Polarization effect in stopping of swift partially screened heavy ions: Perturbative theory

We report a calculation of the polarization correction - also called Barkas or Z(1)(3) effect - to the electronic energy loss of screened heavy ions a bove the Bohr velocity. For distant collisions we extend the classical calc ulation of Ashley et al. (Phys. Rev. B 5 (1972) 2393) to a screened project ile, while for close collisions we make use of an argument by Lindhard. The variation of energy loss with impact parameter is illustrated for Li on C for different charge fractions and projectile velocities upsilon. Stopping numbers are shown at fixed charge fraction as a function of velocity, and f or a velocity-dependent equilibrium charge. The polarization correction is found to be sizable over most of the Velocit y range covered by Bohr theory (the classical regime), but for the three io ns for which results are reported. Li, C and Ar, very similar curves are fo und when stopping numbers are plotted against the Bohr parameter m upsilon( 3)/Z(i)e(2)omega, where omega is a characteristic resonance frequency of th e target. The polarization correction is similar in magnitude to the leading term ove r a wide velocity range below m upsilon(3)/Z(1)e(2)omega similar or equal t o 3. Therefore the perturbative approach presented here, which includes onl y one order beyond the leading term. cannot be expected to deliver very acc urate results. This work will therefore be followed up by a nonperturbative evaluation of the stopping cross-section. (C) 2000 Elsevier Science B.V. A ll rights reserved.