Collisional alignment and orientation

There are two equivalent experimental arrangements for determining the alignment parameters of an excited atom At in the process 

The first arrangement uses photon—electron coincidence techniques. 

The second, time-reversed, process uses scattering of electrons by atoms optically pumped by a laser, i.e. 

Both methods can give identical information on Ai excited in process (8.1). The schemes for measuring directional correlations between two particles resulting from a collision process by coincidence techniques are discussed in some detail in chapter 2. Similarly details of optical pumping techniques can also be found in chapter 2. 

We now introduce the general concepts and definitions behind studies of alignment and orientation in electron—atom collisions. Let us consider the scattering process (8.1) in some detail. We must first understand the concept of coherence. We illustrate it by considering the initial state of the atom 

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So far we have discussed various studies on the relaxation of the population bPo of excited state rovibrational levels in elastic and inelastic collisions. To this end the intensity of fluorescence was measured in one or the other way. If an analysis of the state of polarization of the radiation is performed, one may obtain information on the behavior of alignment and orientation of the molecular angular momenta in elastic and inelastic collisions. If we register, under collisional conditions, the polarization properties of a directly laser-induced rovibrational level of the molecule, then, according to (2.35) and (2.39), it is possible to determine the rates... 

Manabe, T., Yabuzaki, T. and Ogawa, T. (1981). Observation of collisional transfer from alignment to orientation of atoms excited by a single-mode laser, Phys. Rev. Lett., 46, 637-640. 

Two recent papers [1,2] provide updated views of advances in the production of intense and continuous beams of aligned molecules. In [1], it was demonstrated that in the prototypical case of a seeded supersonic expansion of a molecular beam of benzene, besides acceleration and cooling, orientation of the molecular plane also occurs because of the anisotropy of the intermolecular forces which govern collisions. This work is reviewed in Sec.2.1. Previous studies on the collisional alignment of the rotational... 

Though at high E/N the ion transport depends in a complex way on collision integrals of various orders (2.2.1), all those quantities in classic transport theory are orientationally averaged. This implies free rotation of ions regardless of E, which is true only for atomic ions or at sufficiently low E. Polyatomic ions in gas can be oriented with respect to E by two unrelated mechanisms—collisional alignment considered here and dipole alignment (2.7). 

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