Multiple photoionization and fragmentation of ${\mathrm{C}}_{60}$ in the $18–280\text{−}\mathrm{eV}$ range

We examined the relative ionization and fragmentation probabilities of ${\mathrm{C}}_{60}$ after excitation by monochromatized synchrotron radiation in the energy range of $18–280\mathrm{eV}$. The energy dependence of the relative cross sections of $\mathrm{C}_{60}{}^{q+}$ $(q=1–4)$ and the associated fragments $\mathrm{C}_{60–2n}{}^{q+}$ ($n=1–6$ for $q=2,3$ and $n=1,2$ for $q=4$) are analyzed. We present the ionization and appearance energies for those ions and fragments as well as the partial cross sections for the ${\mathrm{C}}_{60}$ ions.

Figure 1

(Color online) Sketch of the experimental setup.

Figure 2

(Color online) ${\mathrm{C}}_{60}$ time-of-flight spectrum taken at a photon energy of $210\mathrm{eV}$ on a nonlinear mass-to-charge ratio scale. Note that the ion and fragment peaks of the residual gases, shown in the inset, are small and do not affect the areas of the ${\mathrm{C}}_{60}$ peaks.

Figure 3

(Color online) The $\mathrm{C}_{60}{}^{2+}∕\mathrm{C}_{60}{}^{+}$ ratio taken at $23\mathrm{eV}$ as a function of the front MCP voltage. The best MCP voltage corresponds to the minimum value which was extracted via a polynomial fit (solid line). The minimum of the polynomial fit is marked with an arrow and corresponds to $3915\mathrm{V}$.

Figure 4

(Color online) A typical example of a Voigt-profile fit applied to our ion-yield spectrum in the region of doubly charged ions taken at $255\mathrm{eV}$. The fit curve (solid line) and data points (open circles) are both multiplied by 10 relative to the dotted line and filled circles, respectively.

Figure 5

(Color online) Double-to-single photoionization cross-section ratio as a function of photon energy. The filled triangles represent the relative cross sections of the sum of all double-ionized fragments peak areas. The filled circles represent just the $\mathrm{C}_{60}{}^{2+}$ peak, the open circles represent experimental results of Drewello et al. [1] scaled by a factor of 4, while the open diamonds represent the results of Reinköster et al. [6] scaled by a factor of 1.3. A $280\mathrm{eV}$ point from the results of Aksela et al. [4] and Karvonen et al. [5] is shown as an open triangle.

Figure 6

The Wannier exponent $\alpha$ as a function of the upper limit $E_{\max }$ of the fit range using the formula ${\sigma }^{2+}∕{\sigma }^{+}\propto {(\mathrm{h}\nu -19.0)}^{\alpha }$ where $\mathrm{h}\nu$ is the photon energy in eV. The minimum of the fit range is the double-ionization threshold of $19.00\mathrm{eV}$.

Figure 7

(Color online) Double-to-single photoionization cross-section ratios of $\mathrm{C}_n{}^{2+}$ fragments for $n=58,56,54,52,50,48$ as a function of photon energy.

Figure 8

(Color online) A closeup of Fig. 7 in the appearance energy region of the $\mathrm{C}_n{}^{2+}$ fragments for $n=58,56,54,52,50,48$. The markings show the appearance energies as labeled in the figure. The curves for the different fragments are labeled on the right-hand side of the frame.

Figure 9

(Color online) Triple-to-single photoionization ratios as a function of photon energy. The circles represent the cross-section ratios of the sum of all triply charged fragments. The diamonds represent the cross-section ratios of just the $\mathrm{C}_{60}{}^{3+}$ fragment, while the triangles represent the data for the $\mathrm{C}_{60}{}^{3+}$ ratios as presented by Reinköster et al. [6], but scaled by a factor of 7.2.

Figure 10

(Color online) Triple-to-single photoionization ratios of $\mathrm{C}_n{}^{3+}$ fragments for $n=58,56,54,52,50,48$ as a function of photon energy.

Figure 11

(Color online) Relative cross sections of triple-ionized fragments $\mathrm{C}_n{}^{3+}$ $(n=58,56,54,52,50,48)$ near their appearance energies. The markings show the appearance energies as labeled in the figure. The curves for the different fragments are labeled on the right-hand side of the frame.

Figure 12

(Color online) Quadruple-to-single photoionization ratios of ${\mathrm{C}}_{60}$ and its fragments. The top line shows the quadruple-to-single ionization ratio of all quadruply charged fragments. The other data display the ratio for selected masses as indicated in the figure.

Figure 13

(Color online) Cross sections of $\mathrm{C}_{60}{}^{q+}$ ions $(q=1–4)$. The top line depicts the photoionization cross section of $\mathrm{C}_{60}{}^{+}$, followed by $\mathrm{C}_{60}{}^{2+}$, $\mathrm{C}_{60}{}^{3+}$, and $\mathrm{C}_{60}{}^{4+}$. The gray data points for $\mathrm{C}_{60}{}^{+}$ in the $26\text{to}110\mathrm{eV}$ region are those from Reinköster et al. [6].