Molecular Growth Inside of Polycyclic Aromatic Hydrocarbon Clusters

Letter pubs.acs.org/JPCL

Molecular Growth Inside of Polycyclic Aromatic Hydrocarbon Clusters Induced by Ion Collisions Rudy Delaunay,†,‡ Michael Gatchell,*,¶ Patrick Rousseau,*,†,‡ Alicja Domaracka,† Sylvain Maclot,†,‡ Yang Wang,§,∥ Mark H. Stockett,¶ Tao Chen,¶ Lamri Adoui,†,‡ Manuel Alcamí,§,∥ Fernando Martín,§,∥,⊥ Henning Zettergren,¶ Henrik Cederquist,¶ and Bernd A. Huber† †

CIMAP (UMR6252 CEA/CNRS/Ensicaen/Unicaen), Bd Henri Becquerel, BP 5133, 14070 Caen cedex 5, France Université de Caen Basse-Normandie, Esplanade de la Paix, CS 14032, 14032 Caen cedex 5, France ¶ Department of Physics, Stockholm University, AlbaNova University Center, S-10691 Stockholm, Sweden § Departamento de Quı ́mica, Módulo 13 and ⊥Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain ∥ Instituto Madrileño de Estudios Avanzados en Nanociencias (IMDEA-Nanociencia), Cantoblanco, 28049 Madrid, Spain ‡

S Supporting Information *

ABSTRACT: The present work combines experimental and theoretical studies of the collision between keV ion projectiles and clusters of pyrene, one of the simplest polycyclic aromatic hydrocarbons (PAHs). Intracluster growth processes induced by ion collisions lead to the formation of a wide range of new molecules with masses larger than that of the pyrene molecule. The efficiency of these processes is found to strongly depend on the mass and velocity of the incoming projectile. Classical molecular dynamics simulations of the entire collision processfrom the ion impact (nuclear scattering) to the formation of new molecular speciesreproduce the essential features of the measured molecular growth process and also yield estimates of the related absolute cross sections. More elaborate density functional tight binding calculations yield the same growth products as the classical simulations. The present results could be relevant to understand the physical chemistry of the PAH-rich upper atmosphere of Saturn’s moon Titan.

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in a phenyl radical (C6H5) reaction with vinylacetylene (C4H4).11 Starting from the naphthyl cation, the reaction with benzene leads to the PAH anthracene (C14H10).10 Here, we report on collisions of keV ions with PAH clusters of pyrene ([C16H10]k) that lead to the formation of larger molecules (C16+mHx with 1 ≤ m ≤ 21) inside of the cluster. This process strongly depends on the amount of transferred energy and the way that it is transferred. Experimental results are well-reproduced by classical and density functional tight binding (DFTB) molecular dynamics simulations. The present results show that keV ions efficiently induce molecular growth that, for example, may be relevant to understand the physical chemistry of the PAH-rich upper atmosphere of Titan.13−15 In the experiment, cold (T ≈ 100 K) neutral PAH clusters are produced with a log-normal size distribution with a maximum at clusters containing 30−50 molecules depending on the vapor pressure in the gas aggregation source. The intensities at small cluster sizes are low.16 Neutral clusters interact with a pulsed beam of positive atomic ions (see details in the Supporting Information (SI) and ref 17). Figure 1 displays the mass spectrum of the cations

olycyclic aromatic hydrocarbon (PAH) molecules and other types of carbon-containing particles are present in different environments, either in space or in the Earth’s atmosphere. In the latter case, PAHs and soot particles are produced in large amounts in combustion processes1 and are generally believed to be environmental and health hazards.2 The routes for forming small grains in space and the rich molecular inventory of our universe3,4 are still not well understood. In both cases, knowledge about mechanisms behind growth processes, starting from individual molecular building blocks (small carbon molecules, hydrocarbons, PAHs, etc.), and yielding larger particles are crucial. The different PAH growth mechanisms, considered in combustion science, rely on hydrogen abstraction followed by addition of a small hydrocarbon group (acetylene, phenyl, methyl, or vinyl).5 Starting from the condensed phase, oligomers of PAH molecules are formed by pyrolysis.6,7 Growth of PAH molecules can also be induced by laser irradiation of suitable solids.6,8 Small PAH molecules are formed in the gas phase via lowtemperature ion−molecule reactions.9−11 Polymerization of hydrocarbons induced by electron and photon impact ionization of molecular clusters has also been demonstrated.12 Naphthalene (C10H8) has been formed from the benzene ion (C6H+6 ) in a two-step reaction involving acetylene (C2H2)9 and © 2015 American Chemical Society

Received: February 25, 2015 Accepted: April 8, 2015 Published: April 8, 2015 1536

DOI: 10.1021/acs.jpclett.5b00405 J. Phys. Chem. Lett. 2015, 6, 1536−1542

Letter

The Journal of Physical Chemistry Letters

Figure 1. Experimental mass spectra for 24 keV O6+ projectiles colliding with pyrene clusters. (a) Total mass spectrum, (b) zoom-in of the mass spectrum due to the detection of a single positively charged ion, and (c) zoom-in of the mass spectrum due to coincident detection of two or more charged products.

produced in collision of 24 keV O6+ ions with pyrene clusters [C16H10]k, which were analyzed by means of time-of-flight mass spectrometry. Beside the ionization of these weakly bound van der Waals systems, a rich spectrum of new species is observed. In the total mass spectrum (Figure 1a), the peaks labeled [C16H10]+n (2 ≤ n ≤ 7) are attributed to intact pyrene clusters. They mainly stem from sequential evaporation of intact pyrene molecules (monomers) from larger clusters. The PAH molecules are weakly bound to each other by 1 eV or less.18 The pyrene dimer has a binding energy of roughly 0.5 eV (see the SI). Some of the monomer emission processes occur on the microsecond time scale when the ionized clusters are accelerated in the time-of-flight spectrometer. This gives long tails toward larger masses in the spectra (see Figure 1). The molecular dissociation observed below the pyrene monomer is due to significant heating of individual molecules by penetrating ion trajectories. Growth products are mostly visible between the monomer (202 amu) and the dimer (404 amu). They correspond to the addition of one or several carbons and some hydrogen atoms covalently bound to pyrene [C16+mHx]+ (1 ≤ m ≤ 15). To fragment an individual pyrene molecule before the cluster dissociates, which occurs on picosecond time scales, an energy on the order of several tens of eV seems to be necessary, as estimated from separate quantum molecular dynamics calculations (Sergio Dı ́az-Tendero; Humberto da Silva, Jr.; Fernand Spiegelman; Mathias Rapacioli, private communications). As an energy of only a few eV is sufficient to dissociate the cluster, it is highly unlikely that the observed [C16+mHx]+ peaks are due to loosely bound complexes. Instead, they are most likely covalently bound. In Figure 1b and c, the mass spectra of single-stop events (only one charged product detected per collision) and multiplestop events (at least two charged products detected per collision) are shown, respectively. The single-stop events are mostly due to ion trajectories passing at fairly large distances outside of the clusters, which become singly charged and weakly heated.16 Thus, individual molecules do not fragment, although the cluster itself dissociates by sequential emission of

intact pyrene molecules. No growth product is observed in such collisions (Figure 1b). The multiple-stop events are due to closer collisions in which at least two electrons are removed from the cluster. The stronger interaction with the projectile causes stronger heating of the clusters than in the cases of the single-stop events. Moreover, some of the penetrating collisions may lead to knockout of one or several atoms from individual molecules.19−21 These fragments may then react with neighboring molecules during the cluster dissociation to create new molecular species with masses larger than the pyrene monomer corresponding to the [C16+mHx]+ peaks in Figure 1c. We have performed experiments where 10 keV protons, 11.25 keV 3He+, 24 keV N3+, 24 keV O2+, and 12 keV Ar2+ projectiles collide with pyrene clusters. The corresponding multiple-stop mass spectra of the reaction products are shown in the left column of Figure 2. There are significant differences in the intensities of the molecular growth products. With light projectiles (protons and He), we observe only very few products due to bond-forming reactions while much richer molecular growth distributions are measured for heavier projectile ions. In the case of 10 keV proton collisions, a peak assigned to the molecule [C15Hx]+ is observed just to the left of the monomer peak. When PAHs are heated, the energy redistribution over their internal degrees of freedom leads to statistical fragmentation via the lowest-energy dissociation channels, which are H and C2H2 loss.22,23 Such statistical decay processes are usually much slower than typical cluster dissociation times, which are in the picosecond range. In contrast, the prompt single C loss is due to nonstatistical fragmentation that occurs on the femtosecond time scale as the projectile ion knocks out one of the carbon atoms in the molecule.21 The so-formed [C15Hx]+ fragment is highly reactive, and it effectively forms covalent bonds with neighboring molecules in the cluster before it dissociates. Indeed, a peak assigned to [C31Hx]+ (see the upper panel of Figure 2) may be attributed to such a reaction. In the case of 11.25 keV 3He+ collisions, additional growth products are detected associated with [C16+mHx]+ (13 ≤ m ≤ 1537


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Figure 2. Experimental (left panels) and simulated (right panels) mass spectra for 10 keV H+, 11.25 keV 3He+, 24 keV N3+, 24 keV O2+, and 12 keV Ar2+ projectiles colliding with clusters of pyrene molecules. Experimental data show only events where two or more cationic products are detected per collision.

electronic stopping in the molecular growth processes, we compare the distributions of the growth products for different projectiles (see Figure 2). When going from the top (H+) to the bottom (Ar2+), the projectile mass increases (mp = 1 → 40 amu) while the velocity decreases (v = 0.6 → 0.1 au). In the case of collisions with H+ or He+ ions, the energy transfer is dominated by electronic stopping processes, whereas nuclear scattering dominates for Ar2+ impact (see, for example, ref 26). The momentum transfer to the system increases when the projectile mass (velocity) increases (decreases). This strongly suggests that it is the momentum transfer to individual atoms leading to single- or multiple-atom knockout that drives the molecular growth process rather than a general heating of the system. Measurements with two different oxygen projectiles, having the same velocity but different charge states (24 keV O2+ and 24 keV O6+ ions), show growth product distributions that are rather similar (see the SI). This indicates that the projectile charge state plays a minor role for the ion-induced reactivity that we observe. This is consistent with the results of Schenkel et al., who showed that the total energy transfer to a thin

15) molecules. Moreover, a feature just to the right of the monomer peak is due to reactions between a single carbon atom (or possibly CHx groups) and an intact pyrene molecule. With 24 keV N3+, 24 keV O2+, and 12 keV Ar2+ projectiles, wide growth product distributions are observed with peaks corresponding to [C16+mHx]+, where 1 ≤ m ≤ 21. Note that these distributions extend to masses that are larger than that of the dimer. Following the same reasoning as that for the peaks between the monomer and dimer, we conclude that these peaks are also due to covalent systems. For obvious reasons, these products must have been formed in interactions between (at least) three fragments/intact molecules. It is not possible to separate contributions from weakly bound dimers [C16H10]+2 and covalently bound [C32Hx]+, which have similar masses. The collision of ions with matter can lead to energy deposition through two different mechanisms: scattering by the nuclei and scattering by the electronic cloud. These processes are referred to as nuclear and electronic stopping processes, respectively. The energy transfer depends on the mass, the velocity, and the charge state of the incoming projectile.24,25 In order to investigate the relative importance of nuclear and 1538


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Figure 3. Three molecular growth reactions from DFTB simulations of pyrene clusters following collisions with 12 keV Ar (see the text).

of one molecular fragment and one intact pyrene molecule. These reactions take place within a few tens of femtoseconds after the initial collision and before the cluster begins to dissociate on the picosecond time scale. Self-consistent charge density functional tight binding (SCCDFTB)31 molecular dynamics calculations are used to benchmark the results of the classical molecular dynamics simulations. DFTB offers the advantage of providing more accurate molecular properties, comparable to DFT calculations, in systems like those studied here32,33 as well as allowing for the inclusion of charges. Trajectories resulting in growth products in the classical simulations were selected, and the same reactions were followed for 1 ps using DFTB molecular dynamics. In order to speed up the simulations, only the reacting fragments identified in the classical calculations are considered, excluding the surrounding cluster molecules that do not take part in the reaction. DFTB simulations with cationic systems show similar reactions as the ones obtained in the classical simulations of neutral systems. However, the bonds in the DFTB simulations are more sensitive to cleavage at high temperatures than those in the classical force field calculations. We find from the simulations that systems with internal temperatures below ∼4000 K will remain intact, while systems with higher internal temperatures may fragment after the initial bond-forming reaction. Separate electronic stopping calculations34−37 indicate internal temperatures of about 2000 K. Thus, most of the molecular growth products stay intact, which is consistent with the close similarity of the simulated and measured growth product distributions. Examples of three growth processes occurring in a cluster of nine pyrene molecules after collision with a 12 keV Ar projectile are shown in Figure 3. The top row is an example of the most common reaction observed in the classical simulations, a stray C atom absorbed by an intact pyrene molecule. The resulting [C17H10]+ molecule undergoes a few isomerization processes during the 1 ps simulation time but remains intact according to the classical and the DFTB simulations. The second row shows a case where a fragment consisting of 10 C atoms and 6 H atoms reacts with an intact molecule, resulting in [C26H16]+. The mass of this fragment places it near the center of the growth product distribution seen

carbon foil changes by only a small amount (about 15%) when varying the charge of oxygen ions at a velocity of 0.3 au from 2 to 6.24 In the right column of Figure 2, we show results from classical molecular dynamics simulations for the five projectiles used in the experiment. We have simulated the entire process including the interaction between the projectile and all atoms in the cluster, the breaking of bonds, rearrangements of atoms, and formations of new bonds (molecular growth). Here, the cluster target contains nine pyrene molecules, and the calculations have been performed with the LAMMPS software suite.27 The C−C, C−H, and H−H interactions are described with the AIREBO force field,28,29 which gives realistic descriptions of bond formation and cleavage. Interactions between the projectile and all atoms in the clusters are modeled using the Ziegler−Biersack−Littmark (ZBL) potential;30 a screened Coulomb potential is also used to model Rutherfordlike scattering (nuclear stopping) in ion−solid interactions.30 There are striking similarities between the experimental results and the simulations where effects of electronic stopping processes are included indirectly by giving the clusters an internal temperature before the collision. Nevertheless, molecular dynamics simulations reproduce the main features of the molecular growth, in particular, the shape of the distributions and their changes when going from one projectile to the other one. This strongly suggests that the subsequent bond-forming reactions are initiated by nuclear stopping processes causing prompt, nonstatistical knockouts of one or several atoms from individual pyrene molecules. The similarity between the experimental and the simulation results, obtained on microsecond and picosecond time scales, respectively, also suggests that most of the covalent products formed in the cluster are stable at least on the time scale of microseconds. Multiple-atom knockout can result in fragments that react with intact molecules giving intermediate masses in the distribution between that of the monomer and dimer. This is less likely than single-atom knockout for all collision systems studied in this work and is the reason for the lower intensities of peaks in the middle of the U-shaped molecular growth distributions in Figure 2. Nearly all of the reactions resulting in products with a mass between the monomer and dimer are due to coalescence 1539


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sections for molecular growth and typical ion impact velocities suggest substantial growth rate constants of 10−8 cm3 s−1 depending on the cluster size. This indicates that the observed growth processes may play a role for the chemistry of Titan’s atmosphere.43 In conclusion, the present results demonstrate that molecular growth may be efficiently induced by collisions of keV ions with pyrene clusters. The growth efficiency strongly depends on the mass and velocity of the projectile according to (mp)1/2/v. Experimental results are well-reproduced by classical molecular dynamics simulations, which emphasize the role of prompt knockout of C and/or H atoms from one or several molecules in the cluster due to the interaction with the projectile. Subsequently, reactive fragments may interact with intact neighboring molecules as shown by classical and quantum (DFTB) molecular dynamics simulations. These allow us to deduce absolute cross sections for molecular growth. Ion impact on molecular clusters could be relevant for the molecular growth in, for example, planetary atmospheres such as the one of Titan. Furthermore, this opens new perspectives to study the molecular growth from linear hydrocarbons to cyclic ones and PAHs in an astrophysical environment.

in the mass spectra (Figure 2). The bottom row illustrates the case when a CH group has been knocked out from one of the pyrene molecules. The residual fragment forms a single bond with a neighboring intact molecule where one ring closes into a pentagon giving [C31H19]+. From the classical molecular dynamics simulations, the absolute cross sections for growth processes are determined by counting the events that lead to products with masses larger than that of the pyrene monomer. This number is then related to the total number of trajectories launched through a given cross-sectional area in the simulations. Absolute cross sections per molecule in the cluster for molecular growth products in the mass range between the monomer and dimer are compared for 12 keV Ar collisions clusters containing 9, 18, and 36 pyrene molecules. The intensity distributions on the peaks between the monomer and dimer are similar in all cases (see the SI). Absolute molecular growth cross sections, σg, from the simulations are shown in Figure 4. These are the cross sections

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ASSOCIATED CONTENT

S Supporting Information *

Comparison between the mass spectra of the two oxygen projectiles and details of the experimental procedure and of the theoretical simulations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.G.) *E-mail: [email protected] (P.R.).

Figure 4. Absolute growth cross sections, σg, for forming covalently bound hydrocarbon molecules heavier than pyrene in the simulations as a function of the square root of the projectile mass divided by the velocity (mp)1/2/v. The red points correspond to cross sections for five projectiles colliding on [C16H10]9, and the blue squares are from simulations with 11.25 keV 3He, 24 keV O, and 12 keV Ar colliding on [C16H10]18. The black lines show a common linear fitting to the data points belonging to both the n = 9 and 18 data sets.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The experimental studies have been performed at the lowenergy ion beam facility ARIBE at GANIL (Caen, France). The simulations have been performed at Stockholm University, Sweden and at the Universidad Autónoma de Madrid, Spain. The authors thank Fabien Noury and Stéphane Guillous for their technical support. The authors acknowledge assistance by and scientific discussions with Jean-Yves Chesnel, Jaroslav Kočišek, Alain Méry, Jean-Christophe Poully, Jimmy Rangama, Lucas Schwob, and Violaine Vizcaino. In particular, we would like to thank Sergio Dı ́az-Tendero, Humberto da Silva Jr., Fernand Spiegelman, and Mathias Rapacioli for fruitful discussions and for sharing unpublished results of quantum molecular dynamics calculations. Research was conducted in the framework of the International Associated Laboratory (LIA) “Fragmentation DYNAmics of complex MOlecular systems  DYNAMO” and the COST actions “XUV/X-ray light and fast ions for ultrafast chemistry” (XLIC - CM1204). This work was supported by the Swedish Research Council (Contract No. 621-2012-3662 and 621-2012-3660), the Spanish MINECO Projects FIS2013-42002-R and CTQ201343698-P, and the CAM Project NANOFRONTMAG. We acknowledge allocation of computer time at the Centro de Computación Cientı ́fica of the Universidad Autónoma de Madrid (CCC-UAM).

for forming new molecular species with at least 17 C atoms as a function of projectile mass and velocity. The data points in the figure are for simulations with [C16H10]9 (red circles) and [C16H10]18 (blue squares) clusters. For a given cluster size, the growth cross section is proportional to (mp)1/2/v and the number, n, of pyrene molecules in the cluster (solid lines labeled n = 9 and 18). The fitted relation for clusters containing 9 and 18 molecules is scaled to arbitrary sizes according to σg = 0.0017(2) × n × (mp)1/2/v × 10−16 cm2 using atomic units. In Saturn’s magnetosphere, keV oxygen ions are produced by water dissociation and accelerated in the fields of the magnetosphere.38 With solar wind oxygen ions,39 they constitute the heavy components of the keV ions that interact with Titan’s upper atmosphere. These ions deposit substantial amounts of energy in the ionosphere and may possibly contribute to the formation of heavy molecular ions.40 Indeed, PAH molecules and clusters are most likely present in Titan’s ionosphere, as indicated by mass spectrometry41 and infrared spectroscopy42 performed during the Cassini mission. Charged PAH clusters are suggested to be parts of the large ionic species observed in the atmosphere of Titan.41 The absolute cross 1540


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Molecular Growth Inside of Polycyclic Aromatic Hydrocarbon Clusters

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