Fragmentation of $\mathrm{C}{{\mathrm{F}}_4}^{q+}$ $(q=2,3)$ induced by 1-keV electron collisions

We report on an investigation on the fragmentation dynamics of $\mathrm{C}{{\mathrm{F}}_4}^{q+}$ $(q=2,3)$ induced by 1-keV electron collisions utilizing an ion momentum imaging spectrometer. From the time-of-flight correlation maps five dominating dissociation channels of $\mathrm{C}{{\mathrm{F}}_4}^{2+}$ as well as one three-body fragmentation channel of $\mathrm{C}{{\mathrm{F}}_4}^{3+}$ are identified. The kinetic energy release (KER) distributions for these channels are obtained and compared with the data available in the literature. The Dalitz-like momentum diagram and the Newton diagram are employed to analyze the breakup mechanism in the three-body fragmentation channel. We found that, for $\mathrm{C}{{\mathrm{F}}_4}^{2+}$ dissociation into ${\mathrm{F}}^{+}+\mathrm{C}{{\mathrm{F}}_2}^{+}+\mathrm{F}$, ${\mathrm{F}}^{+}+\mathrm{C}{\mathrm{F}}^{+}+2\mathrm{F}$, and ${\mathrm{F}}^{+}+{\mathrm{F}}^{+}+{\mathrm{CF}}_2$, the concerted breakup is the dominating process. Channel ${\mathrm{C}}^{+}+{\mathrm{F}}^{+}+3\mathrm{F}$ is dominated by the initial charge separation, i.e., $\mathrm{C}{{\mathrm{F}}_4}^{2+}\to {\mathrm{F}}^{+}+\mathrm{C}{\mathrm{F}}^{+}+2\mathrm{F}\to {\mathrm{C}}^{+}+{\mathrm{F}}^{+}+3\mathrm{F}$. With the help of the native frame method, we assigned one sequential pathway and two concerted pathways for channel $\mathrm{C}{{\mathrm{F}}_4}^{3+}\to {\mathrm{F}}^{+}+{\mathrm{F}}^{+}+\mathrm{C}{{\mathrm{F}}_2}^{+}$. The branching ratios of these pathways are determined. The momentum correlation of the fragments and the deduced KER distribution indicate that different excited states of $\mathrm{C}{{\mathrm{F}}_4}^{3+}$ with different geometries are responsible for these three pathways. The Coulomb explosion model simulation shows that most of the events in this channel are produced by $\mathrm{C}{{\mathrm{F}}_4}^{3+}$ ions that have deformed geometries from the neutral ${\mathrm{CF}}_4$ molecule.

Figure 1

Schematic diagram of the experimental setup. PSD means position-sensitive detector.

Figure 2

Definition of the Dalitz-like momentum diagram. The arrows at each point show the momentum correlation of the three fragments.

Figure 3

(a) TOF spectrum of ${\mathrm{CF}}_4$ produced by 1-keV electron impact; (b) TOF correlation map of the first hit ions vs the second ions; (c) TOF correlation map of the first hit ions vs the third ions. The red dashed oval in (b) is used to select the events of channel ${\mathrm{F}}^{+}+\mathrm{C}{{\mathrm{F}}_3}^{+}$.

Figure 4

Schematic diagram illustrating the threshold energies and dissociation limits of $\mathrm{C}{{\mathrm{F}}_4}^{2+}$. Appearance potentials are adopted from the results of Codling et al. [9]. Dissociation limits are derived by using thermochemical data from Rosenstock et al. [48].

Figure 5

Kinetic energy distributions for channel $\mathrm{C}{{\mathrm{F}}_4}^{2+}\to {\mathrm{F}}^{+}+\mathrm{C}{{\mathrm{F}}_3}^{+}$.

Figure 6

Dalitz-like momentum diagrams, Newton diagrams, and kinetic energy (KE) distributions for incomplete Coulomb fragmentation of $\mathrm{C}{{\mathrm{F}}_4}^{2+}$: (a)–(c), channel $\mathrm{C}{{\mathrm{F}}_4}^{2+}\to {\mathrm{F}}^{+}+{\mathrm{F}}^{+}+{\mathrm{CF}}_2$; (d), (e) channel $\mathrm{C}{{\mathrm{F}}_4}^{2+}\to {\mathrm{F}}^{+}+\mathrm{C}{{\mathrm{F}}_2}^{+}+\mathrm{F}$; (g)–(i) $\mathrm{C}{{\mathrm{F}}_4}^{2+}\to {\mathrm{F}}^{+}+\mathrm{C}{\mathrm{F}}^{+}+2\mathrm{F}$; (j), (k) Newton plots filtered by kinetic energy of the neutral fragment(s).

Figure 7

Kinetic energy distributions for channel $\mathrm{C}{{\mathrm{F}}_4}^{2+}\to {\mathrm{C}}^{+}+{\mathrm{F}}^{+}+3\mathrm{F}$.

Figure 8

Channel $\mathrm{C}{{\mathrm{F}}_4}^{3+}\to {\mathrm{F}}^{+}+{\mathrm{F}}^{+}+\mathrm{C}{{\mathrm{F}}_2}^{+}$: (a) Dalitz-like momentum diagram, (b) Newton diagram, and (c) KER of $\mathrm{C}{{\mathrm{F}}_3}^{2+}$ vs θ ($\mathrm{C}{{\mathrm{F}}_3}^{2+}$, ${{\mathrm{F}}_a}^{+}$) native frame plot by assuming sequential breakup of $\mathrm{C}{{\mathrm{F}}_4}^{3+}$ via ${{\mathrm{F}}_a}^{+}+\mathrm{C}{{\mathrm{F}}_3}^{2+}$. ${{\mathrm{F}}_a}^{+}$ and ${{\mathrm{F}}_b}^{+}$ are the ${\mathrm{F}}^{+}$ ions with higher and lower kinetic energy, respectively.

Figure 9

Dalitz -like momentum diagrams, Newton diagrams, and KER distributions for regions A–C in Fig. 8.

Figure 10

Dalitz-like momentum diagram (a), Newton diagram (b), and KER distribution (c) obtained from the Coulomb explosion model simulation.