Ions colliding with mixed clusters of ${\mathrm{C}}_{60}$ and coronene: Fragmentation and bond formation

We have studied collisions between 22.5 keV ${\mathrm{He}}^{2+}$ ions and mixed clusters $\left[\right({\mathrm{C}}_{60}\left){}_m\right({\mathrm{C}}_{24}$${\mathrm{H}}_{12}\left){}_n\right]$ of $m$ ${\mathrm{C}}_{60}$ and $n$ coronene molecules where $m$ and $n$ range up to about ten. Surprisingly, the cluster fragmentation behavior in distant collisions is dramatically different for pure coronene clusters ($m=0$) and clusters containing a single ${\mathrm{C}}_{60}$ molecule $(m=1)$. In the latter case, the clusters may be ionized without also being fragmented on the experimental time scale of tens of microseconds. This does not occur for pure coronene clusters, but is a main characteristic of pure fullerene clusters. For ion trajectories penetrating the mixed cluster, we observe covalent bond formations between ${\mathrm{C}}_{59}$ or ${\mathrm{C}}_{58}$ and ${\mathrm{C}}_{60}$, but not between coronene fragments and ${\mathrm{C}}_{60}$, or between ${\mathrm{C}}_{60}$ fragments and coronene. These results are explained by means of classical molecular dynamics simulations of collisions inside the fragmenting mixed clusters.

Figure 1

Schematic of the experimental setup which has a cluster gas aggregation source with separate ovens for ${\mathrm{C}}_{60}$ and coronene. The molecules are cooled by He buffer gas at liquid ${\mathrm{N}}_2$ temperatures allowing them to condense into clusters. The jet of neutral clusters crosses with the pulsed 22.5-keV ${\mathrm{He}}^{2+}$ beam in the extraction region of a time-of-flight mass spectrometer.

Figure 2

Example of the initial positions of a simulated binary collision between ${\mathrm{C}}_{59}$ and ${\mathrm{C}}_{24}$${\mathrm{H}}_{12}$ inside the fragmenting molecular cluster. The velocities $v_1$ and $v_2$ are set to give a center-of-mass collision energy between 0.1 and a few eV. The impact parameter $b$ is defined from the center of each molecule, here $b=5$ Å.

Figure 3

Mass spectrum for 22.5-keV ${\mathrm{He}}^{2+}$ colliding with mixed clusters of ${\mathrm{C}}_{60}$ (720 amu) and coronene (${\mathrm{C}}_{24}$${\mathrm{H}}_{12}$, 300 amu). The peaks are labeled by the numbers $j$, $k$ of ${\mathrm{C}}_{60}$ and coronene molecules, respectively, after ionization or after ionization and fragmentation. Some of the less intense peaks are not labeled. Note the $\times 100$ zoom in factor for mass-to-charge ratios above 1100 amu/e.

Figure 4

Integrated intensities (color scale, left panels) and widths (FWHMs of fitted Gaussian, color scale, right panels) of peaks in the mass spectra due to 22.5 keV ${\mathrm{He}}^{2+}$ + $\left[\right({\mathrm{C}}_{60}){}_m$(Cor)${}_n]$ $\to$ $\left[\right({\mathrm{C}}_{60}){}_j$(Cor)${}_k]$${}^{+}$ for ${\mathrm{C}}_{60}$:Cor mixtures close to 1:1 (top panels) and 1:3 (lower panels). The largest intensities and peak widths are those belonging to the molecular monomers, with intensities up to $\sim$100 000 and peak widths up to $\sim$0.5 eV. The labels $j,k$ are defined in the upper left panel and an example is given in the lower left panel: “4,6” refers to the $\left[\right({\mathrm{C}}_{60}){}_4$(Cor)${}_6]$${}^{+}$ cluster.

Figure 5

Intensities of peaks in mass-to-charge spectra as functions of cluster size for pure clusters of ${\mathrm{C}}_{60}$, [${\mathrm{C}}_{60}]$${}_m$ from Ref. [26] (solid blue line), or for pure coronene clusters [${\mathrm{C}}_{24}$${\mathrm{H}}_{12}]$${}_n$, from Ref. [33] (solid red line), and for mixed clusters containing zero, one, or two ${\mathrm{C}}_{60}$ molecules from the present experiment (black lines). Statistical error bars are smaller than the data points.

Figure 6

Comparison of the ${\mathrm{C}}_{60}$ dimer region (top panel) and the ${\mathrm{C}}_{60}$ + ${\mathrm{C}}_{24}$${\mathrm{H}}_{12}$ mass region (lower panel) from the spectrum due to 22.5-keV collisions of ${\mathrm{He}}^{2+}$ with mixed clusters $\left[\right({\mathrm{C}}_{60}\left){}_m\right({\mathrm{C}}_{24}$${\mathrm{H}}_{12}\left){}_n\right]$ in the $j$:$k\sim 1$:1 case. In the top panel, we clearly see evidence of molecular fusion products as a result of the knockout of one (yielding ${\mathrm{C}}_{119}^{+}$) or two (${\mathrm{C}}_{118}^{+}$) C atoms from a ${\mathrm{C}}_{60}$ cage. We do not detect any evidence of ${\mathrm{C}}_{59}^{+}$ reacting with coronene (lower panel). The optimized structure of ${\mathrm{C}}_{60}$ + ${\mathrm{C}}_{24}$${\mathrm{H}}_{12}$ with binding energy $\sim 0.8$ eV is shown in the lower panel, while the structure of the covalently bound ${\mathrm{C}}_{119}$ dumbbell dimer from Refs. [34, 36] is shown in the upper panel (cf. text).

Figure 7

Relative reaction probabilities for formations of covalently bound mixed ${\mathrm{C}}_{60}$ + coronene dimers per randomly oriented collision as a function of collision energy. We have normalized to the ${\mathrm{C}}_{59}$+${\mathrm{C}}_{60}$ $\to$ ${\mathrm{C}}_{119}$ reaction probability at 0.1 eV. The error bars show the statistical uncertainty of each point. Note the large factor ($\sim$50 for most energies) between the red and the green data series. All other data sets show no reactivity at these energies (the lines are all superimposed at zero probability). Molecular structures for ${\mathrm{C}}_{119}$ and the ${\mathrm{C}}_{23}$${\mathrm{H}}_{12}$(1) + ${\mathrm{C}}_{60}$ $\to$ ${\mathrm{C}}_{83}$${\mathrm{H}}_{12}$ product are shown to the right.