Characteristic x rays from multiple-electron capture by slow highly charged ${\mathrm{Ta}}^{q+}$ ions from He and Xe atoms

Characteristic x rays, emitted by slow (velocity $\nu \sim 0.3\mathrm{a.u.}$) highly charged ${\mathrm{Ta}}^{q+}$ $(q=41–49)$ ions after multiple capture from He or Xe atoms, were investigated in coincidence with recoil ions. The x-ray spectra are distinctly different for single and double capture as well as for different target ionization potentials (He and Xe). For the open $M$-shell configurations of the projectile $(q>45)$ the spectra are dominated by cascade transitions. There is no evidence for direct radiative transitions from Rydberg states into the $M$ shell, indicating dominant capture into angular momentum states that forbids such transitions. From such features we get, by comparison with spectra obtained from the calculation of the radiative cascades, the angular momentum values of the captured electron. A simple model, including autoionization, is used to relate double- and single-capture x-ray spectra and to explain their differences. The x-ray spectra with projectiles having no initial $M$ vacancies $(q<45)$ can have an important contribution from the internal dielectronic excitation (IDE) process. It is found that this channel is drastically reduced for double capture from He and closed for projectiles with $q=41$, as expected from our calculations of the core-excited states in ${\mathrm{Ta}}^{(q-1)+}$. The x-ray spectra and photon yields in these heavy systems are thus determined by a competition between the relaxation channels: radiative relaxation, IDE, and, for multiple capture, autoionization.

Figure 1

X-ray spectra for single- and double-capture events in collisions of ${\mathrm{Ta}}^{q+}$ $(q=41,43,44,46,49)$ with He, black line and grey line, respectively. (a), (b), and (c) $(q=41,43,44)$ are for a closed $M$ shell in the Ta core. (d) and (e) $(q=46,49)$ are for an open $M$ shell in the Ta core.

Figure 2

X-ray spectra for single and double-capture events in collisions of ${\mathrm{Ta}}^{q+}(q=42,43,44,46,49)$ and $Xe$ target, black line and grey line, respectively. Figures (a), (b), and (c) $(q=42,43,44)$ are for closed $M$ shell in $Ta$ core. Figures (d) and (e) $(q=46,49)$ are for open $M$ shell in $Ta$ core.

Figure 3

X-ray spectra from single- and double-capture events with normalized intensities (see text) are presented by dark and gray wiggling lines, respectively. By the solid dark line we present the Gaussian fit to the double-capture x-ray spectra. Open squares present the synthetic x-ray spectrum for one electron cascading in the ${\mathrm{Ta}}^{43+}$ core potential from the $n=9$ state.

Figure 4

X-ray spectra from ${\mathrm{Ta}}^{46+}$ single capture from He and Xe are dark wiggly lines. X-ray spectra from ${\mathrm{Ta}}^{46+}$ double capture from He and Xe are thick grey wiggly lines. Figure (b): to guide the eye, we plotted fits to the measured single and double capture spectra as solid dark and white lines, respectively.

Figure 5

(Color online) The grey wiggly lines show measured x-ray spectra for double capture. The other lines show the calculated spectra ($\mathcal{l}=1,2$ etc.). The double-capture (DC) spectra are calculated using Eq. (2), and the single-capture (SC) spectrum is calculated using Eq. (1).