Three-body fragmentation mechanism of ${\mathrm{C}}_2{{\mathrm{H}}_4}^{3+}$ produced by $18\text{−}\mathrm{keV}/\mathrm{u}{\mathrm{Ne}}^{8+}$ impact

In this article we present the three-body fragmentation dynamics of ${\mathrm{C}}_2{{\mathrm{H}}_4}^{3+}$ caused by $18\text{−}\mathrm{keV}/\mathrm{u}{\mathrm{Ne}}^{8+}$ ion impact. Utilizing the cold target recoil ion momentum spectroscopy, the complete kinematical information of three fragmentation channels, i.e., ${\mathrm{C}}_2{{\mathrm{H}}_4}^{3+}\to {\mathrm{H}}^{+}+{\mathrm{H}}^{+}+{\mathrm{C}}_2{{\mathrm{H}}_2}^{+}, {\mathrm{H}}^{+}+\mathrm{C}{\mathrm{H}}^{+}+\mathrm{C}{{\mathrm{H}}_2}^{+}$, and ${\mathrm{H}}^{+}+{{\mathrm{H}}_2}^{+}+{\mathrm{C}}_2{\mathrm{H}}^{+}$ is obtained. For each channel, both concerted and sequential fragmentation mechanisms are observed and differentiated through the momentum correlation of resultant fragments visualized by the Dalitz plot and the Newton diagram. In particular, for the channel ${\mathrm{C}}_2{{\mathrm{H}}_4}^{3+}\to {\mathrm{H}}^{+}+{\mathrm{H}}^{+}+{\mathrm{C}}_2{{\mathrm{H}}_2}^{+}$, it is found in addition to the sequential pathway that the two protons can be emitted concertedly in two ways, i.e., from the same carbon atom or from the two carbon atoms. The present results show a more diverse molecular fragmentation triggered by ion collisions in contrast to a previous study using intense laser field ionization.

Figure 1

Triple-ion coincidence TOF map of the ${\mathrm{C}}_2{{\mathrm{H}}_4}^{3+}$ ions caused by the $18\text{−}\mathrm{keV}/\mathrm{u}{\mathrm{Ne}}^{8+}$ impact.

Figure 2

(a) The Dalitz plot of fragmentation channel I. Intense areas are surrounded by black, magenta, and red ellipse lines and marked as A–C, respectively. The black, blue, and purple dots on the vertical middle line are momentum correlation patterns calculated by the Coulomb explosion model. ${{\mathrm{H}}_{\mathrm{a}}}^{+}$ presents the first detected proton, and ${{\mathrm{H}}_{\mathrm{b}}}^{+}$ is the second one. (b) Newton diagram of fragmentation channel I. The momentum of ${{\mathrm{H}}_{\mathrm{a}}}^{+}$ is taken as the unit vector, indicated by a red arrow, the relative momentum vectors of ${{\mathrm{H}}_{\mathrm{b}}}^{+}$ and ${\mathrm{C}}_2{{\mathrm{H}}_2}^{+}$ are mapped in the upper and lower halves of the plot. (c)–(e) Newton diagrams with Dalitz filters A–C, respectively. In (e) ${{\mathrm{H}}_{1\mathrm{st}}}^{+}$ and ${{\mathrm{H}}_{2\mathrm{nd}}}^{+}$ represent the protons produced in the first and second steps of the corresponding sequential fragmentation process.

Figure 3

Distribution of the angle between momentum vectors of ${{\mathrm{H}}_{\mathrm{a}}}^{+}$ and ${{\mathrm{H}}_{\mathrm{b}}}^{+}$ (red squares) and that of ${{\mathrm{H}}_{\mathrm{a}}}^{+}$ and ${\mathrm{C}}_2{{\mathrm{H}}_2}^{+}$ (black circles) of the concerted fragmentation in region A of channel I. The red and black curves are the Gaussian-fitting results.

Figure 4

Chemical structure of the neutral ethylene molecule.

Figure 5

(a) The Dalitz plot of fragmentation channel II. The fragmentation events located at different regions are marked by D–F, corresponding to different fragmentation mechanisms. (b) The Newton diagram of channel II. The momentum of fragment ${\mathrm{H}}^{+}$ is set as a unit vector and plotted at the horizontal axis as a red arrow. The momenta of fragments $\mathrm{C}{\mathrm{H}}^{+}$ and $\mathrm{C}{{\mathrm{H}}_2}^{+}$ are plotted at the upper and lower halves. The structures marked by black, red, and golden lines correspond to the fragmentation events in regions D–F in Fig. 6, respectively. (c) The Newton diagram for the data in region F. The momentum of fragment $\mathrm{C}{{\mathrm{H}}_2}^{+}$ is set as the unit vector.

Figure 6

(a) The Dalitz plot of fragmentation III. Two different momentum correlation patterns are marked by black and golden lines and denoted as regions G and K, respectively. (b) The Newton diagram of fragmentation III. The momentum of ${\mathrm{H}}^{+}$ is set as unit vector. The momenta of fragments ${{\mathrm{H}}_2}^{+}$ and ${\mathrm{C}}_2{\mathrm{H}}^{+}$ are plotted at the upper and lower halves. The structures marked by black and golden lines correspond to the fragmentation events in regions G and K in (a), respectively. (c) Simulated Dalitz plot for the sequential fragmentation pathway IIIb. The momenta of fragments are calculated by solving the kinetic equations in the sequential process.