## 4 Discussion  
As mentioned above, two pulses (with spatial overlap) produced more high charge state iodine ions than a single pulse even with the same total pulse energy. The reason is that the ions generated by the first pulse and further ionized by the second pulse are associated with higher ionization potentials which are closer to the photon energy and therefore have larger ionization cross sections compared to the neutral particles. Since the life time of the iodine M1 shell is only \(\sim\)435 as [14], much shorter than the X-ray pulse duration, the suppression of every high charge states at short delay time is likely due to the depletion of valence shell electrons which hinders the Auger decay, rather than X-ray induced photoabsorption frustration caused by inner-shell electron depletion which hinders sequential photoionization [15; 16]. Indeed, with a photon energy of 1916 eV, photoionization of M1 shell electron alone can only lead to up to 1\({}^{26+}\), since the binding energy of a 3s electron of 1\({}^{26+}\) (with all valence vacancies) is \(\sim\)1680 eV [17]. At long delay times, the population in iodine high charge states increased, because the long delay times allow for the completion of the electron transfer from carbon sites to the methyl group, restocking the valence electrons at iodine needed for the Auger decay. We obtained the average charge of the atomic iodine ions for different pump-probe delay times, as shown in Figure 7a. With a single FEL pulse, the average charge of iodine is \(\sim\)7 amu; with the second pulse, a charge of \(\sim\)9 amu was established around 60 fs. The experimental results show a charge saturation at \(\sim\)30 fs. We simulated the charge evolution in a pump-probe experiment, as shown in Figure 7b. The charge by a single X-ray pulse was set to be \(\sim\)7 amu and a charge built up time of \(\sim\)30 fs was used. Since the photoionization cross section for carbon is more than one order of magnitude less than that of iodine at 1619 eV, photoionization of carbon was excluded in the simulation. Hence, the carbon charge mainly resulted from the charge transfer between the iodine and the methyl group, which can charge the carbon site up to 4+ [10; 11]. As seen in Figure 7b, the charge on carbon is not affected by the probe pulse arriving 50 fs later after the pump pulse, because the charge transfer was hindered due to the large C-I bond separation at the latter time of the molecular dissociation. From the TOF spectral peaks with single pulse, we estimated the KEs for 1\({}^{25+}\), 1\({}^{26}\), and 1\({}^{27+}\) (corresponding to horizontal lines in Figure 4, see figure caption), as well as the KEs for the carbon ions and their associated iodine ions on average. The results are presented in Table 1. Also shown in Table 1 for comparison are the KEs of carbon ions estimated from the results reported in [11]. Since the C-I bond breaks within the first 10 fs of the X-ray pulse [11], we expect the KE for C\({}^{+}\) which was dominantly produced at an early time during the FEL pulse to be similar between the experiments in current work and in [11]. This is indeed the case as shown in Table 1. However, for higher carbon charge states  
Figure 7: **(a)** Experimental average charge of iodine as a function of pump–probe delay time. The ‘0’ delay corresponds to the case of no overlap between the two X-ray beams (see text); **(b)** Simulation of charges as a function of time for different ionic species. Pulse duration of 50 fs and delay time of 50 fs were used. The \(x\)-axis is time with zero position at the onset of the first pulse. (2+, 3+), the KEs in the current work are much lower than those in [11].