Fig. 1. Black line: the experimental spectrum of Fujioka et al.^[13]. Blue line: the theoretical result of an optically thin model. f, Li, i and r denote the position of the forbidden line, satellite lines, the intercombination line and the resonance line, respectively.

Fig. 2. Comparison of the collisional excitation cross-section with the results of Refs. [26–29]. The transitions include 1s2s ³S 1s2p ¹P₁, 1s2s ³S 1s2s ¹S₁, 1s2p ³P 1s2p ¹P₁ and 1s2s ¹S 1s2p ¹P₁ of He-like Si.

Fig. 3. Contributions of atomic processes under the conditions of the Fujioka et al. photoionization experiment for five selected levels. The atomic processes are listed on the x-axis. The blue processes are related to the radiation field, the dark-gray processes are controlled by collisions and the green processes are autoionization and dielectronic capture. Solid black lines represent the populating contributions for the levels, and the red dashed lines represent the depopulating contributions for the levels. The contribution friction of each process is also labeled.

Fig. 4. Evolution of fractions of charge states from C-like ion to bare nuclei in the time-dependent model with a Gaussian radiation pulse (red line). The radiation pulse is adapted as a Gaussian distribution with FWHM of 160 ps and =80 ps^[13,16,20].

Fig. 5. Evolution of process contributions to 1s2p ¹P₁. Upper panel: populating contributions. Lower panel: depopulating contributions. The radiation pulse is also plotted.

Fig. 6. Black line: the experimental spectrum of Fujioka et al.^[13]. Green line: theoretical spectrum of time-dependent model. Red line: theoretical spectrum of optically thick model.

Table 1. Transition energies and Einstein A coefficients of some intense silicon lines between 1820 eV and 1865 eV. The resonance, intercombination and forbidden lines are marked as , and , respectively.