Strong orientation effects observed in the δ-electron emission probability in fast ion-atom collisions

The δ-electron emission probability d3P(b, Ee, Φe, qf)/dEedΦedqf as a function of the electron energy Ee, impact parameter b, and projectile final charge state qf has been measured for 0.53 MeV/u F8+ + Ne at an electron emission angle Φe = 00 and d2P(b, Ee, Φe)/dEedΦe for 0.25 MeV/u He2+ + Ne at Φe = 900, and are compared to previous results for 0.53 MeV/u F8+ + Ne at an electron emission angle Φe = 900. For both collision systems at Φe = 900 a minimum in the emission probability d2P towards small b has been found and both shapes of the b-dependence appear quite similar, whereas for F on Ne a dramatic change in the relative b dependence of the probability d2P between Φe = 900 and Φe = 00 has been observed. All measured dependences can be explained qualitatively in the framework of strong collisionally oriented Ne-L substrates. (orig.)