Statistical fragmentation of doubly charged anthracene induced by fluorine-beam impact at 3 keV

The fragmentation scheme of the doubly charged anthracene molecule (C${}_{14}$H${}_{10}$${}^{2+}$) has been studied via monocharged fluorine impact at 3 keV using the CIDEC method (collision-induced dissociation under energy control). Doubly or singly charged fragments resulting from the loss of neutrals (H, C${}_2$H${}_2$) or C${}_x$H${}_y$${}^{+}$ ($x$ $=$ 1–5; $y$ $=$ 1–5) have been measured versus the excitation energy of the parent molecule C${}_{14}$H${}_{10}$${}^{2+}$. The branching ratio of the C${}_x$H${}_y$${}^{+}$ emission process was found to be 16$\%$. For the major neutral evaporation channels via $n$H or $n$C${}_2$H${}_2$ ($n$ $=$ 1, 2) emission, the measured population distributions have been fitted using a Rice-Ramsperger-Kassel (RRK) statistical dissociation and cascade model. The simulated rate-energy dependences of the two primary competing pathways, i.e., the loss of H or C${}_2$H${}_2$ from C${}_{14}$H${}_{10}$${}^{2+}$, present a crossing point at 13 eV. This feature is characteristic of two different fragmentation mechanisms: a direct cleavage for the loss of H for energy above the crossing point and a rearrangement process for the loss of C${}_2$H${}_2$ bellow the crossing point.

Figure 1

Experimental setup.

Figure 2

TOF spectrum in F${}^{+}$ $+$ C${}_{14}$H${}_{10}$→F${}^{-}$ $+$ C${}_{14}$H${}_{10}$${}^{2+*}$ DCT collisions calibrated to $m$/$q$ value of the detected ions. The molecular structure of anthracene is shown in the inset of the figure.

Figure 3

(a) 2D spectrum in F${}^{+}$ $+$ C${}_{14}$H${}_{10}$→F${}^{-}$ $+$ C${}_{14}$H${}_{10}$${}^{2+}$ collisions. Horizontal axis: $m$/$q$ of the recoil ions; vertical axis: excitation energy of the parent C${}_{14}$H${}_{10}$${}^{2+}$ before dissociation. (b) Projection of the 2D spectrum to the horizontal axis. (c) Projection of the 2D spectrum to the vertical axis.

Figure 4

(a) An enlarged view of the 2D spectrum from Fig. 3(a). (b)–(e) Partial projection to the excitation energy axis of the main spots assigned respectively to C${}_{14}$H${}_{10}$${}^{2+}$, (C${}_{14}$H${}_{10}$-2H)${}^{2+}$, (C${}_{14}$H${}_{10}$-C${}_2$H${}_2$)${}^{2+}$, and (C${}_{14}$H${}_{10}$-2C${}_2$H${}_2$)${}^{2+}$. The top scale and the bottom scale are converted respectively to the kinetic energy loss $\Delta$$E$ of the anions F${}^{-}$ and the excitation energy of the parent C${}_{14}$H${}_{10}$${}^{2+}$ ions.

Figure 5

Excitation energy distribution corrected from experimental broadening after deconvolution. The dots linked with a dashed line show the sum of these four distributions corresponding approximately to the measured total excitation energy distribution of the parent C${}_{14}$H${}_{10}$${}^{2+}$.

Figure 6

Simplified dissociation scheme of C${}_{14}$H${}_{10}$${}^{2+}$ including the main dissociation channels only.

Figure 7

Calculated breakdown curves : Relative probability versus the excitation energy for C${}_{14}$H${}_{10}$${}^{2+}$ parent ions to decay to (C${}_{14}$H${}_{10}$-H)${}^{2+}$, (C${}_{14}$H${}_{10}$-2H)${}^{2+}$, (C${}_{14}$H${}_{10}$-C${}_2$H${}_2$)${}^{2+}$, and (C${}_{14}$H${}_{10}$-2C${}_2$H${}_2$)${}^{2+}$ for a time windows $t_{\mathrm{extr}}$ $=$ 1 μs.

Figure 8

Comparison between experimental (solid lines) and simulated (dashed lines) distributions for C${}_{14}$H${}_{10}$${}^{2+}$, (C${}_{14}$H${}_{10}$-H)${}^{2+}$, (C${}_{14}$H${}_{10}$-2H)${}^{2+}$, (C${}_{14}$H${}_{10}$-C${}_2$H${}_2$)${}^{2+}$, and (C${}_{14}$H${}_{10}$-2C${}_2$H${}_2$)${}^{2+}$. Experimental distribution of (C${}_{14}$H${}_{10}$-2H)${}^{2+}$ includes the small contribution of (C${}_{14}$H${}_{10}$-H)${}^{2+}$.

Figure 9

Dissociative rate constant $k$ as a function of the excitation energy for the emission of H and C${}_2$H${}_2$.