Ions Colliding with Cold Polycyclic Aromatic Hydrocarbon Clusters

We report the first experimental study of ions interacting with clusters of polycyclic aromatic hydrocarbon (PAH) molecules. Collisions between 11.25 keV ${}^3\mathrm{He}^{+}$ or 360 keV ${}^{129}\mathrm{Xe}^{20+}$ and weakly bound clusters of one of the smallest PAH molecules, anthracene, show that ${\mathrm{C}}_{14}{\mathrm{H}}_{10}$ clusters have much higher tendencies to fragment in ion collisions than other weakly bound clusters. The ionization is dominated by peripheral collisions in which the clusters, very surprisingly, are more strongly heated by ${\mathrm{Xe}}^{20+}$ collisions than by ${\mathrm{He}}^{+}$ collisions. The appearance size is $k=15$ for $[{\mathrm{C}}_{14}{\mathrm{H}}_{10}{]}_k^{2+}$.

Figure 1

Left panels: Mass-to-charge spectra for ${\mathrm{He}}^{+}/{\mathrm{Xe}}^{20+}+{\mathrm{C}}_{14}{\mathrm{H}}_{10}$ (monomer) collisions (molecular structure in the inset). Middle panels: Mass-to-charge spectra below $190\mathrm{amu}/\mathrm{e}$ for ${\mathrm{He}}^{+}/{\mathrm{Xe}}^{20+}+[{\mathrm{C}}_{14}{\mathrm{H}}_{10}{]}_k$ (cluster) collisions. Insets show relative ${\mathrm{C}}_n{\mathrm{H}}_x^{+}$ intensity distributions as functions of $n$ for the cluster (filled symbols) and monomer target (open symbols) scaled by the ratio between the cluster and monomer fragmentation efficiencies. Right panels: Size-to-charge spectra for fragment clusters, $[{\mathrm{C}}_{14}{\mathrm{H}}_{10}{]}_{j<k}^{q+}$, produced in $\mathrm{He}+/{\mathrm{Xe}}^{20+}+[{\mathrm{C}}_{14}{\mathrm{H}}_{10}{]}_k$ collisions. Insets: zoom-ins for $j>4$ with logarithmic intensity scales.

Figure 2

Zoom-ins on the ${\mathrm{C}}_{14}{\mathrm{H}}_{10}^{+}$- and ${\mathrm{C}}_2{\mathrm{H}}_x$-loss peaks for ${\mathrm{He}}^{+}/{\mathrm{Xe}}^{20+}+{\mathrm{C}}_{14}{\mathrm{H}}_{10}$ (grey curves) and ${\mathrm{He}}^{+}/{\mathrm{Xe}}^{20+}+[{\mathrm{C}}_{14}{\mathrm{H}}_{10}{]}_k$ (black curves) collisions. The insets are zoom-ins on the ${\mathrm{C}}_2{\mathrm{H}}_x$ loss peaks. The asterisks mark the presence of carbon-13 isotopes.

Figure 3

Relative intensities of $[{\mathrm{C}}_{14}{\mathrm{H}}_{10}{]}_{j<k}^{+}$ clusters produced in ${\mathrm{Xe}}^{20+}$ (squares) and ${\mathrm{He}}^{+}$ (triangles) collisions with clusters of $k$ anthracene molecules $[{\mathrm{C}}_{14}{\mathrm{H}}_{10}{]}_k$.