New evidence for resonant-transfer-and-excitation for 16S, 20Ca, and 23V ions colliding with helium

Significant new evidence is presented for resonant-transfer-and-excitation (RTE) in ion-atom collisions. This process occurs when a target electron is captured simultaneously with excitation of the projectile followed by deexcitation via photon emission. RTE, which is analogous to dielectronic recombination (DR), proceeds via an inverse Auger transition, and is expected to be resonant for projectile velocities corresponding to the energy of the ejected electron in the Auger process. Cross sections for projectile K x-ray emission coincident with single electron capture have been measured for 70 to 160 MeV 16S13+, 100 to 360 MeV 20Ca16+17+18+ and 180 to 460 MeV 23V19+20+21+ ions colliding with helium. Strong resonant behavior, in agreement with theoretical calculations of RTE, is observed as a function of beam energy in the measured cross sections. For Ca and V ions two maxima are observed in the energy dependence of the measured coincidence cross sections. Theoretical calculations show that these maxima are correlated to two groups of intermediate resonant states in the RTE process for which the excited and captured electrons occupy levels with principal quantum numbers n = 2,2 and n = 2, greater than or equal to 3. 13 references, 6 figures