Ion-Photon Angular Correlations in Slow Atomic Collisions

Abstract

Detailed information about atomic collision processes may be obtained from angular correlation studies. A typical example for such an experiment is the charge exchange process $$ {{H}^{ + }} + Ar \to H\left( {2p} \right) + A{{r}^{ + }} $$ where H(2p) (lifetime 1.6 ns) decays after the collision to its ground state by emission of Lyman-α photons. As a result of the collision process the spatial distribution of the angular momenta of the excited H(2p) state in general is non-isotropic; information about this anisotropy may be obtained from a measurement of the angular distribution or by polarization analysis of the emitted photons. The anisotropy of the excited state is conveniently described in terms of state multipoles1. The so-called orientation (dipole) vector alignment (quadrupole) tensor (Q = 0,1,2)2 are related to mean values of an orbital angular momentum operator L by1: $$ \begin{array}{*{20}{c}} { / = \sqrt {{3/2}} } \\ { / = \sqrt {{1/2}} \left( { } \right)} \\ { / = \sqrt {{3/2}} \left( { } \right)} \\ { / = \sqrt {{3/2}} \left( { } \right)} \\ \end{array} $$ where the monopole \( = \sigma \left( {2p} \right)\sqrt {3} \) , and σ(2p) the differential cross section.