Nonstatistical enhancement of the 1s2s2p ⁴P state in electron transfer in 0.5–1.0-MeV∕u C^4,5+ + He and Ne collisions

Nonstatistical enhancements for formation of the metastable 1s2s2p ⁴P state compared to the similarly configured 1s2s2p ²P₋ and 1s2s2p ²P₊ states are observed following single electron transfer to C⁴⁺(1s2s ³S) and double electron transfer to C⁵⁺(1s) ions, respectively. Previously, similar enhancements were observed in the population of the ⁴P metastable state resulting from electron transfer in ∼1 MeV∕u F^7,8++He and Ne collisions [ J. A. Tanis et al. Phys. Rev. Lett. 92 133201 (2004)]. The enhancements were attributed to a dynamical Pauli exchange mechanism involving projectile and target electrons having the same spin alignment. Recently, it was suggested that the enhancement might be due to cascade effects following electron transfer to states with n>3. To further understand the observed enhancement of the ⁴P state, new investigations of Auger emission spectra in collisions of 0.5–1 MeV∕u C^4,5+ with He and Ne targets have been conducted. Experimental ratios of the resulting ⁴P intensities to the sum of the ²P₋ and ²P₊ intensities for the collision systems studied give values three to four times larger than expected based solely on spin statistics for single transfer to C⁴⁺(1s2s ³S) and double transfer to C⁵⁺(1s). Theoretical calculations suggest that about half of the observed enhancement is caused by cascade effects.