Fragmentation patterns of multicharged $\text{C}_{60}{}^{r+}$ $\left(r=3–5\right)$ studied with well-controlled internal excitation energy

We have studied the relaxation of triply charged ${\text{C}}_{60}$ obtained in collisions ${\text{F}}^{2+}+{\text{C}}_{60}\to {\text{F}}^{-}+\text{C}_{60}{}^{3+\ast }$ at low impact energy $\left(E=6.8\text{keV}\right)$. Depending on the excitation energy, these initial parent ions decay following a variety of channels, such as thermal electronic ionization, evaporation of ${\text{C}}_2$ units, asymmetrical fission, and multifragmentation. Using a recently developed experimental method, named collision-induced dissociation under energy control, we were able to measure the energy deposited in $\text{C}_{60}{}^{3+\ast }$ for each collision event and to obtain an excitation energy profile of the parent ions associated with each decay channel. In our chosen observation time scale of the order of $1\mu \text{s}$, evaporations and asymmetrical fissions of $\text{C}_{60}{}^{3+,4+}$ occur when the internal energy is in the range from 40 to 100 eV. The multifragmentation becomes dominant for multicharged $\text{C}_{60}{}^{4+,5+}$ parent ions from 100 to 210 eV. In the case of $\text{C}_{60}{}^{4+}$, the multifragmentation channel is opened at low energy (40 eV). Therefore, in the energy range 40–100 eV, the asymmetrical fission, evaporation, and multifragmentation channels are in competition.

Figure 1

(Color online) (a) EX-RI spectrum recorded in ${\text{F}}^{2+}+{\text{C}}_{60}\to {\text{F}}^{-}+\text{C}_{60}{}^{3+*}$ collisions. Horizontal axis: TOF of recoil ions. Vertical axis: right scale, electrostatic analyzer voltage; left scale, excitation energy of the initial $\text{C}_{60}{}^{3+*}$ parent ions. (b) Projection of (a) onto the horizontal axis. The spots and the peaks between $\text{C}_{60}{}^{3+}$ and $\text{C}_{60}{}^{4+}$ in (a) and (b) are reduced by a factor of 7.

Figure 2

(Color online) (a) EX-RI spectrum recorded in ${\text{F}}^{2+}+{\text{C}}_{60}\to {\text{F}}^{-}+\text{C}_{60}{}^{3+*}$ collisions with extra conditions to ensure that the parent of the fragmentation process is $\text{C}_{60}{}^{3+}$. (b) Projection of (a) onto the horizontal axis showing the evaporation sequence of $\text{C}_{60}{}^{3+}$ fragmentation parent ions.

Figure 3

(Color online) The center of the excitation energy profile for the evaporation channels leading to $\text{C}_{60-2m}{}^{3+}$ and $\text{C}_{60-2m}{}^{4+}$ fullerene ions from the fragmentation parents $\text{C}_{60}{}^{3+}$ (circle) and $\text{C}_{60}{}^{4+}$ (square).

Figure 4

(Color online) Symbols linked with solid lines: experimental population distributions of evaporation products, $\text{C}_{58}{}^{3+}$, $\text{C}_{56}{}^{3+}$, and $\text{C}_{54}{}^{3+}$ fullerene fragments, as a function of the excitation energy of the parent ions $\text{C}_{60}{}^{3+}$. Symbols linked with dashed lines: theoretical modeling. The lines in this figure are to guide the eyes.

Figure 5

(Color online) (a) EX-RI spectrum associated to the $\text{C}_{60}{}^{4+*}$ intermediate parent ions recorded in collisions ${\text{F}}^{2+}+{\text{C}}_{60}\to {\text{F}}^{-}+\text{C}_{60}{}^{3+*}$ with an extra condition that one electron from TEI be detected. (b) Projection of (a) onto the horizontal axis. Evaporation, AF, and MF processes are observed. The large cross section for the production of $\text{C}_{60}{}^{3+}$ leads to a weak peak in this spectrum due to the electrical noise on the PIPS detector.

Figure 6

(Color online) Correlation spectrum of recoil fragments. The TOF of the heavy and light fragments from AF of $\text{C}_{60}{}^{4+*}$ intermediate parent ions are plotted, respectively, along the horizontal and vertical axes.

Figure 7

(Color online) Center $E_c$ of the excitation energy distributions for AF channels from $\text{C}_{60}{}^{4+*}$ intermediate parent ions. The values are given for all fission channels observed in Fig. 6.

Figure 8

(Color online) Same as Fig. 7, but the excitation energy values are given versus the number of emitted light charged fragments.

Figure 9

(Color online) Center of the excitation energy distributions of initial parents $\text{C}_{60}{}^{3+}$ for MF processes related to the detection of a light fragment $\text{C}_n{}^{+}$ $\left(n=1–15\right)$ from $\text{C}_{60}{}^{4+}$ (squares) and $\text{C}_{60}{}^{5+}$ (triangles) intermediate parent ions. Values related to the detection of $\text{C}_n{}^{2+}$ $\left(n=15,19,23\right)$ from $\text{C}_{60}{}^{4+}$ (circles) and $\text{C}_{60}{}^{5+}$ (diamonds) intermediate parent ions.

Figure 10

(Color online) (a) EX-RI spectrum associated to the intermediate parent ions $\text{C}_{60}{}^{5+}$ recorded in collisions ${\text{F}}^{2+}+{\text{C}}_{60}\to {\text{F}}^{-}+\text{C}_{60}{}^{3+*}$ with an extra condition that two electrons from TEI be detected. (b) Projection from (a) onto the horizontal axis. MF process is observed for $\text{C}_{60}{}^{5+}$.

Figure 11

Mass distribution associated to different excitation energies from 70 eV to 230 eV for the MF channels from the $\text{C}_{60}{}^{4+}$ intermediate parent ions.

Figure 12

(Color online) Comparison between the TOF peaks of $\text{C}_4{}^{+}$ due to the AF (circles) and due to the MF processes (squares) from $\text{C}_{60}{}^{4+}$ intermediate parent ions.