Observation of an extremely-long-lived metastable level in a Ti-like system via an $L$-shell dielectronic recombination measurement in highly charged $3d^n$ ions of tungsten

In this paper we report the L-shell dielectronic recombination measurement in highly charged $3d^n$ ions of tungsten by employing a fast electron beam-energy scanning technique at Shanghai EBIT. The studies of the LMM DR resonance strengths of Ar-like up to Mn-like tungsten were implemented through the experiment as well as a fully relativistic configuration interaction method in the flexible atomic code. In the analysis of DR spectrum, an isolated resonant peak was discovered coming from the DR processes via an extremely-long-lived metastable level in a Ti-like system. This work indicated the population of this Ti-like metastable level in the present EBIT plasma condition was as large as 45%.

Figure 1

2D scatter plot of x-ray intensity as a function of electron beam energy ($x$ axis) and photon energy ($y$ axis).

Figure 2

The recombination excitation function of Cl- up to Fe-like LMM DR resonances. (a) The experimental curve (black) and fitted curve (red) without metastable level contribution. (b) The DR resonances through the ground (black solid) and metastable level (red dash) of Ti-like. (c) The experimental curve (black) and fitted curve (red) with Ti-like metastable level contribution. The gray dashed lines show the contribution of $n=3$ RR.

Figure 3

Levels diagram for the ground ${\text{[core]}}^{\prime }\left(3d_{3/2}^4\right)$ and first excited configuration ${\text{[core]}}^{\prime }\left(3d_{3/2}^{3}3d_{5/2}\right)$ of Ti-like W ions.

Figure 4

The electron density dependence for the population of the ground and first excitation configurations of Ti-like W ions with the electron impact energy of 7.9 keV. The shadow shows the possible electron density of ${10}^{11}–{10}^{12}{\text{cm}}^{-3}$ in Shanghai EBIT.