## 4 Discussion  
As shown in the polycyclic structure of Figure 2a-g, it is clear that fragment ions with the same charge state come from different reaction channels. The corresponding KER values have been extracted and labeled in Figure 2a'-g'. It can be seen that all these ion kinetic energy distributions have both high and low kinetic energy components. Generally, the multiphoton dissociative ionization (MPDI) process would produce fragment ions with low KER (<0.5 eV), while the Coulomb explosion (CE) process would produce fragment ions with high KER [44]. Here, we concentrate on these channels in the CE processes. \[\mathrm{KER(A^{P+})/KER(B^{H+})=M(B^{H+})/M(A^{P+})}, \tag{1}\]  
\[\mathrm{KER\_total=14.4 pq/r}, \tag{2}\]  
In the classical two-body CE model, two fragmented ions should satisfy momentum conservation, as explained by Equation (1), where \(\mathrm{A^{P+}}\) and \(\mathrm{B^{H+}}\) represent the two fragment ions, \(\mathrm{p/q}\) denote the charge states, and M stand for the mass of the fragment ions. Equation (2) represents the total KER of a CE channel, where r indicates the internuclear distance when the CE starts. Based on the equations given above, the possible CE channels are listed in Table 1.  
The channel (n, m) represents the predicted CE process, \(\mathrm{IBr^{(n+m)+}\to\mathrm{I^{n+}+Br^{m+}}}\). The energy collected in the previous ionization step is included in the KER that accumulates from each channel, as seen by the R\({}_{\mathrm{c}}\) changing with the charge states in Table 1. It is generally believed that the time for the double ionization to occur is more than a dozen fs. On this time scale, the molecular structure (e.g., the distance between nuclei) does not change. Therefore, the starting point of multi-electron ionization or the site where the third electron is ionized should still be at the equilibrium internuclear distance (R\({}_{\mathrm{e}}\) = 2.47 A). Next, some theoretical calculations are performed to verify the above assumptions. Figure 3 shows the dissociation potential energy (PE) curves of the ground electronic state and excited states of \(\mathrm{IBr^{2+}}\) ion where the PE curves of the ionic \(\mathrm{IBr^{2+}}\) are calculated at the def2 TZVP/EOM-CCSD level and the pink rectangles show the \(\pm\)0.05 A region around the R\({}_{\mathrm{e}}\). The charge-repulsive (CP) curves, as described by Equation (2), are also included. Similar to the relationship shown in ref. [45], the PE curve and CP curve will overlap when the nuclei are spread far apart enough. Here, we have defined E(\(\infty\)) = 0 eV, so that the green line (3.78 eV) represents the measured KER of the channel (1, 1). If \(\mathrm{IBr^{2+}}\) were to experience \begin{table}
\begin{tabular}{c c c c c c} \hline \hline (n, m) & **CE Channels** & **KER\_P\({}^{m+}\) (eV)** & **KER\_B\({}^{m+}\) (eV)** & **KER\_total (eV)** & **R\({}_{\mathrm{c}}\) (Å)** \\ \hline (1, 1) & \(\mathrm{IBr^{2+}\to\mathrm{I^{+}+Br^{+}}}\) & 1.58 & 2.20 & 3.78 & 3.81 \\ (1, 2) & \(\mathrm{IBr^{2+}\to\mathrm{I^{+}+Br^{2+}}}\) & 3.25 & 4.32 & 7.57 & 3.80 \\ (2, 1) & \(\mathrm{IBr^{2+}\to\mathrm{I^{+}+Br^{2+}}}\) & 2.72 & 4.47 & 7.19 & 4.01 \\ (2, 2) & \(\mathrm{IBr^{2+}\to\mathrm{I^{+}+Br^{2+}}}\) & 5.40 & 8.12 & 13.52 & 4.26 \\ (2, 3) & \(\mathrm{IBr^{5+}\to\mathrm{I^{+}Br^{2+}}}\) & 7.21 & 12.24 & 19.45 & 4.44 \\ (3, 1) & \(\mathrm{IBr^{4+}\to\mathrm{I^{+}Br^{+}}}\) & 4.26 & 6.39 & 10.65 & 4.06 \\ (3, 2) & \(\mathrm{IBr^{5+}\to\mathrm{I^{+}Br^{2+}}}\) & 7.49 & 11.43 & 18.92 & 4.57 \\ (3, 3) & \(\mathrm{IBr^{5+}\to\mathrm{I^{+}Br^{2+}}}\) & 10.28 & 17.04 & 27.32 & 4.74 \\ (4, 2) & \(\mathrm{IBr^{6+}\to\mathrm{I^{+}Br^{2+}}}\) & 9.88 & 15.07 & 24.95 & 4.62 \\ (4, 3) & \(\mathrm{IBr^{2+}\to\mathrm{I^{+}+Br^{2+}}}\) & 14.29 & 21.10 & 35.39 & 4.88 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The possible CE channels for the molecular ions \(\mathrm{IBr^{m+}}\) (x = 2–7). a succeeding Coulomb explosion at \(\rm R_{e}\), the channel would be expected to release a total kinetic energy of 5.83 eV.