Observation of electric octupole emission lines strongly enhanced by the anomalous behavior of a cascading contribution

We present extreme ultraviolet spectra of Ag-like ${\mathrm{W}}^{27+}$ observed with an electron beam ion trap. In the spectra, the $4f_{7/2,5/2}–5s$ electric octupole transitions are identified. Our theoretical investigation shows that the emission line intensity is strongly and specifically enhanced at the atomic number 74 by the anomalous behavior of the cascading contribution to $5s$ via $5p\leftarrow 5d$.

Figure 1

EUV spectra of tungsten ions obtained with a compact electron beam ion trap with electron energies of 770, 800, and 870 eV. The upper panel shows the CR model spectrum for ${\mathrm{W}}^{27+}$. The intensity in the upper panel is defined as the product of the $5s$ fractional population (dimensionless) and the $E3$ transition probability (${\mathrm{s}}^{-1}$).

Figure 2

Atomic number dependence of the transition probability (blue) and the intensity (red) of the $4f_{7/2}$–$5sE3$ transition. The definition of the intensity is the same as that in Fig. 1. It is noted that the ground state is $5s$ and the upper state is $4f$ for $Z<62$, whereas the ground state is $4f$ and the upper state is $5s$ for $Z\geqq 62$.

Figure 3

(a) Inflow and depopulation rates for the metastable $5s$ level (left axis). Fractional population of the $5s$ level determined by $F_{\mathrm{in}}/f_{\mathrm{out}}$ is also plotted (right axis). See also Eq. (1). (b) Branching ratios to $5p$ in the decay of the $5d_{3/2}$ (open diamond) and $5d_{5/2}$ (closed diamond) levels.

Figure 4

(a) Energy level diagram of $5d_{3/2}$ (connected blue bars) and ${\left(4d^{9}4f^2\right)}_{3/2}$ (red bars) states. The ${\left(4d^{9}4f^2\right)}_{3/2}$ state has 15 energy levels. (b) Reduced matrix elements of $E1$ transition between $5d_{3/2}$ and $4f_{5/2}$ states. Open circles show $5d_{3/2}$ (blue) and ${\left(4d^{9}4f^2\right)}_{3/2}$ (red) components, respectively. The blue solid circles are the sum of the two components.

Figure 5

Calculated energy levels and the population flows for Ag-like ${\mathrm{W}}^{27+}$. An electron energy of 870 eV and a density of ${10}^{10}{\mathrm{cm}}^{-3}$ are assumed in the CR model calculation. The solid and open arrows represent radiative and collisional flows, respectively. The number associated with each arrow represents the flow in ${\mathrm{s}}^{-1}$.