Observation of indirect ionization of ${\mathrm{W}}^{7+}$ in an electron-beam ion-trap plasma

In this work, visible and extreme ultraviolet spectra of ${\mathrm{W}}^{7+}$ are measured using the high-temperature superconducting electron-beam ion trap (EBIT) at the Shanghai EBIT Laboratory under extremely low-energy conditions (lower than the nominal electron-beam energy of 130 eV). The relevant atomic structure is calculated using the flexible atomic code package based on the relativistic configuration interaction method. The grasp2k code, in the framework of the multiconfiguration Dirac-Hartree-Fock method, is employed as well for calculating the wavelength of the M1 transition in the ground configuration of ${\mathrm{W}}^{7+}$. A line from the ${\mathrm{W}}^{7+}$ ions is observed at a little higher electron-beam energy than the ionization potential for ${\mathrm{W}}^{4+}$, making this line appear to be from ${\mathrm{W}}^{5+}$. A hypothesis for the charge-state evolution of ${\mathrm{W}}^{7+}$ is proposed based on our experimental and theoretical results; that is, the occurrence of ${\mathrm{W}}^{7+}$ ions results from indirect ionization caused by stepwise excitation between some metastable states of lower-charge-state W ions, at the nominal electron-beam energy of 59 eV.

Figure 1

Spectra of tungsten obtained by SH-HtscEBIT at nominal electron-beam energies of 55, 58, 59, 70, 90, and 130 eV in the range 559–623 nm. Accumulation time of each spectrum is 2 h. Line at 574.49(3) nm is the M1 transition between the fine-structure levels in the $4f^{13}5s^{2}5p^6{}^{2}F$ ground term of ${\mathrm{W}}^{7+}$.

Figure 2

Partial energy-level diagram of ${\mathrm{W}}^{5+}$ with the lowest (in energy) 20 energy levels from the flexible atomic code calculations.

Figure 3

Partial energy-level diagram of ${\mathrm{W}}^{6+}$ including energy levels with relatively high population. Shown are one energy level belonging to the $4f^{14}5s^{2}5p^6$ configuration (black), 16 energy levels belonging to the $4f^{13}5s^{2}5p^{6}5d^1$ configuration (red), five energy levels belonging to the $4f^{14}5s^{2}5p^{5}5d^1$ configuration (green), and eight energy levels belonging to the $4f^{13}5s^{2}5p^{5}5d^2$ configuration (blue). Total populations of each configuration are marked with blue numbers, which are calculated by fac at electron-beam energy 55 eV and density $1.0\times {10}^{11}/\mathrm{c}{\mathrm{m}}^3$.

Figure 4

Partial energy-level diagram of ${\mathrm{W}}^{7+}$ with lowest (in energy) 15 energy levels from fac calculations. The red arrow represents the M1 transition between the ground configuration $4f^{13}5s^{2}5p^6$.

Figure 5

Hypothesis of charge-state evolution of ${\mathrm{W}}^{7+}$ from ${\mathrm{W}}^{4+}$. Bold red lines represent the ground level of each charge state, while lines of other colors represent the metastable level.

Figure 6

Spectra of tungsten obtained at SH-HtscEBIT with nominal electron-beam energies 70, 73, 75, and 79 eV in the EUV range 17–26 nm.

Figure 7

Experimental and simulated spectra of ${\mathrm{W}}^{7+}$ ions in the EUV range 19–21 nm. Experimental spectra are obtained at a nominal electron-beam energy of 79 eV ($E_{\mathrm{corr}}=71.9\mathrm{eV}$) while the simulated spectra are obtained by CRM at electron-beam energy 70 eV and density $1.0\times {10}^{11}/\mathrm{c}{\mathrm{m}}^3$ with energy spread 3.5 eV.