Kinematically complete study of electron transfer and rearrangement processes in slow ${\mathrm{Ar}}^{16+}$-Ne collisions

The complete kinematics of single- and double-electron capture from neon to ${\mathrm{Ar}}^{16+}$ was measured with a reaction microscope at a projectile energy of 3.2 keV/u (velocity $v_p=0.36$ a.u.). Not only the change of the electronic binding energies (the $Q$ value) and the projectile scattering angles, but also (in the case of auto-ionization) the three-dimensional momentum vectors of the emitted electrons were determined. For single-electron capture, the $Q$-value spectrum shows strong population of both $n=7$ and 8 states on the projectile, and weak contributions to $n=6$ and 9 are also observed. In the case of double-electron capture, auto-ionizing double capture (ADC) dominates and the populations of $(n,n^{\prime })=(5,7),(6,6),(6,7)$ and $(6,8)$ are observed, while true double capture (TDC) populates the $(5,7)$ state and asymmetric states of $(5,n^{\prime })$ with $n^{\prime }>10$. The experimental cross sections for Auger decay with the electron energy $E_e$ plotted as a function of the $Q$ value suggest the occurrence of target excitation accompanying the population of configurations $(5,7)$ and $(6,6)$. No essential difference is found in the differential cross sections for ADC and TDC, and the angular distributions suggest that two-step processes dominate the double capture.

Figure 1

$Q$-value spectrum for SEC in ${\mathrm{Ar}}^{16+}\text{-Ne}$ collisions at 3.2 keV/u. The top numbers represent the principal quantum number of the final projectile state ${\mathrm{Ar}}^{15+}\left(n\right)$. The boxes indicate the $Q$ values based on the MCHF calculation for the binding energies, with their widths related to the respective $l$-splitting.

Figure 2

Partial cross sections for the SEC channels in ${\mathrm{Ar}}^{16+}\text{-Ne}$ collisions at 3.2 keV/u, compared with the predictions of a MCLZ model.

Figure 3

Differential cross sections for SEC channels in ${\mathrm{Ar}}^{16+}\text{-Ne}$ collisions at 3.2 keV/u. The arrows show the locations of the half Coulomb angles. The solid lines represent the calculations of a MCLZ model by using the method of Andersson et al. [32]. They were folded with the present angular resolution and normalized to approximately match the experimental peak heights.

Figure 4

Doubly differential cross sections for (a) TDC and (b) ADC as a function of the $Q$ value and scattering angle in ${\mathrm{Ar}}^{16+}\text{-Ne}$ collisions at 3.2 keV/u. The solid lines denote the locations of the half Coulomb angles. The three rows of boxes at the top indicate the expected $Q$-value ranges for the capture in the $(n,n^{\prime })$ states with $n=7$ (top row), $n=6$ (middle row), and $n=5$ (bottom row) [33]. For each row, the left-most box corresponds to an $n^{\prime }$ as given by the number and it increments towards higher $Q$ values. The observed $(n,n^{\prime })$ states are identified at the bottom of each graph. ${(n,n^{\prime })}^{*}$ and ${(n,n^{\prime })}^{**}$ represent DEC associated with target excitation. See the text for the small squares, triangles, and circle.

Figure 5

Differential cross section for ADC as a function of the $Q$ value and electron kinetic energy in the projectile frame in ${\mathrm{Ar}}^{16+}\text{-Ne}$ collisions at 3.2 keV/u. The numbers in parentheses represent the main quantum numbers $(n,n^{\prime })$ of the two transferred electrons in the doubly excited projectile state, while ${(n,n^{\prime })}^{*}$ and ${(n,n^{\prime })}^{**}$ denote DEC associated with target excitation. See text for the lines.

Figure 6

Experimental $Q$-value spectra for (a) TDC and (b) ADC in ${\mathrm{Ar}}^{16+}\text{-Ne}$ collisions at 3.2 keV/u. The numbers above represent the stabilization ratios. The solid curves show the Gauss fits for individual $(n,n^{\prime })$ shells. The numbers in parentheses represent the main quantum numbers $(n,n^{\prime })$ of the two transferred electrons in the doubly excited projectile state, while ${(n,n^{\prime })}^{*}$ and ${(n,n^{\prime })}^{**}$ denote DEC associated with target excitation.

Figure 7

Coulomb potential curves for the system ${\mathrm{Ar}}^{16+}$-Ne. Dashed and solid curves are related to single and double capture, respectively. The circle, squares, and triangles show two-step transitions via $n=6$, 7, and 8, respectively.

Figure 8

Doubly differential cross sections for the population of (a) ${(5,7)}^{**}$, (b) ${(6,6)}^{*}$, and (c) $(6,7)$ as a function of the $Q$ value and scattering angle in ${\mathrm{Ar}}^{16+}\text{-Ne}$ collisions at 3.2 keV/u. Events related to (a) ${(5,7)}^{**}$, (b) ${(6,6)}^{*}$, and (c) $(6,7)$ were picked out in coincidence with an electron energy range (in projectile frame) of 0–19, 19–38, and 38–58 eV (see Fig. 5), respectively.

Figure 9

Differential cross sections for TDC $(•)$ and ADC $(△)$ in ${\mathrm{Ar}}^{16+}\text{-Ne}$ collisions at 3.2 keV/u. The experimental curves have been normalized independently to a peak height of unity. The numbers in parentheses represent the main quantum numbers $(n,n^{\prime })$ of the two transferred electrons in the doubly excited projectile state, while ${(n,n^{\prime })}^{*}$ and ${(n,n^{\prime })}^{**}$ denote DEC associated with target excitation.