Experimental and theoretical study of double-electron capture in collisions of slow C 4 + ( 1 s 2 S 1 ) with He ( 1 s 2 S 1 )

The doubly differential cross sections (DDCS) of double electron capture in collisions of slow ${\mathrm{C}}^{4+}$ with He were measured in the energy range of 240, 320, and $440\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ (in laboratory frame) using a crossed-beam apparatus and represented as a two-dimensional contour map. The double-electron capture into the ${\mathrm{C}}^{2+}(1{s}^{2}\phantom{\rule{0.2em}{0ex}}2{s}^{2}^{1}S)$ state was found to be dominant in the present energy range. The St\"uckelberg oscillation structures, which result from interference among two different paths on the interaction potential curves, were clearly observed in the DDCS at the present low impact energies. The differential cross sections were also calculated in an ab initio molecular orbital framework for ${(\mathrm{C}\mathrm{He})}^{4+}$, and show a good agreement with the present experimental results.