Threshold collision induced dissociation of pyrene cluster cations

Abstract

We report threshold collision induced dissociation experiments on cationic pyrene clusters, for sizes n=2 to 6. Fragmentation cross-sections were recorded as a function of the collision energy and analyzed with a statistical model. This model can account for the dissociation cascades and provides values for the dissociation energies. These values, of the order of 0.7 to 1 eV, are in excellent agreement with those previously derived from thermal evaporation. They confirm the charge resonance stability enhancement predicted by theoretical calculations. In addition, a remarkable agreement is obtained with theoretical predictions for the two smaller sizes n=2 and 3. For the larger sizes, the agreement remains good, although the theoretical values obtained for the most stable structures are systematically higher by 0.2 eV. This offset could be attributed to approximations in the calculations. Still, there is indication in the results of an incomplete description of the role of isomerization and/or direct dissociation upon collisions. Finally, by-product clusters containing dehydrogenated species are found to dissociate at energies comparable to the non-dehydrogenated ones, which shows no evidence for covalent bonds within the clusters.

Dynamical simulation of collision-induced dissociation of pyrene dimer cation

“…We have also plotted in Figure 5 the model cross section that successfully reproduced the threshold collision induced dissociation experimental results [69]. This model cross section is obtained by considering that the collision energy transfer is given by the LOC model and the expression for the cross section is given by:…”

“…The collision products are then analysed by time of flight mass spectrometry. Further details about the experiment can be found elsewhere [32,69,82,83].…”

“…The occurrence of collision or dissociation is decided at each time step of the ion trajectory based on the collision and dissociation probabilities. In our previous study [69] we treated the energy transfer upon collision by using the Line of Center model (LOC) [84]. In this model, the transferred energy is the kinetic energy along the line of centers.…”

“…The first two ones correspond to dissociation occurring during the MD simulation only or with the MD+PST model. The dissociation at infinite time accounts for all MD trajectories where the amount of energy transferred to the internal dimer rovibrational modes ∆E P y2 int is larger than the dissociation energy of 1.08 eV (value from references [59,69]). It can be regarded as the dissociation occurring after an infinite time neglecting any cooling processes, such as thermal collisions or photon emissions.…”

“…High energy collisions of PAH clusters with energetic ions have been simulated by Gatchel et al [67,68] at the semi-empirical and DFTB levels. Recently experiments at lower collision energies were performed [69]. These were analysed by treating statistically the dissociation after collision energy deposition.…”