# Electron-Induced Fragmentation Dynamics of 1-Methylpyrene (C17H12) Dications and Trications: C2H$_{x}$$^{q+}$ Release Pathways

Eszter Dudás, Mathias Rapacioli, Patrick Moretto-Capelle, Anthony

Scemama, Jean-Philippe Champeaux

► To cite this version:

Eszter Dudás, Mathias Rapacioli, Patrick Moretto-Capelle, Anthony Scemama, Jean-Philippe Champeaux. Electron-Induced Fragmentation Dynamics of 1-Methylpyrene (C17H12) Dications and Trications: C2H$_{x}$$^{q+}$ Release Pathways. Journal of Physical Chemistry A, In press, 10.1021/acs.jpca.5c08076 . hal-05518458

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Submitted on 19 Feb 2026

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Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
Electron-Induced Fragmentation Dynamics of
1-Methylpyrene ($C_{17}H_{12}$) Dications and Trications:
$C_{2}H_{x}^{q+}$ Release Pathways

Eszter Dudás¹*, Mathias Rapacioli², Patrick Moretto-Capelle¹, Anthony Scemama² and Jean-Philippe Champeaux¹

¹Laboratoire Collisions Agregats Reactivité, CNRS

(118 Rte de Narbonne Toulouse, Haute-Garonne, FR 31062)

²Laboratoire de Chimie et Physique Quantiques, CNRS

(118 route de Narbonne Toulouse, Haute-Garonne, FR 31062)

KEYWORDS: Astrochemistry, Electron-induced dissociation, PAH dications and trications, Fragmentation dynamics, $C_{2}H_{x}$ fragments, ToF-VMI

Corresponding author: eszter.dudas.work@gmail.com
**Abstract**

The dissociation behavior of 1-methylpyrene (MP) was investigated using the SWEET experimental system under electron-induced collisions to elucidate the fragmentation dynamics of polycyclic aromatic hydrocarbon ions (PAH). Molecular interaction with 200 eV electrons initiated multiple ionization and fragmentation pathways resulting in molecular cation production. The unfragmented monocation $MP^+$ is identified as the most abundant one, the intact dication $MP^{2+}$, the de-ethylated dication, and a stable trication $MP^{3+}$ were also observed, demonstrating the formation and stability (>ms) of highly charged molecular ions. Among the smaller observed hydrocarbon fragments, neutral $C_2H_x^0$ and cationic $C_2H_x^+$ species were the predominant dissociation products. Experimental findings were compared with density functional theory based tight binding molecular dynamics simulations. Measurements of the cation signals as a function of the incident electron energy (17–35 eV) provided appearance energies for the dications and correlated cations. The higher experimental appearance energy values relative to simulated vertical ionization energies suggest the involvement of autoionization processes from initially populated electronic exited states of the molecule.

**1. Introduction**

Spectacular advances in mm-wave, notably the Atacama Large Millimeter Array (ALMA),¹ and infrared (IR), such as the James Webb Space Telescope (JWST) telescopic observations² have illuminated the chemical complexity of circumstellar environments. These regions are notably rich in complex organic molecules, including PAHs and their aliphatic derivatives.

Nevertheless, models of the physical-chemical mechanisms governing these astrophysical environments, as well as the precise role of PAHs in the cosmic carbon cycle, remain incomplete.
A recent and significant discovery lies in the identification of small hydrocarbon cations, such as $CH_3^+ 3$ and $C_2H_2^0 4.5$ within the protoplanetary disk of a young star located in the Orion cloud.

While photochemistry is commonly invoked to model the formation processes of these cationic species, the possibility of these cations being produced via the interaction of stellar winds with pre-existing PAHs in the surrounding medium is largely under-explored.

These stellar winds are composed of high-energy ions and electrons, typically in the keV range for ions⁶ and in the eV range for electrons,⁷ emitted by the nascent star. It is important to note that bipolar jets, emanating from the accretion disk, can also propagate these energetic particles over vast distances ranging from few au to several light-years,⁸ and deep into the host molecular cloud.

The fragmentation pathways of molecular dications and trications have long been a central topic in studies of molecular fragmentation dynamics, both experimentally and through molecular dynamics simulations. Early investigations by Kingston et al. examined the abundances of multiply charged ions in a homologous series of aromatic compounds with one to four carbon rings using a modified VC Micromass ZAB-2F mass spectrometer. Their results revealed characteristic fragmentation routes for the doubly and triply charged species, emphasizing the influence of charge state on preferred dissociation channels.⁹ Subsequent studies have shown that ultrafast intramolecular charge migration and proton transfer processes can strongly affect the fragmentation pattern. Time-resolved experiments by Lee et al., employing pump-probe spectroscopy coupled with a double-sided velocity-map imaging (VMI) spectrometer, explored the electronic relaxation dynamics of fluorene, phenanthrene, and pyrene.¹⁰ Their observations in the XUV-IR range indicated sub-picosecond relaxation across densely populated electronic states, consistent with earlier findings of Marciniak et al.¹¹
The fragmentation dynamics and corresponding kinetic energy release (KER) associated with
molecular dissociation have also been systematically explored over the past decades. Lee *et al.*
directly measured the total KER (TKER) of the dissociations and analyzed the contributions from
Coulomb repulsion and charge-state effects.¹² Wang *et al.* linked higher charge states to enhanced
Coulombic repulsion and correspondingly increased kinetic energies using VMI techniques.¹³
Sundarajan *et al.* reported that photoinduced dissociation of bowl-shaped hydrocarbons primarily
yields sequential H losses followed by dominant C₂H₂ elimination in sumanene.¹⁴ Furthermore,
dissociative electron ionization dynamics, probed using multimass VMI combined with pulsed
electron impact, have been investigated for a variety of molecular systems.¹⁵⁻¹⁷ These studies
reveal that the measured fragment ion kinetic energy distributions are largely independent of the
incident electron energy, suggesting that initially excited electronic states undergo rapid internal
relaxation into metastable dissociative states before fragmentation.¹⁸

From the theoretical side, the fragmentation of PAHs can be investigated with static approaches
when the study is restricted to a few well-identified dissociation channels. In such case, *ab initio*
schemes can derive the corresponding energy barriers.¹⁹⁻²² Molecular dynamics (MD) simulations
offer an alternative approach. Their main advantage is that they naturally include entropic effects,
rather than focusing solely on dissociation pathways associated with the lowest potential energy
barrier. In addition, they account for the competition among numerous isomerization and
fragmentation channels, as expected for extended molecular systems at high internal energies. The
drawback lies in the fact that these approaches usually rely on semi-empirical potentials and that
dissociations at low energies can hardly be simulated in a reasonable computational cost.
Nevertheless, this approach has been shown to fairly reproduce experimentally-determined PAH
fragmentation pathways.²³⁻²⁸
The present work focuses on investigating the fragmentation pathways and dissociation dynamics of 1-methylpyrene dications and trications formed via electron-induced ionization. We identify the most prominent fragmentation routes and study the TKER of these fragmentation channels. This approach enables a detailed examination of charge-state-dependent fragmentation pathways, the potential formation of metastable intermediate states. Additionally, it permits the determination of the ionization thresholds of 1-methylpyrene and the appearance energies of the resulting product cations. Particular attention was given to the formation of small hydrocarbon cations (e.g., $C_2H_x^{q+}$), which provides important insight into both the underlying interaction mechanisms and the physical characterization of such processes in an astrophysical context. Comparison of the experimental results with molecular dynamics simulations further supports the interpretation of the observed fragmentation behavior.

## 2. Experimental Method

Collision-induced dissociation of neutral 1-methylpyrene molecules upon interaction with low-energy electrons was investigated using an event-by-event multiple correlation spectroscopy approach that couples time-of-flight mass spectrometry (TOFMS) with VMI.

<figure><img src="image_2.png" /><figcaption>1. Figure: 1-methylpyrene molecule</figcaption></figure>
The experimental system, referred to as STellar wind and Electrons interactions on astrophysical molecules: Experiment and Theory (SWEET), has been described in detail previously.²⁹ Only a brief overview is provided here with a schematic representation in Figure 2.

1-methylpyrene (≥ 96%, [abcr GmbH]) was stored in solid form within a reservoir inside a vacuum chamber maintained at ~1.3x10⁻⁵ Pa. The sample was gently heated to 65 °C to promote sublimation, generating a continuous molecular beam of neutral 1-methylpyrene with through a pinhole of approximate diameter of 2 mm at the center of the vacuum chamber.

Electron impact ionization was performed using a commercial electron gun (Kimball Physics® ELG-2). The electron beam was pulsed by homemade fast-circuit²⁹ operating at a repetition rate of 20 kHz. Each pulse, with a duration of a few nanoseconds, delivers fewer than 1000 electrons. Owing to the low molecular beam density, the probability of interaction per pulse remained well below unity, thereby ensuring predominantly single electron-molecule collision events.

The resulting molecular ions and fragment species were extracted into a custom-built 250 mm TOF spectrometer following a delay of approximately 150 ns relative to the electron pulse trigger, preventing perturbation of the electron packet during the interaction. Ion arrival times and spatial impact coordinates (x,y) were recorded using a pair of 40 mm microchannel plates (MCP) (Hamamatsu Ltd.®). They are coupled to a Roentdek DLD40 two-dimensional delay-line reading anode, enabling event-by-event reconstruction of dissociation pathways and fragment-fragment correlations where applicable. From the measured position and time-of-flight of each charged fragment, it is possible to reconstruct its full three-dimensional velocity vector.³⁰

To compensate for the KER of ionic fragments, which otherwise degrades TOF resolution, an additional extraction delay of several microseconds (dependent on the fragment mass) can be
introduced.³¹ However, this comes at the cost of losing the ability to correlate charged fragments on an event-coincident basis.

<figure><img src="image_3.png" /><figcaption>2. Figure: Schematic representation of the experimental system.</figcaption></figure>

3. Theoretical approach

The fragmentation mass spectra have also been simulated from molecular dynamics simulations.
Calculations were performed within the Born Oppenheimer molecular dynamics framework on the
electronic ground state of the 1-methylenepyrene dications and the initial energy-was randomly
spread over the nuclear degrees of freedom. It assumes that ionization leaves the molecule in an
excited electronic state which is followed by electronic relaxation and intramolecular ro-
vibrational redistribution before the dissociation occurs. The MP dication (MP²⁺) electronic
ground state energy was computed at the Density Functional based Tight Binding (DFTB)
level.³²,³³ This is an approximated DFT scheme presenting a much lower computational cost
arising from the use of parameterized integrals expressed in a minimal atomic valence basis set as
required to perform long MD simulations, while inheriting from DFT the ability to describe bond
breaking and forming. The simulations were performed using the mio parameters dataset,³⁴ and a
Fermi temperature of 1500 K governing the filling of molecular orbitals, to avoid convergency
issues at the dissociation.²⁶ Five initial internal energies of 18, 20, 22, 25 and 30 eV were selected arbitrarily for simulation of MP²⁺. We mention here that the internal energies of MP²⁺ should not be confused with either the experimentally measured electron energies or the deposited energy. The deposited energy is partly consumed in the double ionization of the MP molecule and therefore consists of the internal energy plus the double-ionization potential:

$$Deposited \ energy = internal \ energy + double \ ionization \ potentiel \qquad 1$$

Furthermore, the high simulated internal energies have been chosen to observe fragmentation within a reasonable simulated time, the latter being smaller than the experimental timescales.

Energies lower than 18 eV were not investigated as the 1-methylenepyrene dications remained stable in simulations at this energy. For each internal energy, a set of 40 simulations of 500 ps have been performed making use of a timestep of 0.1 fs. The fragments were identified from the last snapshot coordinates and the associated charges were recorded. We note that the value of the fragment charges is questionable, due to DFT-based issues describing non-integer numbers of electrons,³⁵ which, in practice resulted in partial charges on the fragments. All calculations have been performed with the deMonNano software.³⁶

# 4. Results and Discussion

## 4.1 Single-hit-events

A TOF mass spectrum of 1-methylpyrene was recorded at an incident electron energy of 200 eV. The single-hit events, in which the MCP detects only one cation at a time, are shown in Figure 3 as a function of TOF, with the corresponding mass-to-charge (m/z) ratios indicated above the spectrum. Enlarged views of four selected spectral regions are also presented, with enhanced mass resolution made possible by the use of delayed extraction.
<figure><img src="image_4.png" /><figcaption>**3. Figure:** ToF mass spectrum of 1-methylpyrene, single-hit-events, with four enlarged regions. The red dotted traces represent spectra acquired under delayed extraction conditions, which provide enhanced resolution in the regions of interest. Panel a: triply ionized MP, panel b: doubly charged fragment formed by the loss of a neutral ethyl group, panel c: doubly ionized MP, panel d: simple ionized MP.</figcaption></figure>

The observed peaks correspond to sequential hydrogen losses of varying magnitude originating from different carbon sites within the molecule. Panel d displays the singly charged molecular ion of 1-methylpyrene, dominated by a peak at *m/z* = 216 and accompanied by a satellite at *m/z* = 217 arising from the natural isotopic abundance of ¹²C (98,93%) and ¹³C (1.07%). For 1-methylpyrene this distribution yields approximately 85% of *M*<sub>216</sub> and 15% of *M*<sub>217</sub>. Peaks at *m/z* = 215 and 213 indicate successive hydrogen atom loss. The stable doubly charged dication is shown in panel c, the *m/z* = 108 corresponds to the intact 1-methylpyrene dication, and the presence of half-integer
$m/z$ values confirms the formation of odd-mass fragments typical of doubly charged species. For example, the feature at $m/z = 105.5$ arises from a doubly charged fragment resulting from the loss of five hydrogen atoms from MP. Panel *b* highlights a doubly charged fragment at $m/z = 94.5$ (189 Da neutral mass) formed by the loss of a 27 Da unit (C-CH$_3$), flanked by additional fragments exhibiting hydrogen loss. Finally, a weak signal, magnified in panel *a*, reveals the presence of a stable triply charged $MP^{3+}$ ion, accompanied by a series of fragments corresponding to the successive loss of up to three hydrogen atoms. The relative abundances of the singly, doubly, and triply charged species are 79.8%, 19.3%, and 0.9%, respectively, under single-hit detection conditions.

*Ab initio* vertical ionization energy (VIE) calculations of 1-methylpyrene were investigated using two complementary computational approaches. First, DFT-based VIE calculations were performed at the 6-311G basis set, making use of the General Atomic and Molecular Electronic Structure System (GAMESS)$^{37}$ quantum chemistry package. The calculated results consistent with the stable existence of $MP^{+}$, $MP^{2+}$ and $MP^{3+}$ species observed in our single-hit experimental measurements, as presented in Figure 3. In parallel, higher-level *ab initio* calculations were carried out using the Q-Chem program$^{38}$ to provide a more accurate description of the ionization process. The molecular geometry of 1-methylpyrene was fully optimized at the B3LYP/6-311G** level and subsequently kept fixed. The first ionization energy of the neutral molecule was computed with the Coupled Cluster Singles and Doubles method at the CCSD/aug-cc-pVDZ level (7.28 eV) and refined at the CCSD(T)/aug-cc-pVDZ level, yielding 7.33 eV. The computed theoretical VIEs are summarized in Table 1.

**Table 1.** Vertical ionization energies referenced to the optimized geometry of the neutral singlet molecule.
<table><thead><tr><th>Charge</th><th>Multiplicity</th><th>DFT (eV)</th><th>CCSD(T) (eV)</th></tr></thead><tbody><tr><td>1</td><td>doublet</td><td>6.94</td><td>7.33</td></tr><tr><td>2</td><td>singlet</td><td>18.14</td><td>18.76</td></tr><tr><td>2</td><td>triplet</td><td>19.13</td><td>19.73</td></tr><tr><td>3</td><td>doublet</td><td>34.49</td><td></td></tr></tbody></table>

The first ionization potential is in good agreement with the experimental value 7.27 ±0.01 eV reported by West *et al.*<sup>39</sup>

### 4.1.1 Experimental uncertainties in appearance-energy estimates

The first uncertainty component arises from the intrinsic energy width of the pulsed electron gun, which was characterized using a high-resolution calibration measurement.<sup>40</sup> This instrumental broadening results in electron energy uncertainty of ±0.15 eV across all measurements.

The second component reflects the statistical uncertainty associated with the threshold fitting procedure applied to the experimental data. These fit-derived uncertainties are reported individually for each measurement. The uncertainty related to the counting statistics (i.e., the counting rate uncertainty of cationic events) is negligible compared to the two aforementioned sources. Therefore, these two dominant terms define the total combined uncertainty of the appearance energy estimates.

### 4.1.2 Measurement of 1-methylpyrene dication appearance energy

The appearance energies for the doubly ionized 1-methylpyrene, the doubly charged fragment resulting from the loss of a neutral $C_2H_x^0$ group and the specific dissociation energy threshold of the $MP^{2+} \rightarrow M_{27}^+ + M_{189}^+$ fragmentation channel were experimentally determined. The investigations started at 17.3 eV just below the DFT double ionization energy estimation (at 18.14 eV) and progressed in 0.5 eV energy increments up to 29.3 eV. The measured intensities of
$MP^{2+}$ were normalized to those of the corresponding singly charged molecular ion $MP^{+}$, in order to remove potential effects from variations in electron-beam intensity, molecular-jet density, and ionization cross sections across different electron energies. Once the dataset was obtained, the ion yield (IY) curve were fitted by a power law onset model:

$$ IY(E) = \begin{cases} 0 & E < E_0 \\ A \times (E - E_0)^p & E \ge E_0 \end{cases} \qquad 2 $$

Where, $E_0$ is the appearance energy, A is a scaling factor and p corresponds to an empirical exponent. The experimental setup was calibrated using helium, whose ionization energy of 24.58 eV is well established,⁴¹ and a deviation of $\Delta E = +0.7$ eV from the expected value was observed in our measurements. Since the appearance energies of 1-methylpyrene and its fragments lie within the same energy range, a correction of $-0.7$ eV was applied to all reported appearance energies. Figure 4 depicts the appearance energy analysis of the 1-methylpyrene dication, yielding an experimental value of $22.2 \pm 0.8_{-0.9}^{+0.8}$ eV (95%CI). The fitted empirical exponent ($p = 1.95$) indicates a relatively steep onset of the $MP^{2+}$ yield, implying that once the energetic threshold is exceeded, ionization to the dication state becomes increasingly probable. This result was benchmarked using the ThreSpect software developed by Jurkowski et al.⁴²,⁴³ and the resulting appearance energy of $22.3 \pm 0.4_{-0.35}^{+0.4}$ eV (95%CI) is in good agreement with the present value and is shown in Figure 4 (red crosses).
<figure><img src="image_5.png" /></figure>

**4. Figure:** Intensity of appearance energy in function of the electron energy for *MP*²⁺. Green circles: experimental data with error bars, blue continuous line: ion yield curve fitted by power law onset model, red crosses: ion yield curve fitted by ThreSpect software.

To the knowledge of the authors, the appearance energy of *MP*²⁺ has not been reported in the literature yet. However, the obtained ionization energy lies significantly above the calculated vertical double ionization energies, *E*<sub>IDFT</sub><sup>2+</sup> = 18.14 eV and *E*<sub>IDFT</sub><sup>2+</sup> = 19.13 eV for the singlet and triplet states, respectively. Notably, variations in appearance potentials have been already reported for triphenylene-d12 depending on the ionization technique, with electron-impact ionization typically yielding ionization energies up to 4 eV higher than those obtained by photoionization.⁴⁴ This discrepancy may arise from the fact that the collision leaves the molecule in an electronic excited state few eV above the electronic doubly ionized ground state energy. A delayed ionization is still energetically possible but the competition with other processes, such as internal conversion and energy redistribution over the nuclear degrees of freedom and possible fragmentation would prevent the experimental observation of the dication at these energies. The intense appearance of *MP*²⁺ observed in Figure 4 could indicate that the double ionization occurs through the involvement of a specific autoionization state of *MP*⁺.
### 4.1.3 Appearance energy of 1-methylpyrene de-ethylated dication

A higher appearance energy at $30.5 \pm_{1.4}^{0.4} \text{ eV}$ was measured for the doubly charged fragment associated with the loss of a neutral $C_2H_x$ group forming the $(MP - C_2H_x^0)^{2+}$ dication. Figure 5 presents the obtained fit, with once again a relatively high fitted empirical exponent $p = 1.74$. Benchmarking with the ThreSpect software gave a consistent value of $31.40 \pm 0.7 \text{ eV}$.

<figure><img src="image_6.png" /></figure>

**5. Figure:** Intensity of appearance energy as a function of the electron energy for $MP^{2+} \rightarrow C_{15}H_x^{2+} + C_2H_x^0$.

To explore whether the difference in appearance energy ($8.29 \pm 0.33$) between $MP^{2+}$ and the $(MP - C_2H_x^0)^{2+}$ dication could be related to the dissociation energy, DFT calculations were performed. Neutral $C_2H_2^0$ loss was considered at several molecular sites (see Figure 6) and the geometries of the resulting $(MP - C_2H_x^0)^{2+}$ species were optimized at the B3LYP/6-311G** level using GAMESS. The calculated dissociation energies span the 5.76-7.87 eV range, with several values comparable to the experimentally observed appearance-energy difference. This suggests that the energetic cost of neutral $C_2H_x^0$ loss may contribute significantly to the higher appearance energy of the fragmented dication.
<figure><img src="image_7.png" /></figure>

6. Figure: DFT calculations of the relaxed geometries of $(MP - C_2H_x^0)^{2+}$ followed the emission of a neutral $C_2H_x^0$ fragment.

## 4.2 Double-hit-events

Figure 7 depicts the double-hit correlation map, (i.e., the MCP detects 2 cations per event), recorded at 200 eV incident electron energy to ensure the opening of the greatest number of dissociative channels. The pink circles highlight several correlation islands that represent the favorable multivalent cation fragmentation channels.
<figure><img src="image_8.png" /><figcaption>**7. Figure:** 2-ion fragmentation correlation map. The most prominent correlation islands are circled in pink.</figcaption></figure>

**Table 2.** Correlation islands, their corresponding fragment origins, and the percentage distribution of the six most intense correlation groups. Note: the production of groups c and d are considered together since they overlap.
<table><thead><tr><th>Group</th><th>Mass & charge</th><th>Percentage (%)</th><th>Dissociation mechanism</th></tr></thead><tbody><tr><td>a</td><td>27<sup>+</sup>/189<sup>+</sup></td><td>32.8</td><td>MP<sup>2+</sup> fragmented via a metastable state into 2 cations</td></tr><tr><td>b</td><td>15<sup>+</sup>/200<sup>2+</sup> (H<sup>0</sup>)</td><td>5.9</td><td>MP<sup>3+</sup> lost a H<sup>0</sup> and fragmented into a cation and a dication</td></tr><tr><td>c</td><td>27<sup>+</sup>/189<sup>2+</sup></td><td>21.6</td><td>MP<sup>3+</sup> fragmented via a metastable state into a cation and a dication</td></tr><tr><td>d</td><td>26<sup>+</sup>/189<sup>2+</sup> (H<sup>0</sup>)</td><td></td><td>MP<sup>3+</sup> lost a H<sup>0</sup> and fragmented into a cation and a dication via a metastable state</td></tr><tr><td>e</td><td>50<sup>+</sup>/86<sup>+</sup></td><td>18.3</td><td>MP<sup>2+</sup> fragmented into C<sub>4</sub>H<sub>x</sub><sup>+</sup>/C<sub>7</sub>H<sub>x</sub><sup>+</sup> after the loss of neutral fragment(s)</td></tr><tr><td>f</td><td>63<sup>+</sup>/75<sup>+</sup></td><td>21.4</td><td>MP<sup>2+</sup> fragmented into C<sub>5</sub>H<sub>x</sub><sup>+</sup>/C<sub>6</sub>H<sub>x</sub><sup>+</sup> after the loss of neutral fragment(s)</td></tr></tbody></table>

### 4.2.1 Appearance energy of correlated simply charged $M_{27}^+$ and $M_{189}^+$

Utilizing the event-by-event detection capability of the SWEET experimental system, the vast majority of recorded events (>98%) in the 20–40 eV energy range correspond to single-hit occurrences. Furthermore, the recorded double-hit event counts must be separated from “the fortuitous” events. Fortuitous events occur when two cations are detected for the same event but originating from two different molecules, provoked by two electrons from the same pulse of the electron gun. The procedure for correcting such fortuitous coincidences has been detailed previously.³⁰ In the present case, 63% of the recorded counts within the $M_{27}^+/M_{189}^+$ correlation island were identified as non-fortuitous events and an appearance energy at $33.9\pm_{0.04}^{0.4}$ eV was determined. This value is notably higher than both the appearance energy of $MP^{2+}$ and the energy of the optimized $M_{27}^+ + M_{189}^+$ fragmentation products (26.14 eV as obtained from DFT calculations at the B3LYP/6-311G** level). This discrepancy suggests that the fragmentation proceeds via a
transient state associated with a higher effective energy barrier. However, such a transient state
has not yet been identified in our DFT simulations.

<figure><img src="image_9.png" /></figure>

**8. Figure:** Intensity of appearance energy as function of the electron energy for $MP^{2+} \rightarrow M_{27}^{+} + M_{189}^{+}$

**4.2.2 Metastability**

When a dissociation occurs between two bodies, the conservation of momentum requires that:

$$ \vec{p} = \vec{0} = \vec{p}_1 + \vec{p}_2 = \vec{v}_1 \cdot m_1 + \vec{v}_2 \cdot m_2 \qquad 3 $$

Where, $\vec{p}$ is the momentum and $\vec{v}_i$ and $m_i$ denote the velocity and mass of fragment $i$ respectively.
The measured TOF $T_i$ of a fragment can be expressed as the sum of the nominal TOF $T_{0i}$ and a
correction term that accounts for the ion's initial velocity component (+/-), which can therefore
be positive or negative, along the TOF z axis:

$$ T_i = T_{0i} - \frac{v_{iz} \cdot m_i}{q_i \cdot E} \qquad 4 $$

Here, $q_i$ refers to the charge of the fragment and $E$ is the electric field strength in the acceleration region of the TOFMS. The slope of the correlation islands is directly linked to the broadening of
the $T_i$ peaks and by combining equations (3) and (4) it is apparent that it depends solely on the charge ratio of the two ions:

$$ \frac{\Delta T_1}{\Delta T_2} = -\frac{q_1}{q_2} \qquad 5 $$

The upper part of Figure 9.b presents the enlarged view of group (a) in Figure 7, illustrating a fragmentation channel with slope = -1. This process corresponds to the dissociation of a doubly charged 1-methylpyrene dication into two singly charged fragments without the emission of a neutral species:

$$ MP^{2+} \rightarrow M_{27}^{+} + M_{189}^{+} \qquad 6 $$

<figure><img src="image_10.png" /></figure>

**9. Figure:** 2-body fragmentation: $MP^{2+} \rightarrow M_{27}^{+} + M_{189}^{+}$ proceeding through a metastable state.

Panel a: scheme of the numerical trajectory simulations to establish the correlation between
metastable lifetime ($\delta\tau$) and experimentally observable TOF. Panel b: upper part shows an enlarged view of the correlation map presenting the fragmentation group (a) and the metastable dissociation events appear as a tail on the right-hand side. The corresponding simulated metastable lifetime values plotted over with the pink diamonds. Lower part: fitting the intensity of TOF 1st fragment to an exponential decay.

Figure 9.b presents an enlarged view of the two-body fragmentation of $MP^{2+} \rightarrow M_{27}^{+} + M_{189}^{+}$ proceeding through a metastable state. The central peak corresponds to prompt (instantaneous) fragmentation, while metastable dissociation events appear as a tail on the right-hand side of this peak. The dissociation happens in the acceleration region of the TOFMS similarly as it discussed in Wei et al.⁴⁵ The metastable decay leads to longer TOF(1) values for the lighter fragment, as the parent $MP^{2+}$ ion (m/z= 108) partially shields and decelerates the $M_{27}^{+}$ fragment (m/z =27) prior to charge separation. Conversely, the heavier fragment reaches the detector at shorter TOF(2), since $MP^{2+}$ traverses the extraction field faster than the $M_{189}^{+}$ fragment (m/z = 189). To extract the metastable lifetime, a relationship between the fragmentation delay time ($\delta\tau$) and experimentally accessible observables, namely the TOF of one or both ionic fragments, must be established. Numerical simulation of the TOFMS was performed (see Figure 9.a for the schematic) to determine the correlation between $\delta\tau$ and TOF(1), which was found to be linear:

$$ \delta\tau = \frac{1}{\alpha}TOF(1) - \text{const.} \qquad 7 $$

The obtained fragmentation delay times are plotted on the upper panel of Figure 9.b with pink diamonds. The lower panel of Figure 9.b shows the intensity of the first-arriving (lighter) fragment as a function of TOF(1). An exponential fit to this distribution is used to extract the metastable lifetime, $\delta\tau = 94$ ns, of the two-body fragmentation of $MP^{2+} \rightarrow M_{27}^{+} + M_{189}^{+}$, according to:
$$I(\text{TOF}(1)) = A \cdot \exp\left[-\frac{\text{TOF}(1) - \text{const.}}{\alpha \cdot \delta\tau}\right] \qquad 8$$

The observed metastability indicates that the dissociation does not follow a pure Coulomb repulsion between the two singly charged fragments. In other words, the reaction path explores a dissociation pathway involving one or more energy barriers on the potential energy surface (PES).$^{46,47}$

### 4.2.3 Dicatonic 1-methylpyrene fragmentation dynamics

Figure 10 displays the three-dimensional velocity distribution for the 2-body fragmentation of $MP^{2+}$ dictations into two correlated cations: $M^{+}_{27}$ and $M^{+}_{189}$. The event-by-event VMI technique developed in SWEET experiment enables the determination of the complete velocity vector for each detected fragment.$^{29}$ As shown in the upper panels of Figure 10, the distribution appears homogeneous (spherical) across all spatial directions, indicating isotropic fragmentation. For the lighter fragment ($m/z = 27$), velocities are approximately an order of magnitude higher, with a mean value of 3769 m.s$^{-1}$ with a distribution width of $\sigma = 654$ m.s$^{-1}$, as illustrated in the corresponding cut view. In contrast, the heavier fragment ($m/z = 189$) exhibits a significantly lower mean velocity of 622 m.s$^{-1}$ with a distribution width of $\sigma = 199$ m.s$^{-1}$, due to momentum conservation (i.e. in the ratio of mass of each fragment).
<figure><img src="image_11.png" /></figure>

**10. Figure:** Initial velocity distribution of $M_{27}^+$ and $M_{189}^+$ fragments upon dissociation

Ultimately, using the obtained velocities, the TKER of the $M_{27}^+/M_{189}^+$ fragmentation channels was investigated for each event:

$$ TKER = \frac{m_1(v_{1x}^2 + v_{1y}^2 + v_{1z}^2)}{2} + \frac{m_2(v_{2x}^2 + v_{2y}^2 + v_{2z}^2)}{2} \qquad 9 $$

The resulting TKER distribution, plotted in Figure 11, provides insight into the partitioning of the available energy following dicationic fragmentations. The obtained result 2.3 ± 0.5 eV is comparable to the measured values of ~1.9-2.6 eV for studies on PAH dications such as fluorene,
phenanthrene, and pyrene by Lee *et al.*¹², suggesting a similar degree of charge delocalization and internal energy partitioning during fragmentation.

<figure><img src="image_12.png" /><figcaption>11. Figure: Measured TKER distribution of the 2-body fragmentation $MP^{2+} \rightarrow M_{27}^{+} + M_{189}^{+}$</figcaption></figure>

### 4.2.4 Tricationic 1-methylpyrene fragmentation dynamics

Based on the same idea, Figure 12 zooms into the group (c) and (d) of Figure 7. Here the fragmentation correlation can be fitted on slope of -0.5, revealing that the initial specie was a triply charged 1-methylpyrene ($MP^{3+}$). Group (c) demonstrates the fragmentation without the emission of a neutral species:

$$MP^{3+} \rightarrow M_{27}^{+} + M_{189}^{2+} \qquad 10$$

Meanwhile, group (d) corresponds to fragmentation pathways in which either a neutral loss occurs prior to charge separation, or two charged fragments are initially formed, followed by neutral H emission from one of them:

$$ \begin{aligned} MP^{3+} & \rightarrow M_{215}^{3+} + H^{0} \rightarrow M_{26}^{+} + M_{189}^{2+} + H^{0} \\ MP^{3+} & \rightarrow M_{27}^{+} + M_{189}^{2+} \rightarrow M_{26}^{+} + M_{189}^{2+} + H^{0} \\ MP^{3+} & \rightarrow M_{26}^{+} + M_{190}^{2+} \rightarrow M_{26}^{+} + M_{189}^{2+} + H^{0} \end{aligned} \qquad 11 $$

The corresponding metastable lifetimes, derived from the TOF histogram, are significantly shorter than those of the doubly charged 1-methylpyrene species. Values of 31 ns and 45 ns were obtained for the $M_{27}^{+}/M_{189}^{2+}$ and $M_{26}^{+}/M_{189}^{2+}$ fragmentation channels, respectively. This reduction
reflects the higher charge state of the parent ion, which enhances Coulomb repulsion inside the molecule and leads to earlier fragmentation than in case of $MP^{2+}$ ion ($\delta t = 94$ ns).

<figure><img src="image_13.png" /></figure>

**12. Figure:** 2-body fragmentations of tricationic MP. Upper panel: enlarged view of the correlation map presenting the fragmentation group (c): $MP^{3+} \rightarrow M_{27}^{+} + M_{189}^{2+}$ and group (d): $MP^{3+} \rightarrow M_{27}^{+} + M_{189}^{2+} + H^{0}$. And the metastable dissociation events appear as tail on the right-hand side. Corresponding metastable lifetime values are plotted for the $M_{27}^{+} + M_{189}^{2+}$ fragmentation in pink diamonds and for the $M_{27}^{+} + M_{189}^{2+} + H^{0}$ fragmentation in brown diamonds.

The distributions of the respective TKERs of the two tricationic fragmentations are plotted in Figure 13. The results of 4.2 ± 0.5 eV and 4.6 ± 0.5 eV for the $M_{27}^{+} + M_{189}^{2+}$ and $M_{27}^{+} + M_{189}^{2+} + H^{0}$ fragmentation channels respectively, are comparable to the measured values of ~3-5.3 eV for
PAH trications,¹² and the higher total kinetic energies that were obtained for higher charges are
also in agreement with the results of Wang *et al*.¹³

<figure><img src="image_14.png" /><figcaption>13. Figure: Measured TKER distribution of 2-body fragmentations. Lower part with red: $MP^{3+} \rightarrow M_{27}^{+} + M_{189}^{2+}$, upper part with green: $MP^{3+} \rightarrow M_{27}^{+} + M_{189}^{2+} + H^{0}$</figcaption></figure>

**4.3 Comparison with theoretical simulations**

Figure 14 presents the mass spectra resulting from MD simulations conducted for $MP^{2+}$ with internal energies ranging from 18 to 30 eV. Each spectrum is built from the fragments extracted from last snapshot of all the MD simulations performed at a given internal energy. Each fragment is reported with an intensity proportional to its charge and the histogram is then normalized by the number of MD simulations leading to a mass spectrum which integrates to a value of 2, i.e., the total charge of the simulated full system. At first glance, these spectra illustrate the progressive opening of multiple fragmentation channels and the charge redistribution among a larger set of units as the internal energy increases.

At 18 eV, the system only contains the parent dication $MP^{2+}$, which remains stable. The first calculated fragmentation channel, namely the formation of $C_{2}H_{x}^{+}/C_{15}H_{x}^{+}$ pairs, appears at 20 eV and remains the most favorable pathway up to 25 eV. A second dissociation channel, namely the loss of two $C_{2}H_{x}^{+/0}$ fragments leaving a $C_{13}H_{x}^{+}$ fragment, appears at 22 eV and is competing with
the former route at 25 eV. At 30 eV, a plethora of dissociation routes compete, resulting in a mass
spectrum where all possible fragments have equivalent intensities, apart from the $C_2H_x^+$ peak,
which is still more favorable, and the near disappearance of molecules with more than 13 carbon
atoms.

<figure><img src="image_15.png" /><figcaption>14. **Figure:** DFTB simulations of $MP^{2+}$ dicationic fragmentation as of the function its internal energy (See Equation 1). All intensities are normalized to the total charge as described in the text.</figcaption></figure>

Figure 15 shows a fragmentation correlation probability map where events accumulated over all simulations with internal energies between 18–30 eV. In the corresponding bubble plot, fragments are organized by carbon number, and the relative likelihood of each fragmentation pathway is derived from the cumulative partial charge distributions. It confirms that $MP^{2+} \rightarrow M_{27}^{+} + M_{189}^{+}$ is
a dominant fragmentation route, followed by the alternative $C_{13}H_x$ fragment produced by the loss of two ethyl groups.

<figure><img src="image_16.png" /></figure>

**15. Figure:** Simulated correlation map of fragmentation events weighted by their partial charge and regrouped by carbon number.

The analysis of the final snapshots of the MD simulations leading to the dominant fragmentation route showed that three different patterns, depicted on Figure 16, have been found for $M_{189}$: a) the formation of a pentagonal cycle, b) the opening of a ring leading to a ramified PAH, or c) a combination of these two patterns.

<figure><img src="image_17.png" /></figure>

**16. Figure:** Three characteristic patterns found for $M_{189}$: (a) pyrene with a pentagonal cycle, (b) a ramified PAH, and (c) a combination of the two former patterns

When comparing the simulations to the experimental results, the prediction of the dominant fragmentation route, $MP^{2+} \rightarrow M_{27}^{+} + M_{189}^{+}$, is in good agreement as it corresponds to the highest
correlation probability in both experimental and theoretical correlation maps. In the experimental data, the coincidence of the loss of two ethyl groups leading to the formation of the fragmentation island $C_2H_x^+/C_{13}H_x^+$ appears much less prominent, than in case of the simulated data. Indeed, in the experiment these events are observable only when the $C_2H_x$ fragment is fully ionized and detected as $C_2H_x^+$ in true coincidence with $C_{13}H_x^+$. Whereas in the simulations the corresponding fragment may carry only a partial charge and still be included. Although these events are weighted by the degree of charge localization, the simulation representation likely overemphasizes the $C_2H_x^+/C_{13}H_x^+$ coincidence compared to the experiment. The minor observation of the $MP^{2+}$ secondary fragmentation channels could also be explained by the fact that their observations would require higher internal energies than for the lowest energy channel and that, at such high energies, the triple ionization (not considered in the simulations) might be the dominant process giving rise to the experimentally observed groups *b*, *c*, and *d* in the correlation map.

Although the former hypotheses are sufficient to explain that experimental spectra are dominated by the lowest-energy fragmentation pathway, one should also mention that the intensity of secondary dissociation paths in the simulations may suffer from limitations of the DFTB potential. One can for instance mention charge localization issues or possible inaccuracies in the calculated dissociation barriers. In addition, one should also keep in mind that the simulations have been performed for the electronic ground state only and for high internal energies to observe fragmentations within a reasonable computational time.

Finally, we note that experimental data confirm the absence of $C_3H_x^+$ fragments, in agreement with the findings of Kingston et al.⁹ in their investigation of other doubly charged PAH ions. This is also the case in simulations up to 22 eV, reinforcing the hypothesis that higher internal energies would lead to the formation of $MP^{3+}$ rather than fragmentation of $MP^{2+}$.
## 5. Conclusions

The dissociation dynamics of 1-methylpyrene were investigated using the SWEET experimental system under electron-induced collisions. Collisions with 200 eV electrons revealed multiple fragmentation pathways, dominated by the loss of neutral $C_2H_x^0$ fragments and cationic $C_2H_x^+$ species. In particular, the detection of $C_2H_x^0$ suggests that the observed signature of these species in proto-stellar environments could be the result of collision-induced dissociation by stellar wind electrons acting on preexisting neutral methylated PAHs in the circumstellar cloud. Contrarily, the $CH_3^+$ has been experimentally observed only as a trace product from the dissociation of tricationic MP, meaning that a direct link between $CH_3^+$ and methylated pyrene cannot be established.

DFTB-based molecular dynamics simulations reproduced the principal $C_2H_x^+/C_{15}H_x^+$ dicationic fragmentation channel and showed good agreement with the experimental mass spectra.

At lower incident electron energies, the appearance energies of dications and correlated fragment cations were measured. The significantly higher experimental appearance energies compared to the simulated vertical ionization energies suggest the involvement of autoionization processes. Planned follow-up measurements using a TOF electron spectrometer will directly probe these discrete autoionizing electron energies.

Finally, the fragmentation dynamics producing $C_2H_x^+$ species were examined, originating from both dicationic and tricationic parent ions, providing deeper insight into the charge-dependent dissociation mechanisms of polycyclic aromatic hydrocarbon cations.

## 6. Acknowledgment

This study supported through the EUR grant NanoX n°ANR-17-EURE-0009 in the framework of the "Programme des Investissements d'Avenir". MR acknowledge HPC mesocenter CALMIP
(UAR CNRS 3667) for allocation of computer resources (p18009). The authors thank Thomas. S.

Hearne for the careful reading of the manuscript.
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<figure><img src="image_18.png" /><figcaption>17. Figure: TOC graphic</figcaption></figure>