Orientation in static inhomogeneous fields

For polar polyatomic molecules one has to ascertain the orientation of molecular symmetry axes (x-axis in Fig. 1.4 for a symmetric top molecule). As was established by Novakoski and McClelland [299] it is more likely for a CF3H molecule to desorb from an Ag(lll) surface with the F end of the symmetry axes pointing directly away from the surface, and a degree of orientation of 0.27 (averaged value of (cos )) was observed. 

So far we have considered various radiational and collisional mechanisms of polarization of molecules. There exist earlier methods applying the action of an external stationary magnetic, later of an external electrostatic, field to beam molecules for producing anisotropic distribution of the angular momentum J and of the molecular axis. 

From the shape of the resonances the rotational magnetic moment values were determined for the simplest diatomics H2, D2, HD and later on for the alkali dimers Li2, Na2, Cs2 (see [87]), and others (see also [155, 294]). The drawbacks of the method were connected with the complexity of the set-up and the lack of selection over vibrational and rotational states. 

Let us consider this case in some detail. If collisions are eliminated in a molecular beam, it is possible to orient molecules (their figure axis) by removing the particles possessing unwanted orientation (analogous to the Stern-Gerlach experiment with a magnetic field). Then, classically, the interaction energy with external electric field is simply 

As an example one may again mention a reaction of type (6.13). In particular, the authors of [210] studied the steric effect in the reactive collisions of a beam of excited metastable Ca (1 2) atoms with oriented symmetric top molecule CH3F for the reaction  

---