Elasticity, internal excitation, fragmentation, and charge transfer during grazing scattering of fast fullerenes from a KCl(001) surface

${\mathrm{C}}_{60}^{+}$ and ${\mathrm{C}}_{70}^{+}$ fullerenes with keV energies are scattered under grazing polar angles of incidence from an atomically clean and flat KCl(001) surface. For this model system of molecule surface interactions, the elastic properties of the fullerenes in front of the surface are studied by polar angular distributions. From the analysis of fragment spectra, the internal excitations of scattered molecules are deduced and excitation mechanisms are identified. Charge fractions indicate a kinematically induced neutralization of the fullerenes. Via an analysis of negatively charged fragments, the transition from a “soft” scattering event with intact outgoing fullerenes to postcollision multifragmentation is analyzed. The data are compared to three-dimensional molecular dynamics simulations based on empirical bond-order potentials.

Figure 1

Polar angular distributions recorded with the MCP detector (normalized intensity encoded in color scale) for scattering of ${\mathrm{C}}_{70}^{+}$ ions with energies indicated under a grazing angle of incidence ${\Phi }_{\text{in}}={2.2}^{°}$ along the high-indexed (random) direction of KCl(001). Distributions of (left) neutral projectiles and (right) positively charged projectiles.

Figure 2

Outgoing normal energy $E_{\perp }^{\text{out}}$ as a function of the incident normal energy $E_{\perp }^{\text{in}}$ for maxima (full symbols) and half-maximum positions (open symbols) of polar angular distributions for scattering of ${\mathrm{C}}_{70}^{+}$ (top) and ${\mathrm{C}}_{60}^{+}$ molecules (bottom) from KCl(001) under the indicated angles of incidence ${\Phi }_{\text{in}}$ and indicated azimuthal orientations with respect to low-indexed directions of the surface. Filled triangles: experimental data of Kimura et al.[19, 21] for scattering of $E=3$ keV ${\mathrm{C}}_{60}^{+}$ from KCl(001). Dotted curves: specular reflection.

Figure 3

Outgoing normal energy $E_{\perp }^{\text{out}}$ as a function of the incident normal energy $E_{\perp }^{\text{in}}$ derived from maxima of angular distributions for scattering of ${\mathrm{C}}_{70}^{+}$ (top) and ${\mathrm{C}}_{60}^{+}$ (bottom) from KCl(001). Filled and open circles: experimental data from Fig. 2 and from Kimura et al.,[19, 21] respectively. Thin curves with open symbols and thick curves with filled symbols: classical 3D trajectory simulations based on BOP by Tersoff[37] and by Erhart and Albe,[38] respectively, for different indicated parameter sets. For details see the text.

Figure 4

Solid curve: fragment size distribution measured via deflection of projectiles from the maximum of the polar angular distribution in the electric field for scattering of 50-keV ${\mathrm{C}}_{70}^{+}$ from KCl(001) under ${\Phi }_{\text{in}}={0.9}^{°}$. Bars: fit of fragment spectrum based on theoretical model for delayed ${\mathrm{C}}_2^0$ emission. Inset: Raw data as recorded with MCP detector. For details see text.

Figure 5

Mean energies $\langle E_{\text{int}}\rangle$ (filled symbols) and FWHM (open symbols) of internal energy distributions derived from positive fragment spectra as a function of incident normal energy $E_{\perp }^{\text{in}}$ for scattering of ${\mathrm{C}}_{70}^{+}$ (upper panel) and ${\mathrm{C}}_{60}^{+}$ (lower panel) under the indicated polar angles of incidence ${\Phi }_{\text{in}}$ along the indicated azimuthal directions from KCl(001). For details see the text.

Figure 6

Collision-induced internal excitation as a function of incident normal energy $E_{\perp }^{\text{in}}$ for scattering of ${\mathrm{C}}_{70}^{+}$ (upper panel) and ${\mathrm{C}}_{60}^{+}$ (lower panel) from KCl(001) under indicated polar angles of incidence ${\Phi }_{\text{in}}$. Symbols: experimental data from Fig. 5. Classical 3D trajectory simulations based on BOP by Tersoff[37] (thin curves with open symbols) or Erhart and Albe[38] (thick curves with filled symbols) for different indicated parameter sets. For details see the text.

Figure 7

Experimental positive ion fractions $P^{+}$ (filled symbols) as a function of incident normal energy $E_{\perp }^{\text{in}}$ for scattering of ${\mathrm{C}}_{70}^{+}$ (upper panel) and ${\mathrm{C}}_{60}^{+}$ (lower panel) molecules under the indicated polar angles of incidence ${\Phi }_{\text{in}}$ along indicated azimuthal directions from KCl(001). Open symbols: upper and lower limits for the fractions of positive ions leaving the surface $P_{\text{surf}}^{+}$. For details see the text.

Figure 8

Thin(thick) bars: fragment size distributions of small, negatively charged ${\mathrm{C}}_n^{-}$ fragments for scattering of ${\mathrm{C}}_{70}^{+}$ molecules with energy $E=45$ keV under ${\Phi }_{\text{in}}={1.2}^{°}$(${2.2}^{°}$) from KCl(001). Inset: raw data for polar angular distributions as recorded with the MCP detector for scattering of ${\mathrm{C}}_{70}^{+}$ molecules with $E=45$ keV under ${\Phi }_{\text{in}}={2.1}^{°}$ from KCl(001). Left bar: neutral projectiles. Negatively charged small ${\mathrm{C}}_n^{-}$ fragments deflected to the right. Distributions for different $n$ overlaps.

Figure 9

Measured negative ion fractions as a function of incident normal energy $E_{\perp }^{\text{in}}$ for scattering of ${\mathrm{C}}_{70}^{+}$ (upper panel) and ${\mathrm{C}}_{60}^{+}$ (lower panel) molecules under indicated polar angles of incidence ${\Phi }_{\text{in}}$ and azimuthal directions from KCl(001) (symbols) compared to results from 3D trajectory simulations for indicated angles and parameters (curves with symbols). For details see text.

Figure 10

Snapshots from 3D trajectory calculations for ${\mathrm{C}}_{70}$ molecules scattered under a polar angle of incidence ${\Phi }_{\text{in}}={2.2}^{°}$ deg with indicated normal energies $E_{\perp }^{\text{in}}$ from a perfect KCl(001) surface (a, b) and a step on a KCl(001) surface (c). Insets: simulated impacts of fragments on the MCP detector without deflection plates. Fragment size encoded in size and color of points (color scale). Small black dots: impacts of large ${\mathrm{C}}_{n>20}$ fragments. For details see text.

Figure 11

Outgoing normal energies $E_{\perp }^{\text{out}}$ from maxima (filled symbols) and half-maximum positions (open symbols) of size-integrated polar angular distributions of small negative ${\mathrm{C}}_{n⩽20}^{-}$ fragments as a function of incident normal energy $E_{\perp }^{\text{in}}$ for scattering of ${\mathrm{C}}_{70}^{+}$ (upper panel) and ${\mathrm{C}}_{60}^{+}$ molecules (lower panel) under the indicated angles of incidence ${\Phi }_{\text{in}}$ along the indicated azimuthal directions from a KCl(001) surface. Symbols: experimental data. Curves with symbols: results from 3D trajectory simulations for indicated parameters. For details see the text.