Figure 1

The energy levels and transitions of Fe XVI (Fe15+) and Fe XVII (Fe16+). The bold red arrows represent the excitation process of the XFEL. The bold black arrows represent the C and 3D emission lines. The thin black arrows represent some minor decay channels of the 2p43/22p1/23s3d5/2 J = 3/2 resonant state. The blue dash arrow represents the autoionization from the Fe XVI. The red horizontal dash line represents the first ionization threshold at 489.31 eV of Fe XVI.