Ion-pair dissociation of highly excited carbon clusters: Size and charge effects

We present measurements of ion-pair dissociation (IPD) of highly excited neutral and ionized carbon clusters ${\mathrm{C}}_{n=2-5}^{(q=0-3)+}$. The tool for producing these species was a high-velocity collision between ${\mathrm{C}}_n^{+}$ projectiles ($v=2.25$ a.u.) and helium atoms. The setup allowed us to detect in coincidence anionic and cationic fragments, event by event, leading to a direct and unambiguous identification of the IPD process. Compared with dissociation without anion emission, we found typical ${10}^{-4}$ IPD rates, not depending much on the size and charge of the ($n,q$) species. Exceptions were observed for ${\mathrm{C}}_2^{+}$ and, to a lesser extent, ${\mathrm{C}}_4^{3+}$ whose IPDs were notably lower. We tentatively interpret IPDs of ${\mathrm{C}}_2^{+}$ and ${\mathrm{C}}_3^{+}$ by using a statistical approach based on the counting of final states allowed by energetic criteria. The model is able to furnish the right order of magnitude for the experimental IPD rates and to provide a qualitative explanation of the lower IPD rate observed in ${\mathrm{C}}_2^{+}$.

Figure 1

Measured ${\mathrm{C}}_2^{-}-{\mathrm{C}}_2^{+}-{\mathrm{C}}^{+}$ production probability normalized to the total excitation and ionization probability of ${\mathrm{C}}_5^{+}$ as a function of the helium target thickness $n\delta x$ (${\mathrm{C}}_5^{+}-\mathrm{He}$ collision at $v=2.25$ a.u). The value at zero thickness is used to extract the ${\mathrm{C}}_2^{-}-{\mathrm{C}}_2^{+}-{\mathrm{C}}^{+}$ channel due to IPD.

Figure 2

Measured branching ratios for ion-pair channels in ${\mathrm{C}}_4$ ($n=4, q=0$, top panel) and ${\mathrm{C}}_5^{+}$ ($n=5, q=1$ bottom panel) as a function of the energy cost (eV). Upward triangles, downward triangles, diamonds, and star correspond respectively to two-fragment, three-fragment, four-fragment, and five-fragment IPD channels.

Figure 3

Measured ion-pair dissociation cross sections $\sigma ((q+1)+,-)$ of ${\mathrm{C}}_n^{q+}$ clusters as a function of $n$ (abscissa) and for various $q$ values (from top left to bottom right, $q=0$, 1, 2, 3).

Figure 4

Measured rates of ion-pair dissociation as compared with total dissociation of ${\mathrm{C}}_n^{q+}$ species as a function of $q$ and for various $n$ values: circles, squares, upward triangles, diamonds are for $n=2$, 3, 4, 5, respectively. Lines are to guide the eye.

Figure 5

Visualization of the PECs (in eV) of the 25 lowest ${}^4{\mathrm{\Sigma }}_u^{-}$ states of ${\mathrm{C}}_2^{+}$ from CASSCF ab initio calculations. The lowest ${\mathrm{C}}^{2+}-{\mathrm{C}}^{-}$ ion-pair channel (dashed-dotted red line) undergoes a cascade of avoided crossings with the highly excited valence states of ${\mathrm{C}}_2^{+}$, as detailed in the zoom on the interaction region. Lower-energy dissociation limits for C-${\mathrm{C}}^{+}, {\mathrm{C}}^{+}-{\mathrm{C}}^{+}$, and ${\mathrm{C}}^{2+}-{\mathrm{C}}^{-}$ are highlighted in colors. The calculations were performed from 10.0 Å to 1.0 Å by steps of 0.01 Å except near avoiding crossings where the step was fixed to 0.001 Å.

Figure 6

Internal energy distribution of excited ${\mathrm{C}}_2^{+}$ (circles) and ${\mathrm{C}}_3^{+}$ (triangles) calculated with the IAE model.