homogeneous matrix, the orientation in an electric field is given by the Langevin function ... 

Substrate and intermediate species adsorb on an electrode surface and orient themselves so that their least hindered sides face the electrode, unless there is another effect such as a polar one. An electrode interface has a layered structure in which a nonuniform electric field (some slope of potential) is generated by polarization of the electrode. An extremely strong electric field of approximately 10 V cm i in the innermost layer might cause a variety of polar effects. For instance, electrochemical one-electron oxidation of o-aminophenol derivatives proceeds adiabatically. On the contrary, the homogeneous reaction is nonadiabatic. This difference in behavior is related to... 

According to Onsager, a reaction field is the electric field arising from an interaction between an ideal nonpolarizable point dipole and a homogeneous polarizable dielectric continuum in which the dipole is immersed [80]. The reaction field is the electric field felt by the solute molecule due to the orientation and/or electronic polarization of the solvent molecules by the solute dipole. 

---