Angular-momentum effects in atom-atom scattering

Abstract

Physical effects associated with change in angular momentum (in particular, $s\ensuremath{\rightarrow}p$ 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 ${p}_{\ensuremath{-}}$ states ($\ensuremath{\Delta}{L}_{\ensuremath{\perp}}=\ensuremath{-}1$) and ${p}_{+}$ states ($\ensuremath{\Delta}{L}_{\ensuremath{\perp}}=+1$) 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 ${\ensuremath{\rho}}_{\mathrm{RHC}}$ and ${\ensuremath{\rho}}_{\mathrm{LHC}}$, measured in coincidence with detection of the designated photon, are shifted with respect to one another by an amount $\ensuremath{\Delta}\ensuremath{\tau}$, where $\ensuremath{\Delta}\ensuremath{\tau}=\frac{2\ensuremath{\theta}(\frac{d\ensuremath{\tau}}{\mathrm{db}})}{D}$. Here $\ensuremath{\theta}$, $\frac{d\ensuremath{\tau}}{\mathrm{db}}$, 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 ${\ensuremath{\rho}}_{\mathrm{RHC}}$ and ${\ensuremath{\rho}}_{\mathrm{LHC}}$, rather than the Stokes parameter ${P}_{3}$. 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, ${\mathrm{Mg}}^{+}$ -He, and ${\mathrm{Mg}}^{+}$-Ar collision systems. The shifts $\ensuremath{\Delta}\ensuremath{\tau}$ between ${\ensuremath{\rho}}_{\mathrm{RHC}}$ and ${\ensuremath{\rho}}_{\mathrm{LHC}}$ at 1-keV incident energy are 17.4 keV deg for Li-Ne, 5.1 keV deg for ${\mathrm{Mg}}^{+}$-He, and 4.3 keV deg for ${\mathrm{Mg}}^{+}$-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.