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We discuss intuitive concepts to describe alignment and orientation effects in collision processes with, or leading to, an atomic np state. For direct excitation one can understand the atomic angular momentum transferred in terms of a rolling ball, and for excitation (de-excitation) in a molecular picture one can 'visualise the alignment angle of the atomic p charge cloud in terms of a transition from a body-fixed molecular picture (small internuclear distances R ) to a space-fixed picture (large R ) . These concepts are illustrated by experimental results for e + Na" and Na+ + Na" collisions.Semiclassical theory is discussed for both the direct and the molecular inelastic processes, giving a theoretical foundation for these models. Detailed results are reported for the time development of the charge cloud in Na++Na* collisions as a model case, illustrating the concept of body-fixed versus space-fixed electron motion and its limitations. Further examples are the molecular process NZ+Na* and the atomic process Xe + Ba" at thermal energies. In all cases long-range rotational (E-II) coupling determines the charge cloud motion.

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The reactions of electronically excited sodium have been studied using the crossed molecular beams method. The apparatus and optical pumping are described in chapter I.Chapter II describes the reactions of Na with the hydroqen halides. The reactions of Na(3S,3P,4D,5S) + HCl have been studied in detail. A large increase in the reactive cross section with electronic energy is observed. A change in the reaction mechanism leads to very different product scattering distributions for Na(3P) vs. Na(4D,5S) scattering. Laser polarization dependences show that this is due to long range electron transfer in Na(4D) + HCl that does not occur for Na(3P) + HCl. No reaction was observed for Na(3S,3P,4D) + HF up to collision energies of 13 kcal/mole. Na(3S,3P) + HBr behave ~uch like the equivalent HCl reactions, except that the reduced endothermicity leads to more ground state reaction for HBr. The reactions of Na(3S,3P,4D) + CH 3 Br are described in chapter IV.As in all previously studied alkali plus alkyl monohalide reactions, the. alkali halide product is predominantly back scattered due to steric restrictions. With increasing electronic excitation, a small relaxation of the restriction is observed, leading to NaBr scattered to lower center-of-mass angles than for the ground state reaction.The reactions of Na{3S,3P) + Cl 2 are described in chapter V. At a collision energy. of 6 kcal/mole, the reactive cross section increases 60% on electronic excitation. while at a hiqher collision energy, 19 kcal/mole, there is an increase of only 16% • A stripping mechanism explains the very similar scattering distributions of each state. r-).

Representative families of excited alkali-metal reactions have been studied using a crossed-beam apparatus. For those alkali-metal-molecule systems in which reactions are also known for ground-state alkali metal and involve an early electron-transfer step, no large differences are observed in the reactivity as Na is excited. More interesting are the reactions with hydrogen halides (HCl); it was found that adding electronic energy into Na changes the reaction mechanism. Early electron transfer is responsible for Na(5S, 4 0 ) reactions, but not for Na(3P) reactions. Moreover, the NaCl product scattering is dominated by the HC1-repulsion in Na(5S, 4 0 ) reactions, and by the NaCl-H repulsion in the case of Na(3P). The reaction of Na with O2 is of particular interest since it was found to be state-specific. Only Na(4D) reacts, and the reaction requires restrictive constraints on the impact parameter and the reactants' relative orientation. The reaction with NO2 is even more complex, since Na(4D) leads to the formation of NaO by two different pathways. However, the identification of NaO as product in these reactions has yet to be confirmed.