Nonstatistical fragmentation of large molecules

We present experimental evidence for the dominance of prompt single-atom knockout in fragmenting collisions between large polycyclic aromatic hydrocarbon cations and He atoms at center-of-mass energies close to 100 eV. Such nonstatistical processes are shown to give highly reactive fragments. We argue that nonstatistical fragmentation is dominant for any sufficiently large molecular system under similar conditions.

Figure 1

Comparison of mass spectra due to ${\mathrm{C}}_{14}{\mathrm{H}}_{10}++\mathrm{He}$ (present work, black curve) and ${\mathrm{He}}^{+}+{\mathrm{C}}_{14}{\mathrm{H}}_{10}$ ([25], gray curve) collisions at center-of-mass energies of 110 eV and 11 keV, respectively. Target densities were chosen to ensure single-collision conditions.

Figure 2

Mass loss spectra for ${\mathrm{PAH}}^{+}+\mathrm{He}$ collisions at 110 eV center-of-mass energy (see the text for details).

Figure 3

(a) Attenuation of PAH${}^{+}$ cation beams in He. (b) Attenuation of coronene cation beams in He, Ne, Ar, and Xe. The lines in (a) and (b) are single-exponential fits. (c) Experimental absolute PAH${}^{+}$ fragmentation cross sections for collisions with He at 110 eV center-of-mass energy, geometrical cross sections, and calculated absolute total and nonstatistical fragmentation cross sections (see text). (d) Experimental absolute Cor${}^{+}$ fragmentation cross sections for He, Ne, Ar, and Xe at a Cor${}^{+}$ laboratory energy of 8.35 keV. Statistical uncertainties are smaller than the experimental data points and lines between the points in (c) and (d) are to guide the eye.

Figure 4

Likely structures of fragments produced by carbon knockout from coronene (C${}_{24}$H${}_{12}$, Cor${}^{+}$). The numbers indicate DFT energies (in eV) for the respective separated systems in relation to the ground state of Cor${}^{+}$. The labels I, II, III, etc. also indicate the fragment peaks to which they contribute. The gray area highlights structures which may contribute to the CH${}_x$-loss peak. Structure X may result from C${}_2$H${}_2$ loss in a purely statistical process.