Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation

A recent study of soft x-ray absorption in native and hydrogenated coronene cations, ${\mathrm{C}}_{24}{{\mathrm{H}}_{12+m}}^{+}m=0–7$, led to the conclusion that additional hydrogen atoms protect (interstellar) polycyclic aromatic hydrocarbon (PAH) molecules from fragmentation [Reitsma et al., Phys. Rev. Lett. 113, 053002 (2014)]. The present experiment with collisions between fast (30–200 eV) He atoms and pyrene (${\mathrm{C}}_{16}{{\mathrm{H}}_{10+m}}^{+}, m=0$, 6, and 16) and simulations without reference to the excitation method suggests the opposite. We find that the absolute carbon-backbone fragmentation cross section does not decrease but increases with the degree of hydrogenation for pyrene molecules.

Figure 1

Schematic of the experimental apparatus.

Figure 2

Absolute cross sections for carbon backbone fragmentation, ${\sigma }_{\text{backbone}}^{\text{fragm}}$, in ${\mathrm{C}}_{16}{{\mathrm{H}}_{10+m}}^{+}$ + He collisions as functions of the center-of-mass collision energy. From top to bottom: Experimental results for $m=16, m=6$, and $m=0$ and results for prompt single- and multiple-carbon knockouts from molecular dynamics simulations for $m=16$ (dotted curve), $m=6$ (dashed curve), and $m=0$ (solid curve) (see text).

Figure 3

Mass spectra from collisions between ${\mathrm{C}}_{16}{{\mathrm{H}}_{10+m}}^{+}$ ($m=0,6,16$) and He at 110 eV center-of-mass collision energy. Labels indicate the number of C atoms in every peak, $n_{\text{C}}$, each with a distribution of the number of H atoms. The insets show how the molecular structures change as H atoms are added to the pyrene molecule. The native pyrene molecule (top panel) is completely planar with only aromatic ${\mathrm{sp}}^2$-type bonds. Hydrogenation results in weaker ${\mathrm{sp}}^3$-type single bonds which bend the molecular structure out of the plane (middle and bottom panels).

Figure 4

Fractions of SCC-DFTB simulations which result in carbon-backbone fragmentation of ${\mathrm{C}}_{16}{{\mathrm{H}}_{10+m}}^{+}$ ($m=0,6,16$) as functions of internal energy (top panel) and vibrational temperature (bottom panel). Twenty trajectories have been simulated for each ion and each energy. The backbone fragmentation probability is higher for the hydrogenated molecules at all energies and temperatures. The insets show typical fragmentation pathways for ${\mathrm{C}}_{16}{{\mathrm{H}}_{10}}^{+}$ (${\mathrm{C}}_2{\mathrm{H}}_2$ loss in the lower panel) and for ${\mathrm{C}}_{16}{{\mathrm{H}}_{16}}^{+}$ (${\mathrm{CH}}_3$ loss in the upper panel) at internal energies in the 15- to 20-eV range (see text).

Figure 5

Left scale: Absolute cross sections for forming products with different numbers ($n_{\text{C}}<16$) of C atoms on the microsecond time scale following 110 eV He + ${\mathrm{C}}_{16}{{\mathrm{H}}_{10+m}}^{+}$ ($m=0,6,16$) collisions. Right scale: The bars show the number of SCC-DFTB simulations leading to a charged fragment containing $n_{\text{C}}$ carbon atoms. We show combined result from 20 simulations (simulation time 1 ns) for each molecule for each internal energy of 15, 18, 20, 22, and 25 eV. Knockout processes are not included in these simulations, explaining the differences between experimental and SCC-DTFB results for $n_{\text{C}}=15$.