Angular-momentum effects in atom-atom scattering

Physical effects associated with change in angular momentum (in particular, s→p transitions) caused by collision-induced excitation at a level crossing in kilovolt-energy ion-atom or atom-atom scattering are investigated. The component of angular momentum involved in this effect is that perpendicular to the plane of collision. The excitation of p_- states (ΔL_⊥=-1) and p₊ states (ΔL_⊥=+1) can be distinguished experimentally by the emission of right-handed (RHC) and left-handed (LHC) circularly polarized photons emitted in a direction perpendicular to the collision plane. It is shown that the reduced differential scattering cross sections ρRHC and ρLHC, measured in coincidence with detection of the designated photon, are shifted with respect to one another by an amount Δτ, where Δτ=2θ(dτdb)D. Here θ, dτdb, and D are, respectively, the angular velocity of trajectory motion, derivative of the deflection function, and slope of the ground-state-excited-state energy difference, all calculated at the level crossing. It is suggested that the more appropriate measurements to make experimentally are ρRHC and ρLHC, rather than the Stokes parameter P₃. The effect should be able to provide an experimental determination of the slope of the energy difference at the level crossing. Calculations have been made on the Li-Ne, Mg⁺ -He, and Mg⁺-Ar collision systems. The shifts Δτ between ρRHC and ρLHC at 1-keV incident energy are 17.4 keV deg for Li-Ne, 5.1 keV deg for Mg⁺-He, and 4.3 keV deg for Mg⁺-Ar systems. The shifts increase in direct proportion to the square root of the incident energy in keV. They are easily measurable even at energies below 1 keV.