6 - Other methods of alignment and orientation of molecules

Summary

The material presented so far was based on the creation of anisotropic distribution of molecular angular momenta under the direct effect of light absorption. We are now going to discuss briefly some ideas and examples of experimental realization of other methods leading to the production of polarized molecules, including those which are not directly connected with light effects, such as polarization caused by collisions and external electric or magnetic fields.

Photodissociation and photoionization

It was assumed in the preceding chapters that optical transitions between bonded states of molecules take place with no change in their chemical composition. Photodissociation and photoionization form a general class of photofragmentation processes in collisions between a molecule and a photon which lead to simple chemical reactions of disintegration into atomic or molecular fragments, or into ions and electrons.

The photodissociation process

The photodissociation process takes place most frequently at excitation of the molecule to a non-bonded state, with subsequent dissociation into products. Since the angular part of the transition probability, according to Chapter 2, is still dependent on the mutual orientation of the Ê-vector of the initiating light beam and on the transition dipole moment d, one may expect spatial anisotropy of angular momenta distribution both in the dissociation products and in the set of molecules which remains undestroyed.

Let us start with the second effect which is most close to the ideas discussed in Chapter 3 in connection with optical polarization by ‘de-population’. Indeed, if the photodissociation process proceeds via one fixed (upper) electronic state, then, firstly, the expressions for angular transition coefficients G(θ,ϕ), as considered in Chapter 2 (see formula (2.8)), remain valid.