Excitation of fullerene ions during grazing scattering from a metal surface

Angular distributions, fragmentation, and charge fractions are studied for grazing scattering of ${\text{C}}_{60}^{+}$ fullerenes with keV energies from a clean and flat Al(001) surface. At low energies for the motion along the surface normal, ${\text{C}}_{60}^{+}$ ions are scattered nearly elastically, whereas for larger normal energies energy loss is substantial. We compare our experimental results with classical trajectory simulations exploiting the Tersoff potential between atoms in the cluster and different types of interaction potentials for the cluster with the surface. The internal energy of scattered clusters is deduced from the analysis of fragments. We observe that the loss of kinetic energy for the motion along the surface normal is transferred to internal excitations of the cluster, whereas the energy transfer to the metal surface is negligible. The charge state distributions for scattered projectiles can be understood by a full neutralization of incident ions at the surface and subsequent delayed electron emission.

Figure 1

Waterfall plot of polar angular distributions for scattering of ${\text{Ar}}^0$ atoms and ${\text{C}}_{60}^{+}$ clusters with energies indicated under a grazing angle of incidence of $2.25°$ from Al(001). Ticks on the vertical axis indicate offsets of zero level for distributions. Arrows indicate outgoing angles for which fragment distributions have been measured (see below).

Figure 2

(Color online) Outgoing normal energy $E_z^{\text{out}}$ as a function of incident normal energy $E_z^{\text{in}}$ for clusters scattered under angles of incidence indicated. Full symbols (open symbols): results obtained from peak (half-maximum) positions of angular distributions. The dotted curve indicates specular reflection.

Figure 3

(Color online) Comparison of data of Fig. 2 (circles) for outgoing normal energy $E_z^{\text{out}}$ as a function of incident normal energy $E_z^{\text{in}}$ for grazing scattering of ${\text{C}}_{60}^{+}$ from Al(001) to results from simulations (curves with symbols). The dotted curve indicates specular reflection. For details see the text.

Figure 4

Fragment-size distribution for scattering of 12.5 keV ${\text{C}}_{60}^{+}$ clusters under ${\Phi }_{\text{in}}=2.85°$ from Al(001). Signal for neutral clusters ${\text{C}}_x^0$ scaled down by factor 200.

Figure 5

(Color online) ${\text{C}}_{60}^{+}$, ${\text{C}}_{58}^{+}$, and ${\text{C}}_{56}^{+}$ fragment fractions normalized to ${\text{C}}_{60}^{+}–{\text{C}}_{52}^{+}$ as a function of normal energy loss $\Delta E_z$ for different angles of incidence indicated.

Figure 6

(Color online) ${\text{C}}_{60}^{+}–{\text{C}}_{52}^{+}$ fragment fractions as a function of normal energy loss $\Delta E_z$ (symbols). Plus signs show results from fits of fragment spectra. For details see the text.

Figure 7

(Color online) Measured fractions of positive ions after scattering of ${\text{C}}_{60}^{+}$ ions from Al(001) (full circles) as a function of incident normal energy $E_z^{\text{in}}$. Data compared to charge fractions derived from fits of fragment spectra using different rate prefactors as indicated. Full symbols: results from fits of both mean value and FWHM of internal energy distribution. Open symbols: results from fits of mean energy only, with FWHM from linear approximation. For details see the text and Fig. 8.

Figure 8

(Color online) Mean internal energy $E_{\text{in}}$ and FWHM of internal energy distribution derived from fits of fragment spectra as a function of normal energy loss $\Delta E_z$ for different rate prefactors indicated. Full symbols: results from fits of both mean value and FWHM of internal energy distribution. Open symbols: results from fits of mean energy only, with FWHM from linear approximation $\text{FWHM}=0.47\Delta E_z+14\text{eV}$. The solid line corresponds to $E_{\text{in}}=\Delta E_z+34.5\text{eV}$, which means full transfer of normal energy loss to internal excitations of clusters. For details see the text.