## 3 Results
### Evolution of KE Distribution  
As seen in Figure 1a, with a single pulse, the KE for high charge states (I\({}^{12+}\)-I\({}^{18+}\)) has a broad distribution, shown as a flat top shape in the TOF spectrum. As the delay time increases, the abundance of ions with small KE increased, adding signal on top of the flat top, eventually leading to a sharp dome  
Figure 4: Ion TOF spectra as a function of pump–probe delay time. The data were from Figure 2a with the \(x\)-axis plotted in TOF. A background trace obtained for the spectrum at 20 fs was subtracted from all spectra. The spectrum at 20 fs delay was magnified by \(\times\)6 for purpose of clarity for presentation. The ’0’ delay corresponds to the case of no overlap between the two X-ray beams (see text). At the top of the figure, the positions for the charge states of atomic iodine were marked and labeled. The red horizontal lines indicate the TOF spread of the forward and backward peaks of I\({}^{25+}\), I\({}^{26+}\), and I\({}^{27+}\), identified in the spectrum at 20 fs or single pulse.  
Figure 3: Ion yield as a function of pump–probe delay time for various iodine charge states. Data were extracted from TOF spectra in Figure 2. The ion yield with a single X-ray pulse was used as a reference spectrum and subtracted from the iodine charge states at all other delay times.  
shape, as seen at long delay times. As to extremely high charge states of iodine (I\({}^{20+}\)-I\({}^{29+}\)), a sharp distribution starts at short delay time. For low charge states (shown in Figure 1b), the distribution is relatively sharp even at single pulse or short delay time, indicating small average KEs. Similarly seen in Figure 2a, the broad KE distribution with a single pulse is represented by an evenly colored rectangular region at around zero delay time. As more low KE ions were detected at longer delay times, the shape of distribution becomes sharper, forming slim columns (see Figure 2).  
We extract the ion KE from the TOF spectra with the method described in Section 2. Figure 5 shows the KE of I\({}^{14+}\) as a function of pump-probe delay time. Each KE spectrum was normalized to its maximum. The map was interpreted without significant distortion of the distribution to smooth out the uneven delay-time steps. The ion species of I\({}^{14+}\) was selected as an example to show the evolution of the KE distribution and other high charge state species show a similar trend (see Figure 1). The difference between the distribution obtained from the forward ions and backward ions is mainly due to the uneven base line caused by spectral overlap with the adjacent peaks. Figure 6 shows the weighted KE averaged over the forward and backward ions. At small delay time, the KE of I\({}^{14+}\) shows a broad distribution with an average KE of 10 eV and a width of \(\sim\)15 eV. At about 60 fs delay time, the KE distribution approaches an asymptotic value of \(\sim\)8 eV with a width of \(\sim\)8 eV.  
Figure 5: Ion KE distribution of I\({}^{14+}\) as a function of pump–probe delay time at a photon energy of 1619 eV and a pulse duration of 50 fs. The color represents the ion signal strength which is normalized to the signal maximum at each delay time. The ‘0’ delay corresponds to the case of no overlap between the two X-ray beams (see text). (**a**) Ions with the direction of the initial velocity towards the detector (‘forward ion’); (**b**) Ions with the direction of the initial velocity against the detector (‘backward ion’).  
Figure 6: Weighted KE averaged over the forward and backward I\({}^{14+}\) ions as a function of pump–probe delay time, extracted from data shown in Figure 5. The ‘0’ delay corresponds to the case of no overlap between the two X-ray beams (see text).