Subshell-resolved electron capture in ${\mathrm{O}}^{4+}$-He collisions near Bohr velocity

Subshell-resolved single-electron capture has been investigated in collisions between ${\text{O}}^{4+}$ ions and He atoms at energies from 7 to 70 keV/nucleon, using high-resolution cold-target recoil-ion momentum spectroscopy. Analysis of the $Q$-value spectra, $n$-shell dependence, relative branching ratios, and angular distributions provides insight into the dominant reaction channels. Capture into the $n=3$ state remains dominant in the entire projectile energy, agreeing with predictions from the molecular Coulombic over barrier model. At low energies, capture preferentially populates the $3s$ subshell, while higher $l$ states like $3d$ become dominant at higher projectile energies. The oscillatory transverse momentum distribution of $3s$ states is attributed to de Broglie wave Fraunhofer diffraction, indicative of the quantum nature of the collision dynamics.

Figure 1

Two-dimensional momentum and longitudinal distributions for single-electron capture in ${\text{O}}^{4+} +$ He collisions at a projectile energy of 7 keV/nucleon. The black, red, and blue text indicates capture into the ground state ${\text{O}}^{4+}$ (${2s}^2$), capture into the metastable state ${\text{O}}^{4+}$ ($2s2p {}^{3}P$), and capture into the ground state with transfer excitation processes, respectively.

Figure 2

$Q$-value spectra for single electron capture in ${\text{O}}^{4+} +$ He collisions at varying projectile energies. The dots denote the experimental data, while the blue dashed curves illustrate the reaction windows calculated using the MCBM method. The curves in various colors result from multipeak Gaussian fits with fixed peak positions. The vertical lines indicate the $Q$-values for electron capture into distinct $n$ states.

Figure 3

Relative branching ratios plotted as a function of projectile energy for single electron capture in ${\text{O}}^{4+} +$ He collisions, with (a) resolution by $n$ shells and (b) subshell resolution for the $n=3$ state. The data points (dots) correspond to experimental measurements, while the solid lines depict the results obtained from the MCBM.

Figure 4

Experimental transverse momentum distributions for electron capture into the $3s, 3p$, and $3d$ states at various projectile energies. The blue dashed line indicates the position of the dark fringe in the Fraunhofer-type diffraction pattern. The red dashed line marks the position of the first bright fringe, which is inferred from the location of the first dark fringe in the Fraunhofer diffraction pattern.