Multiple electron capture, excitation, and fragmentation in $\mathrm{C}{}^{6+}-{\mathrm{C}}_{60}$ collisions

We present experimental and theoretical results on single- and multiple-electron capture, and fragmentation, in ${\mathrm{C}}^{6+}+{\mathrm{C}}_{60}$ collisions at velocities in the $v_{\text{col}}=0.05-0.4$ a.u. range. We use time-of-flight mass spectrometry and coincidence detection of charged fragments to separate pure target ionization from processes in which the ${\mathrm{C}}_{60}$ target is both ionized and fragmented. The coincidence technique allows us to identify different types of fragmentation processes such as ${\mathrm{C}}_{60}^{q+}\to {\mathrm{C}}_{58}^{q+}+{\mathrm{C}}_2$ and ${\mathrm{C}}_{60}^{q+}\to {\mathrm{C}}_{58}^{(q-1)+}+{\mathrm{C}}_2^{+}$. A quasimolecular approach is employed to calculate charge transfer and target excitation cross sections. First-order time-dependent perturbation and statistical methods are used to treat the postcollisional processes: the calculated rate constants for ${\mathrm{C}}_2$ and $\mathrm{C}{}_2^{+}$ emission from the excited and charged fullerene are then used to evaluate the fragmentation dynamics. We show that the target ionization cross section decreases with the induced target charge state and the impact energy. ${\mathrm{C}}_2$ emission from $\mathrm{C}{}_{60}^{q+}$ is found to dominate when $q\le 2$ while $\mathrm{C}{}_2^{+}$ emission dominates when $q\ge 5$, in agreement with the present and previous experimental results.

Figure 1

Schematic of the experimental set up (see text).

Figure 2

Correlation diagram for the relevant molecular orbitals of ${({\mathrm{C}}_{60}+\mathrm{C})}^{6+}$ denoted by their character at infinite separation: $n=5$, 6, and 7 orbitals from $\mathrm{C}{}^{6+}$ and $\pi 2s,2p,2d,2f,2g$, and $2h$ orbitals from ${\mathrm{C}}_{60}$.

Figure 3

Schematic energy profile for the excitation, ${\mathrm{C}}_2$ evaporation, and $\mathrm{C}{}_2^{+}$ asymmetric fission. Dissociation energies, fission barriers, and rate constants are shown.

Figure 4

Mass spectrum due to 30 keV $\mathrm{C}{}^{6+}+{\mathrm{C}}_{60}$ collisions.

Figure 5

Computed absolute charge transfer cross sections (in ${10}^{-16} \mathrm{cm}{}^2$) in ${\mathrm{C}}_{60}+{\mathrm{C}}^{6+}$ collisions as a function of the impact velocity (in a.u.).

Figure 6

Relative charge transfer cross sections $\sigma \left({\mathrm{C}}_{60}^{q+}\right)/\sigma \left({\mathrm{C}}_{60}^{+}\right)$ as a function of the charge state. Open symbols full lines: theoretical simulations, filled symbols dashed lines: experiments, “X”-dashed line: classical over-the-barrier (COB) model. Lines between the data points are to guide the eye.

Figure 7

${\mathrm{C}}_2$ evaporation (left column) and $\mathrm{C}{}_2^{+}$ asymmetric fission (right column) probabilities as functions of the excitation energy and the time after excitation.

Figure 8

${\mathrm{C}}_2$ evaporation and $\mathrm{C}{}_2^{+}$ asymmetric fission probabilities as a function of the time of flight at given two typical values of excitation energy. Internal excitation energy due to the temperature $E_{\text{Temp}}^{*}$, included for $T=800\text{K}$. Solid blue, dashed red, and dotted-dashed green lines represent ${\mathrm{C}}_{60}^{q+},{\mathrm{C}}_{58}^{q+}$, and ${\mathrm{C}}_{58}^{(q-1)+}$, respectively.

Figure 9

Time-of-flight distribution of the cationic products in the 1-STOP spectrum (only one charged fragment detected) after collisions of ${\text{C}}^{6+}$ and ${\text{C}}_{60}$ at $v=0.31\text{a.u.}$. Zoom-ins of regions around ${\text{C}}_{60}^{2+},{\text{C}}_{60}^{3+}$, and ${\text{C}}_{60}^{4+}$ are shown in panels (b), (c), and (d), respectively. Horizontal lines in lower panels show the typical tail corresponding to delayed fragmentation during extraction time.

Figure 10

Time-of-flight of ${\text{C}}_n^{(q-1)+}$ ions measured in coincidence with ${\text{C}}_2^{+}$ in ${\mathrm{C}}^{6+}+{\mathrm{C}}_{60}$ collisions at $v=0.45\text{a.u.}$

Figure 11

Calculated cross sections for producing intact ${\mathrm{C}}_{60}^{q+}$ ions (solid blue curve), and ${\mathrm{C}}_{58}^{q+}$ and ${\mathrm{C}}_{58}^{(q-1)+}$ fragments (dashed red and dashed-dotted green curves, respectively) as functions of time for ${\mathrm{C}}^{6+}+{\mathrm{C}}_{60}$ collisions at five different velocities (see text).