Ultrafast ring-opening fragmentation dynamics of ${\mathrm{C}}_6{{\mathrm{H}}_6}^{3+}$ induced by electron-impact ionization

The fragmentation dynamics of triply charged benzene [(${\mathrm{C}}_6{\mathrm{H}}_6{)}^{3+}$] induced by 260 eV electron-impact ionization are investigated using a multiparticle coincidence momentum spectrometer. By measuring three fragment ions and one outgoing electron in quadruple coincidence, we identify the complete three-body dissociation channels of ${{\mathrm{CH}}_2}^{+} + {\mathrm{C}}_2{{\mathrm{H}}_3}^{+} + {\mathrm{C}}_3{\mathrm{H}}^{+}, {{\mathrm{CH}}_3}^{+} + {\mathrm{C}}_2{{\mathrm{H}}_2}^{+} + {\mathrm{C}}_3{\mathrm{H}}^{+}$, and ${\mathrm{C}}_2{{\mathrm{H}}_2}^{+} + {\mathrm{C}}_2{{\mathrm{H}}_2}^{+} + {\mathrm{C}}_2{{\mathrm{H}}_2}^{+}$. We determine the projectile-energy-loss spectra, Dalitz plots, Newton diagrams, momentum correlation maps, and kinetic-energy release for each fragmentation channel. The analysis of these spectra is supported by an ab initio molecular dynamics simulation, which provides a molecular movie of the dissociation process. Our study reveals sequential mechanisms for all three dissociation channels of (${\mathrm{C}}_6{\mathrm{H}}_6{)}^{3+}$ trication, i.e., an ultrafast ring-opening reaction followed by two subsequent Coulomb-explosion processes.

Figure 1

The projectile-energy-loss spectra for the three considered fragmentation channels. The spectra are normalized to unity at the maximum.

Figure 2

The calculated Dalitz plot considered for channel (i). The purple, red, and black arrows represent the momentum vectors of ${{\mathrm{CH}}_2}^{+}, {\mathrm{C}}_2{{\mathrm{H}}_3}^{+}$, and ${\mathrm{C}}_3{\mathrm{H}}^{+}$, respectively. The dissociation characteristics of the events labeled by solid and dashed lines correspond to the features in Fig. 3 marked by the same lines.

Figure 3

(a) Experimental Dalitz plot and (b) Newton diagram of channel (i) in which the AIMD simulation is shown by the open squares. Momentum correlation spectra (c) between ${\mathrm{C}}_2{{\mathrm{H}}_3}^{+}$ and ${{\mathrm{CH}}_2}^{+}$ and (d) between ${\mathrm{C}}_3{\mathrm{H}}^{+}$ and ${{\mathrm{CH}}_2}^{+}$. The solid and dashed lines correspond to either ${\mathrm{C}}_2{{\mathrm{H}}_3}^{+}$ and ${{\mathrm{CH}}_2}^{+}$ being emitted in the first Coulomb explosion, respectively. (e) Center-of-mass distances between different moieties within the ${\mathrm{C}}_6{{\mathrm{H}}_6}^{3+}$ ion as a function of evolution time after ionization. The molecular structures at specific evolution times are presented in the insets, which include ring opening and Coulomb explosion, with possible hydrogen migration marked by the curved arrows.

Figure 4

(a) Experimental Dalitz plot and (b) Newton diagram of channel (ii) in which the AIMD simulation is shown by the open squares. Momentum correlation spectra (c) between ${\mathrm{C}}_2{{\mathrm{H}}_2}^{+}$ and ${\mathrm{C}}_3{\mathrm{H}}^{+}$ and (d) between ${{\mathrm{CH}}_3}^{+}$ and ${\mathrm{C}}_3{\mathrm{H}}^{+}$. (e) Center-of-mass distances between different moieties within the ${\mathrm{C}}_6{{\mathrm{H}}_6}^{3+}$ ion as a function of evolution time after ionization.

Figure 5

(a) Calculated and (b) experimental Dalitz plots as well as (c) the Newton diagram of channel (iii). (d) Two-dimensional momentum correlation spectra between three ${\mathrm{C}}_2{{\mathrm{H}}_2}^{+}$ ions. (e) Center-of-mass distances between ${\mathrm{C}}_2{{\mathrm{H}}_2}^{+}$ and ${\mathrm{C}}_4{{\mathrm{H}}_4}^{2+}$ ions as a function of evolution time after ionization. Note that since the fragments are indistinguishable, $p$ and $p^2$ randomly denote the momenta and the energies of the three ${\mathrm{C}}_2{{\mathrm{H}}_2}^{+}$ ions.

Figure 6

Measured kinetic-energy-release (KER) spectra for all three dissociation channels. The red and blue vertical arrows represent the simulated KER values.