Threshold Energies for Single-Carbon Knockout from Polycyclic Aromatic Hydrocarbons

Abstract

We have measured absolute cross sections for ultrafast (femtosecond) single-carbon knockout from polycyclic aromatic hydrocarbon (PAH) cations as functions of He–PAH center-of-mass collision energy in the 10–200 eV range. Classical molecular dynamics (MD) simulations cover this range and extend up to 105 eV. The shapes of the knockout cross sections are well-described by a simple analytical expression yielding experimental and MD threshold energies of EthExp = 32.5 ± 0.4 eV and EthMD = 41.0 ± 0.3 eV, respectively. These are the first measurements of knockout threshold energies for molecules isolated in vacuo. We further deduce semiempirical (SE) and MD displacement energies, i.e., the energy transfers to the PAH molecules at the threshold energies for knockout, of TdispSE = 23.3 ± 0.3 eV and TdispMD = 27.0 ± 0.3 eV. The semiempirical results compare favorably with measured displacement energies for graphene (Tdisp = 23.6 eV).

Dissociation of polycyclic aromatic hydrocarbons at high energy: MD/DFTB simulations versus collision experiments

“…3) Most dissociation events occur within a few hundreds of picoseconds, starting from the ionized pyrene. 4) In line with the first point, a wide range of masses are observed and no loss of "CH n " is found in the experiment, showing the absence of non statistic dissociation ("knockout" process) that occurs at the lowest energy collision [17,15,47,48]). This is reproduced by calculations which only describes statistic dissociation events.…”

“…In other environments of the ISM such as the vicinity of a massive star, PAHs may collide with energetic atoms ejected from the star [43,54] while, in shock wave regions, PAHs are exposed to highly energetic collisions [30,29]. To mimic these high energy collisions, experiments, often complemented with molecular dynamics (MD)/force field (FF) simulations, have been achieved (see for instance [17,15,47,48,58,10,23,27,34]).…”

DESIREE electrospray ion source test bench and setup for collision induced dissociation experiments

“…For ions having lost one or several heavy atoms like C and/or N, the spread in energy due to the collisional process is larger than for the loss of H atoms. However, as the beam/collision energy decreases, we observe a broadening and a shift towards lower apparent masses of the peaks in the mass spectra (see Figure 1 in Stockett et al 28 ). This is due to conversion of larger fractions of the available energy into internal energies resulting in a decrease of the translational energy of the fragments.…”

“…Experiments have been performed with this setup for a wide range of molecular ion projectiles, such as native and hydrogenated PAHs [25][26][27][28][29][30][31] , fullerenes 32,33 and biomolecules like tetraphenylporphyrin 34 and nbutylamine retinal protonated Schiff base 35 .…”

Decay pathways for protonated and deprotonated adenine molecules

“…Furthermore, we calculate the cross sections for heavy atom (carbon or nitrogen) knockouts in Rutherfordlike scattering processes by analysing the fragments formed in the collisions 27 . This has been shown to be an important non-statistical fragmentation pathway for stable and large molecular systems such as PAHs [27][28][29] , clusters 25,32,33 and porphyrins 34 in the velocity range considered here. The present model has been shown to overestimate the threshold energy for prompt atom knockout in collisions with helium 35 .…”