The Formation of Polycyclic Aromatic Hydrocarbons in Space

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1 The Formation of Polycyclic Aromatic Hydrocarbons in Space Image Credit: NASA/JPL-Caltech/T. Pyle (SSC) Literature study by Margje Schaveling 1

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3 MSc Chemistry Molecular Simulation and Photonics Literature Thesis The Formation of Polycyclic Aromatic Hydrocarbons in Space An overview of possible in and ex situ formation mechanisms by Margje Schaveling Student number: EC 4 July 2016 Daily Supervisor/Examiner: Dr. Ir. Annemieke Petrignani Thesis Supervisor/Second Examiner: Prof. Dr. Wybren Jan Buma 3

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5 ABSTRACT Polycyclic aromatic hydrocarbons (PAHs) are one of the most complex organic species found in space. Despite the far less than optimal conditions for complex molecule formation, PAHs are abundant throughout space. Multiple formation mechanisms are proposed for interstellar PAHs, both in and ex situ. These pertain to molecular clouds and stellar ejecta, for which physical parameters vary largely. In the stellar ejecta of carbon-rich AGB stars, terrestrial-combustion-like chemical reactions are more likely to take place, of which the products can be transported to the interstellar medium (ISM) by interstellar wind. These reactions include bottom-up sequential reactions involving radicals or polymerisation reactions and a top-down mechanism of sputtering and shattering of dust grains. In cold molecular clouds (10 K), only reactions with no reaction barrier can take place. These include rapid neutral-neutral reactions with a submerged reaction barrier of a phenyl radical and vinylacetylene, potentially leading to ring growth. In more diffuse molecular clouds with UV photons present, photolysis of acetylene can lead to ring growth through ethynyl addition. Furthermore, supernovae (SNe) shocks might also create a top-down mechanism of fragmentation of graphite grains. Despite the many mechanisms proposed, the formation route(s) of PAHs remain elusive. None of the routes or realistic combination thereof can explain the abundance of PAHs in the ISM properly. More experimental, theoretical and observational research is needed to find a conclusive formation mechanism. The James Webb Space Telescope might be of great assistance in this. 5

6 ABBREVIATIONS Amorphous carbon AC Aromatic Infrared Band - AIB Aromatic radicals - ARs Asymptotic giant branch AGB Circumstellar Envelope - CSE Far- ultraviolet FUV Hydrogen abstraction-acetylene addition - HACA Infrared Space Observatory ISO InterStellar Medium ISM James Web Space Telescope JWST Mid-infrared - Mid-IR Nitrogen-substituted polycyclic aromatic hydrocarbons - PANHs Phenyl addition-cyclisation - PAC Photodissociation region - PDR Planetary nebulae PNe Polycyclic Aromatic Hydrocarbon PAH Reflection Nebulae RNe Resonantly stabilised free radicals - RSFRs Silicon Carbide - SiC Spitzer Space Telescope SST SuperNovae - SNe Ultraluminous infrared galaxies - ULIRGs Unidentified infrared emission bands UIRs Very small grains VSGs Young stellar object YSO 6

7 CONTENTS Abstract... 5 Abbreviations Introduction Formation environments polycyclic aromatic hydrocarbons Formation Mechanisms in Stellar ejecta Bottom-up mechanisms The first aromatic ring closure The HACA mechanism Phenyl addition-cyclisation (PAC) Polymerisation reactions Top-down mechanisms Graphene etching on SiC grains Formation Mechanisms in the Interstellar Medium Ion molecule reactions Barrierless and rapid neutral-neutral reactions Formation of monocyclic aromatic molecules Successive ring formation Ethynyl addition Supernovae Shocks in the ISM Discussion Summary & Conclusion Future research Acknowledgement References

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9 1 INTRODUCTION In the mid-1970s astronomical observations of mysterious infrared radiation from space were made by ground-based and airborne studies (Figure 1). 1 7 Relatively broad emission features in the midinfrared (mid-ir) range were detected in bright HII regions, planetary nebulae (PNe) and reflection nebulae (RNe), which were later labelled the unidentified infrared emission bands (UIR bands). When infrared measurements from space could be conducted with the Infrared Space Observatory (ISO) in 1995 and the Spitzer Space Telescope (SST) in 2003, it was revealed that UIRs are omnipresent. They have been observed in HII regions, RNe, young stellar objects (YSOs), PNe, postasymptotic giant branch (post-agb) objects, nuclei of galaxies, photodissociation regions (PDRs) and ultraluminous infrared galaxies (ULIRGs) The UIRs are even stronger in the IR cirrus, the surfaces of dark clouds, and the general interstellar medium (ISM) Figure 2 shows the UIRs in the Orion bar and PNe NGC In these two mid-infrared spectra strong, broad emission features can be observed at 3.3, 6.2, 7.7, 8.6, 11.2 and 12.7 μm. Furthermore, plenty of weaker bands are present at 3.4, 5.2, 5.7, 6.0, 7.4, 12.0, 13.5, 14.2, 15.8, 16.4, 17.0 and 17.4 μm. 11,13,17,18,23 25 Underneath these features broad plateaus and a continuum are present (see ref. 26 and refs. therein). 27 a. b. c. d. Figure 1 a. The 8-13 μ spectra of NGC 7027 and BD by Gillett et al., b. The 8-13 μ spectra of the nuclear region of M82 and NGC 253 by Gillett et al., c. The 8-13 μ spectrum of the emission of HD44179 by Cohen et al., d. The 2-14 μm spectrum of NGC7027 by Russell et al.,

10 Figure 2 The mid-infrared spectra of the photodissociation region (PDR) in the Orion bar and in the planetary nebula NGC 7027 with assignments of the infrared emission features with vibrational modes of PAH molecules at the top. The broad plateaus are assigned to clusters of PAHs and the continuum to nanograins. 10,22,27 Data taken with ISO. Figure reproduced from ref. 27. What are the carriers of these mysterious UIRs? Several factors (explained below) indicate that the IR fluorescence of far-ultraviolet (FUV)-pumped polycyclic aromatic hydrocarbon (PAH) molecules are the most probable origin of these UIRs. PAHs were first suggested by Allamandola and coauthors Tielens and Barker in 1985, who ascertained a good agreement between a laboratory measured Raman spectrum of auto exhaust and the interstellar 6.2 and 7.7 μm bands (Figure 3). 28 This has led to the so-called PAH hypothesis, describing the origin of the UIRs to originate from emitting PAHs after UV excitation. Today, this hypothesis is generally accepted and the UIR features are also called the Aromatic Infrared Bands (AIBs). 26,27,30 33 The first factor supporting PAHs as origin of the UIRs or, as currently known AIBs, is the high feature-to-continuum ratio, exceeding 20 in many sources. 30,31,33,26,27 This indicates a molecular origin since the feature-tocontinuum ratios of bulk materials are much smaller. Additionally, a pronounced red-shaded profile can be seen in several of the IR emission features (6.2-and 11.2-μm features). Such a profile indicates anharmonicity, which imply highly vibrationally excited molecules The AIBs therefore most likely originate from the gas phase as opposed to the solid state of dust particles. The second argument in support of PAHs being responsible for the AIBs is the correlation that exists between the different features the intensities of different features (e.g. 6.2 μm and 7.7 μm) change in a similar fashion when the local conditions are varied. 13,25 This gives strong support for the presence of a family of molecules. Further confirmation for PAHs as the origin of the AIBs is found in the origin of the Figure 3 Comparison of a Raman spectrum of auto exhaust and the spectrum of the Orion bar from 5-10 μm. Figure reproduced from ref

11 vibrations these resemble aromatic C-C or C-H motions, as is shown in Figure 2. The vibrations are therefore associated with aromatic hydrocarbons. 26,31 This leads to the last argument in favour of PAHs - aromatic molecules are stable enough to withstand the harsh conditions of the ISM they are exposed to. 37 The sizes of the detected molecules are derived to lie between 50 and 150 C atoms from the spectral signatures (Figure 4). 13,33,38 To be able to emit in the mid-ir, the carrier(s) of the IR emission features must be hotter than the 15 K of the far-ir emission by nm grains, which are in radiative equilibrium with the interstellar radiation field. This heat is obtained by the absorption of a single FUV-photon. This photon highly excites the molecule and initiates a fluorescence process. After excitation, the molecule relaxes through the emission of IR photons. Analysis of the energetics involved implies that molecular-sized carriers with a size of C atoms causes the spectral mid- IR signature. 27 A further argument for this size of molecules is that molecules smaller than 50 C atoms are quickly destroyed by the normal UV field due to their low heat capacity. 39 Absorption of a photon would lead to fragmentation and thus destruction of the molecule. On the other end, if PAHs grow large enough, they might transform in grains through van der Waals forces. Grains are solid state particles and have a different spectral signature a lower feature-to-continuum ratio. The state between full grain particles and single molecules might possibly explain the underlying plateaus and weak continuum Figure 4 The size of PAHs lies between 50 C and 150 C atoms PAHs have been known for quite some time in a terrestrial context, where often they are stuck together in sooty solids. Astronomical IR observations show that in space PAHs are in the gas phase and mostly charged. 30,31,33,43 Additionally, they are relatively cold when compared to terrestrial conditions. The highly differing physical conditions between terrestrial and astronomical environments makes comparison difficult. Laboratory measurements also were restricted to only few, small PAHs investigated in conditions similar to space. 30,31 For quite some time, this prevented decisive assignment of the UIRs to PAHs. However, since the 1990 s, progress has been made in the research of PAHs. ISO and Spitzer have performed systematic studies of the mid-ir range. 11,13,17 20,23 25 The Midcourse Space Experiment, Infrared Telescope in Space 47,48 and the 8-m class of ground telescopes, such as the Keck Observatory 49 and the Very Large Telescope of the European Southern Observatory, 50,51 added extra opportunities for observations with enhanced spatial and spectral resolution in the IR. Furthermore, dedicated research have been performed, both theoretical and in the laboratory (see refs. 15 and 27 and refs. therein), obtaining more insight in the spectral behaviour and chemical characteristics of PAHs. Laboratory measurements are now moving towards colder PAHs and higher resolution spectroscopy, as well as to the large PAHs, investigating the fragmentation, photolysis, evolution, and spectroscopic behaviour Theoretical calculations are more and more validated by the available experimental results and can now be improved in the application to larger PAHs and to include additional molecular detail, such as anharmonicity. 55 Using 11

12 Figure 5 Composite mid-infrared image of the Orion star-forming region with the 8 μm (red) tracing mostly the emission of PAH molecules in the 7.7-μm feature. The 4.5-μm (green) shows ionized gas and dust. Stars can be seen in the 3.6-μm image (blue). Image taken from ref. 27 all these data, accurate models for the emission of astronomical sources could be developed, which properly incorporate PAHs. 56,57 These models address the likely source of the emission spectrum, including PAHs. The more advanced the models get, the more accurately they can predict the infrared bands and molecular detail. 26,56 Nowadays, it is generally believed that PAHs are an important component of the ISM. 27,57 60 Measurements show that they are very abundant by number relative to hydrogen. With taking up 10% to 20% of the elemental carbon, PAHs play an important role in the ISM. 52,55,57,61,62 They influence the heating of neutral gas, the ionization balance in molecular clouds, and thus the phase structure of the ISM, the ion-molecule chemistry for the formation of simple gas species and the ambipolar diffusion process important for star formation. (ref. 26 and refs. therein). Indeed, the AIBs are strong in regions with young massive stars that can excite the PAHs well, for example HII regions. 22,23 Therefore, PAHs are used as tracers of galactic star formation, especially for massive stars, in both the local as the more distant universe. 26 The example of the Orion region, where massive stars are formed, is shown in Figure 5. The use of PAHs as tracers for galactic star formation implies that PAHs are important for the evolution of galaxies. 27 Furthermore, PAHs do not only play an important role in star formation, but probably also in planet formation. 63 The process towards planet formation, as shown in Figure 6, starts with the clustering of PAHs towards very small grains (VSGs). These grains can in turn assemble to form planetisimals, and eventually planets. 64 Figure 6 PAHs cluster together and form amorphous carbon nanoparticles. These form the basis for planetesimals and eventually planets. Figure adapted from ref. 26 and ISAS/JAXA Furthermore, they may also play an important role in the formation of prebiotic (interstellar) molecules. 65 Weaker bands around the main AIBs, such as the 3.4 and the 6.2 μm features, are believed to point to a presence of a small amount of PAHs that are transformed into nitrogensubstituted PAHs, so-called PANHs. 58,66 69 PANHs in their turn can lead to the formation of biorelevant molecules such as nucleobases. 54 Additionally, PAHs may also have been involved in the 12

13 formation of life through the self-assembly into protocells. 70 Although the components are different than those today, assemblies based on PAHs as prebiotic membrane components could have led to a minimal life form, since PAHs have been the most abundant stable and flexible organic materials present on primitive Earth. 65,70,71 Despite the important role that PAHs play in the ISM, there is no general agreement on the formation mechanism(s) of PAHs. 72 Several hypotheses for formation mechanisms have been proposed, but none can explain the abundance of PAHs in the ISM properly. Also, investigations of formation mechanisms is strongly hindered by the lack of identification of PAH populations. No single individual PAH has been identified to date in space. The only large aromatic species that have been individually identified are C 60 and C 70 fullerenes (Figure 7), which are aromatic but not PAHs. 73,74 They can be formed from PAHs. 52. Therefore, efficient chemical routes leading to the formation of PAHs are still an open issue. 75 Furthermore, both in and ex situ formation mechanisms have been proposed. Varying physical parameters in different environments affect the plausibility of different formation mechanisms, making not only the formation mechanisms debated, but also the formation environment. This literature thesis will focus on the proposed in and ex situ formation mechanisms of interstellar PAHs, specifically in relation to the local environment with differing physical parameters. An overview of current literature on this subject will be presented. Figure 7 C 60 or buckminsterfullerene 1.1 FORMATION ENVIRONMENTS POLYCYCLIC AROMATIC HYDROCARBONS In space many different environments are present with all their own specific physical properties. In the ISM itself diffuse clouds, molecular clouds, prestellar cores and infrared clouds create inhomogeneity with temperatures ranging from 6 to 80 K and densities from 50 cm -3 to 10 4 cm ,76 The ISM is not a complete void and molecules are present. These molecules originate from the remnants of a previous generation of stars. As can be seen in Figure 8, the remnants of old stars provide the material for diffuse clouds. The diffuse clouds will become dense clouds and eventually gravity collapse will initiate the formation of new stars. This finishes the cycle and from the ashes of the old stars, a new generation of stars is created in the ISM. During this process, the diffuse and molecular clouds are not static. Further interstellar matter is added by supernovae (SNe) explosions of massive stars and by the winds of low-mass stars during the asymptotic giant-branch (AGB) phase. The added molecules are created in complete different (circumstellar) environments, such as the photospheres around evolved stars having temperatures up to 3500 K. 66 The molecules in the ISM are thus subject to a wide range of conditions in the different local environments, which facilitates the formation of a varied molecular inventory in the ISM, including PAHs. 13

14 Figure 8 Life cycle of a star from stellar ejecta until rebirth. Credits: ESO, Jean-Charles Cuillandre, Hawaiian Starlight, CFHT, L. Calçada, ESA/Hubble, NASA. Based on ref. 27. The wide range of conditions means that formation of molecules, including PAHs, can take place in many different environments. It is clear from observation that PAHs are ubiquitous in the interstellar medium, e.g. (dense and diffuse) molecular clouds, but is it less clear if they are formed in situ (in the ISM) or ex situ (in CSE). The conditions influence the formation mechanism. There are several hypotheses suggested for the formation of PAHs, where each formation mechanism can contribute differently in different environments. The main question is if the PAHs that are visible in the ISM are formed locally and/or if they are transported there from another formation environment. This is also related to the distinction between bottom up and top down mechanisms in the formation of PAHs and between a thermodynamic and a kinetic equilibrium. The two most plausible environments for the formation of PAHs are the outflow of stellar ejecta and molecular clouds. However, the mechanisms in these environments probably differ a lot due to the different physical parameters of the environments. The stellar ejecta most relevant to PAH formation are the C-rich ejecta of AGB stars. In these ejecta, the typical temperature is around 2500 K in the photosphere, 66 while in molecular clouds temperatures range from 10 to 80 K. 76 In the stellar ejecta the chemistry is thus largely in thermodynamic equilibrium, while in the ISM mostly a kinetic equilibrium is present. Further influence on the formation of PAHs are supernovae (SNe) explosions of massive stars. These send shock waves through the interstellar medium. They compress gas, form dense clouds and distort molecular clouds throughout the galaxy. 77 In the following chapters first the possible formation mechanisms in stellar ejecta will be discussed, starting with the bottom-up mechanisms and then the top-down mechanisms. Then the mechanisms possible in molecular clouds are discussed, including those by SNe explosions. Finally, the plausibility of the discussed mechanisms will be evaluated. 14

15 2 FORMATION MECHANISMS IN STELLAR EJECTA On earth, in combustion processes amorphous carbon (AC) dust or soot is formed from the gas phase, and PAHs are linked to this process. It is believed that the condensation process for soot is initiated with the closure of the first benzene ring, which is then followed by chemical growth towards large PAHs, which form the basis for soot formation (Figure 9). 78 In space, similar conditions might also lead to the synthesis of dust and thus PAHs. The optimal temperature for rapid PAH growth lies between 900 and 1100 K. 79,80 Furthermore, the presence of carbon and hydrogen is needed for the synthesis. In terrestrial combustion processes, these elements come from the fuels used in the combustion process. 81,82 In space, these are already present in the gas phase. Additionally, the pressure should be high enough to yield a collision timescale suitable for PAH formation according to the same process as in combustion. Thus, high temperatures, high densities and an initial carbon- and hydrogen-rich chemical composition in the gas phase are the conditions needed for PAH (and soot) formation according to a similar process as on earth. 67 Figure 9 Dust formation through PAH nucleation These kinds of conditions can be found in circumstellar environments such as ejecta or winds of evolved stars, which provide a large portion of dust in the galaxy. 83 These ejecta have a typical temperature of 2500 K in the photosphere, but when moving away further from the star, the temperature lowers, and also the temperature window between 900 and 1100 K is reached. Pressures of stellar ejecta of evolved stars are typical around 10-6 atm, lower than on earth. Furthermore, ejecta can be both carbon- and hydrogen-rich. 79,80 These requirements for PAH formation by a combustion like-process identify a number of environments where PAH formation may take place. These include the late stages of evolution of low and high mass stars. 67 Especially in the gas layers close to the photosphere of carbon AGB stars that are periodically shocked due to the stellar winds, the conditions are as required for PAH formation. 84 These environments are validated as PAH formation environments by the fact that carbon AGB stars are among the prevalent AC grain makers. When assuming that AC dust formation and the formation of PAHs is also coupled in space (as is it is on earth), this implies that PAH formation takes place in the same environments. 67 However, observationally, there is little direct support for PAHs in outflows from carbon-rich AGB stars. This might not be because of the lack of PAHs, but because of the lack of UV pumping photons in the stellar spectra, possibly also because of the amount of dust present. 85 Generally, when stars are cooler than 4000 K, they show no evidence for the IR emission features. Still, there is some observational support from the carbon-rich AGB star TU Tau, where emission features are likely pumped by the blue binary companion, and recently more AIBs were detected in the carbon rich star HD The ejecta of C-rich AGB stars start in the photosphere with temperatures around 2500 K and pressures around 10-6 atm. These conditions yield a chemistry that is largely in thermodynamic equilibrium. Shock waves that go through the photosphere due to stellar pulsations startle this equilibrium. The shock waves lifts material high above the stellar surface and in the post-shock gas 15

16 the molecules are heated and dissociated by collisions. Most of the molecules will form again during the adiabatic cooling of the gas and the shocked gas layers will also reach a temperature range where chemical nucleation of nanoparticles is initiated by closure of the first benzene ring. The nanoparticles formed will grow into dust particles through chemical processes, which will be returned to the interstellar medium through a stellar wind. This process makes the star return most of its mass to the ISM during a short phase, and the PAHs eventually formed in the stellar ejecta are transported to the ISM. 27,67 In this section the possible pathways towards PAH formation in the stellar ejecta of carbon-rich AGB stars will be discussed. First the bottom-up pathways will be discussed and then the top-down mechanisms. 2.1 BOTTOM-UP MECHANISMS The first aromatic ring closure The first step towards the formation of PAHs in stellar ejecta of carbon-rich AGB stars is the closure of the first single aromatic ring. This is considered the rate limiting step of soot synthesis, and thus of PAH formation, because it marks the passage from non-aromatic structures to aromatic structures. Investigations of soot formation in flames in terrestrial settings have revealed three possible routes for the formation of the first aromatic ring. 67,86 91 The first and prevalent route is the recombination of two propargyl radicals (C 3H 3) to form cyclic and linear benzene and the benzene radical phenyl (C 6H 5) according to the reactions in Scheme 1. Scheme 1 Propargyl recombination reactions These routes were first proposed by Miller & Mellius (1992) 86 and are still used as an explanation nowadays. 92,93 In 2000, an experimental study showed that the direct formation of benzene via the first reaction is prevalent over the latter. 91 Many reaction pathways are possible for the recombination of propargyl into one aromatic ring, with many intermediates, such as fulvene (C 5H 4CH 2). 89,90 The second pathway towards the closure of the first aromatic ring is observed in acetylenic flames and involves, as proposed by Frenklach et al. (1984) 87, the reaction of 1-buten-3-ynyl (1-C 4H 3) with acetylene (Scheme 2). In this reaction, the benzene radical phenyl is formed. Scheme 2 Reaction of 1-buten-3-ynyl with acetylene The third route towards closure of the first aromatic ring is the reaction of 1,3-butadienyl (1-C 4H 5) with acetylene, yielding cyclic benzene, as proposed by Cole et al. (1984) (Scheme 3)

17 Scheme 3 Reaction of 1,3-butadienyl with acetylene The propargyl recombination reaction is believed to be the most prevalent reaction in terrestrial combustion environments. This is because n-c 4H 3 and n-c 4H 5 could not be present in sufficiently high concentrations because they transform rapidly to their corresponding resonantly stabilized isomers, iso-c 4H 3 and iso-c 4H The propargyl radical, however, is a very stable hydrocarbon radical. 94,95 Although the propargyl recombination reaction is the most prevalent one in flames on earth, in circumstellar outflows, it is dependent on the reactants present in the gas. The formation of C 3H 3 results from the reaction of C 2H 2 and methylene, CH 2, whereas vinyl-acetylene, C 4H 4 will form C 4H 3 and C 4H 5 from its reaction with atomic hydrogen. 67 Thus, dependent on the amount of C 2H 2 and methylene present compared to the amount of vinyl-acetylene, the first reaction or the second and third reaction will dominate. Another factor is that reaction of acetylene may be aided by SiC grain surfaces The HACA mechanism After the first ring closure took place and benzene has been formed, further ring growth can occur via several different routes. The first, as proposed by Frenklach et al. (1984) 87, is the growth of polycyclic aromatic structures through the HACA (hydrogen abstraction-acetylene addition) mechanism: sequential hydrogen abstraction and acetylene addition (Scheme 4). The formation of radical sites on the aromatic benzene ring enables C 2H 2 to add and finally form a new aromatic ring. 79,80,96 Scheme 4 HACA mechanism. Based on ref. 67. The key to this reaction mechanism is the reversibility. The H abstraction can be reversed in the same manner, but also other reactions such as the combination with gaseous H can reverse the hydrogen abstraction. 78 The degree of reversibility of the acetylene addition determines if this step will cause molecular growth. The reaction is reversible because of the entropy loss of this addition. The reaction becomes irreversible when the decrease in energy is large enough, e.g. when the temperature is not too high. Additionally, the formation of stable aromatic molecules (PAHs) helps in making this reaction irreversible. 78 These reversibility issues can explain the temperature window for rapid PAH growth. Below 900 K, further growth is inhabited since H abstraction freezes out kinetically. Above 1100 K, H abstraction and C addition are reversible. At 1100 K, C addition is irreversible, but H abstraction is still in fast equilibrium, which means that chemical growth can be rapid. 79,80 Both practical and theoretical research has been performed into the HACA mechanism, making it well established explanation for PAH growth. 78 The HACA mechanism does have a few drawbacks. Both experimental and computational research has shown that its efficiency is not high enough to explain rapid PAH growth in flames. 97,98 Also, after naphthalene, predominantly cyclofused PAHs are formed instead of the more usual aromatic 6-rings. 17

18 2.1.3 Phenyl addition-cyclisation (PAC) Phenyl addition-cyclisation (PAC) is an efficient alternative mechanism for HACA 93 and is especially efficient for ring growth from any fusing site, sites where several aromatic rings are present, of a PAH. 99 This mechanism is shown in Scheme 5 and involves the addition of a phenyl radical at a fusing site of an aromatic species followed by dehydrocyclization. 100 Because this mechanism can generate two new active sites in each step (Scheme 5 c), it can continue growing. HACA, however, is only efficient when ring growth takes place from a triple fusing site of a PAH to produce symmetrical PAHs. An eventual combination of PAC with HACA is important to produce PAHs fast enough because HACA can trap radicals effectively, but PAC can provide efficient growth. 100 An overview of products that can be formed when the PAC and HACA mechanism are collaborating is given in Figure 10. Scheme 5 Examples of phenyl addition-cyclisation: The addition of a phenyl radical at a fusing site of an aromatic species followed by dehydrocyclization. Based on ref

19 Figure 10 PAC reaction routes starting from single, double and triple fusing sites with the corresponding products. Reproduced from ref Successive ring formation processes are suggested to mainly go through sequential reactions of aromatic radicals (ARs) and resonantly stabilised free radicals (RSFRs) such as the phenyl (C 6H 5) radical, propargyl radicals (C 3H 3), and acetylene (C 2H 2). The reason for this is that ARs and RSFRs have a high stability, and can thus reach high concentrations even at elevated temperatures of several thousand Kelvin. Because of those high concentrations they are important reaction intermediates for the formation of PAHs through successive ring formation, which eventually is believed to lead to carbonaceous nanoparticles. 72 Another RSFR that might be involved in PAH formation is benzyl. Recently, computational research 102 has shown that the (self) addition of benzyl might be a viable mechanism for the formation of PAH beyond two rings. More practical research towards this still needs to be performed though Polymerisation reactions Besides sequential reaction of ARs and RSFRs, other mechanisms are possible in stellar ejecta involving polymerisation pathways. The direct polymerisation of PAHs by the initial formation of PAH dimers was proposed by Mukherjee et al. (1994) and is depicted in Scheme

20 Scheme 6 Growth of aromatic structures via the dimerization and coalescence of PAHs proposed by Mukhergee et al. (1994) 103. Based on ref. 67. Alternatively, the polymerisation of polyynes on a surface radical site of any small grain seed is possible (Scheme 7), as proposed by Krestinin et al. (2000).104 The presence of small polyenes (C 4H 2, C 6H 2, C 8H 2) in the sooting zone of flames together with PAHs supports this hypothesis. Scheme 7 Polymerisation of polyyenes on a surface radical site and the sequential transformation into aromatic structures yielding extra radical sites and rapid growth. Based on ref. 67. There is still controversy on the prevalent pathways to AC grain growth. The HACA mechanism is mainly used to explain aromatic ring growth towards PAHs in combustion processes, and thus in stellar ejecta. 67 This is mainly because it is the most researched mechanism. 93 The HACA mechanism cannot explain the formation of small carbonaceous nanoparticles made of a few aromatic rings linked by aliphatic bonds. Furthermore, the HACA pathway only goes up to two benzene rings (naphthalene and acenaphthalene (C 12H 8)) as was recently shown in a theoretical study, and after that predominantly penta 105 More complex PAHs cannot be formed through the HACA mechanism (not even three benzene rings such as anthracene and phenanthrene). Also, the HACA mechanism is too slow to explain the fast experimental PAH- and soot-formation. 97,98,106 Since the HACA mechanism is the central backbone in contemporary PAH synthesis in circumstellar envelopes (CSE) of carbon-rich AGB stars, the omnipresence of PAHs in the ISM cannot be explained by synthesis of PAHs through the HACA mechanism in CSEs alone. A complementary route is needed. This might be one of the other previous discussed routes, PAC or polymerisation. An alternative route towards PAH formation might also exist in the cold environment of the ISM at low temperatures (10 K), as is elaborated on in section 3, 72 but it can also be an alternative top-down mechanism in stellar ejecta as is explained below. 20

21 2.2 TOP-DOWN MECHANISMS Graphene etching on SiC grains An alternative top-down mechanism for producing PAHs is proposed by Merino et al. in This mechanism works through the hydrogen processing of silicon carbide (SiC) dust grains that exist in ISM and CSE. 107 Such SiC dust grains are formed in the inner regions of evolved stars through gasphase condensation, between 1-5 stellar radii (Figure 11). These SiC grains have a significant amount of segregated carbon on their surfaces, with a graphite-like organisation (Figure 11 and Figure 12). Once these SiC grains have been formed, they will travel outwards through the CSE pushed by the radiation pressure coming from the infrared photons of the central star. Eventually they will reach the cold external layers of the envelope, more than 5 stellar radii away. Here they are exposed to ultraviolet radiation because the densities are lower. In this region the grain surfaces are further processed by ultraviolet photons and H atoms produced by H 2 photodissociation. Atomic H interaction at a higher temperature ( K) on a graphene-terminated SiC surface leads to significant surface erosion. This makes the graphene layer at the SiC grain surface etched, leading to the formation of broken graphene flakes (Figure 12). The size of these flakes range from a few carbon rings to large graphene areas. The defects in the graphene layer are spots where the H atoms attacking can be inserted into the graphene structure. This mechanism transfers PAHs and PAH-like species from the solid state to the gas phase. 75 Graphene erosion predominantly occurs between 900 and 1200 K, and a high enough abundance of atomic hydrogen is required. The graphene etching takes place efficiently because it is supported on a SiC grain. Once the PAH is decoupled, etching is no longer an efficient mechanism. 75 Figure 11 The mechanism of PAH formations through graphene etching on SiC grains. The formation mechanism can be divided into four different stages. 1.) The formation of SiC in the gas phase and condensation into micrometer- and nanometre-sized grains (T=2000; 1-5 R*). 2.) The annealing of the SiC dust grains helped by the heat of the star and C-rich phases and graphene (T= K; 1-5 R*). 3.) The exposure of the surface to atomic hydrogen, promoting graphitisation of the C-rich surface and H passivation of the underlying buffer layer (T= K; 5-20 R*). 4.) The etching of the graphene by atomic hydrogen and thermally assisted desorption of PAHs (T=1200K; 5-20 R*). Reproduced from ref

22 Figure 12 The disruption of the graphene surface resulting in small carbonaceous species such as benzene rings (blue), acetylene molecules (black), methylene radical (orange) or PAHs (red). Reproduced from ref. 75 The graphene-etching mechanism is consistent with the fact that PAH emission is prominent in photodissociation regions, where most molecules are photodissociated and dust grains are processed. 75 Furthermore, it is known that dust grains produced in Carbon-rich AGB stars have a high ratio of SiC crystallites. 108 Furthermore, SiC are abundant in evolved stars, but are normally gone in the ISM. This might be because of this etching process which destroys the grains. 109 The results look promising, but further research needs to be performed to establish this mechanism. 22

23 3 FORMATION MECHANISMS IN THE INTERSTELLAR MEDIUM The ISM is a very diverse place. The physical conditions are not comparable with any on earth and densities and temperatures vary a lot. The ISM particle densities range from 50 cm -3 to 10 4 cm -3 in respectively diffuse and dense clouds, and the temperatures range from 10 K to 80 K. 110 The average time between particle collisions is long because of these low densities for diffuse clouds decades and for dense clouds hours. This is relatively short compared to the lifetime of molecular clouds, which are typically stable over a few million years and many collisions may still occur. It is however long enough to allow for alternative, otherwise insignificant, processes to take place, such as the radiative cooling of PAHs producing the AIBs. Diffuse clouds have a visual extinction of around 0.5 with a total hydrogen column density of N H ~10 21 cm -2 and are thus quite transparent to the ambient interstellar radiation field. The high UV radiation field causes a harsh environment, and apart from molecular hydrogen, most carbon, oxygen and nitrogen are in atomic form. PAHs are observed in diffuse clouds through their infrared emission. The UV radiation pumps the PAHs leading to the AIBs. The high UV field, however, can also destroy PAHs and only the larger species are thought to survive. 27 Molecular clouds, particularly dense, provide a friendlier environment. Densities of ~150 cm -3 in diffuse molecular clouds to ~2 x 10 3 cm -3 in cold molecular clouds protect the PAHs from the too harsh UV radiation. The absence of UV pumping in dense molecular clouds, however, hinders observation of the PAHs and they can only be observed at the edges of dense molecular clouds. 27 In molecular clouds the temperatures can be as low as 10 K and a predominantly kinetic equilibrium is present. When PAHs are formed in situ, low-temperature formation mechanisms are required. Usually this implies that only exothermic reaction can occur, although excited (non-thermal) species might supply enough additional internal energy to activate endothermic reactions (when UV photons are still present). Figure 13 Schematic potential energy curves for reactions (a) with and (b) without an energetic barrier along the minimum energy path form reactants to products, and (c) with a submerged barrier. Reproduced from ref There are three kinds of exothermic reactions. The first is a reaction with a substantial barrier (Figure 13a). This barrier is usually caused by the necessity to break bonds and leads to a positive activation energy and the reactions decreases in speed when the temperature decreases. However, if the two reactants involved are radicals, no barrier on the potential energy curve may be present (Figure 13b). 23

24 The potential curve falls into a deep well when the unpaired electrons of both radicals combine in a chemical bond. This causes a negative activation energy. When this happens, the reactions increases in speed when the temperature decreases. There is also a third scenario possible (Figure 13c). In this exothermic reaction pathway, the transition state lies lower in energy than the separated reactants. This means that a barrier does exist in the reaction, but it is located below the energy of the separated reactants. This is called a submerged barrier, and yields a de facto barrierless reaction. This reaction increases in speed at a higher temperature, because the barrier can be easily crossed, but also increase in speed at lower temperature. This increase is because once the intermediate in the first well has been formed, it will not be reversed because of the lack of energy. It can only cross the lower submerged barrier. Further properties of molecular clouds are that at the edges of molecular clouds atomic gas is transformed into molecular gas and the carbon drives a complex hydrocarbon chemistry. This might stimulate the formation of PAHs. 112,113 Additionally, benzene is detected in molecular clouds, the key-building block for PAHs. 72,114 This suggests that molecular clouds might be able to host further PAH formation. 72 In the following sections the possible reactions in molecular clouds are discussed. 3.1 ION MOLECULE REACTIONS Complementary routes for the formation of PAHs have often been sought in ion-molecule reactions. 115 Ion-molecule reactions probably dominate diffuse clouds, where most molecules are ionized. 27 Exothermic reactions between neutrals and ions could proceed very rapidly and at low temperatures due to the long-range attraction between the ion and the (induced) dipole of the molecule. Recent astrochemical models 110 have proposed that the synthesis of benzene and PAHs is dominated by elaborate networks of ion-molecule reactions (Figure 14). An example of such a reaction is the formation of C 6H 5 + ions (of an unknown structure) from methane (CH 4), ethylene (C 2H 4), and propargyl with C 2H 5 + and C 4H 2 + ions. 116,117 The C 6H 5 + react further to form PAH-like cations via multistep pathways

25 Figure 14 Initial sequences of some ion-neutral reactions leading to the production of hydrocarbons in the ISM. Species in boxes have been observed, species in circles are inferred. Reproduced from Ref The extent to which PAHs are formed in the above mentioned ion-molecule reactions could not be confirmed by laboratory experiments. 72 To give an example of this, a comparison of the ion-molecule reactions leading to C 6H 5 + can be made. When only the studied ion-molecule reactions leading to C 6H 5 + are taken into account, the peak abundance of C 6H 5 + decreases orders of magnitude. This also influences the benzene formation from this ion a similar reduction of benzene formed via ionmolecule reactions occurs. Then, the uncertainties for the assumed rate constant for the formation of C 6H 7 + from C 6H 5 + with molecular hydrogen should also be taken into account. 119 This means that benzene cannot be formed in sufficiently high quantities. 120 Thereby PAHs cannot be formed in sufficiently high abundacnes as benzene is the key building block of PAHs BARRIERLESS AND RAPID NEUTRAL-NEUTRAL REACTIONS Astrochemists assumed that ion-molecule reactions are always barrierless and all neutral-neutral reactions have entrance barriers. 121,122 In cold molecular clouds, however, less ionization is present than in diffuse molecular clouds and neutrals play an more important role. Recently, alternative barrierless neutral-neutral reactions have been discovered Formation of monocyclic aromatic molecules A more plausible route to explain the presence of benzene in cold molecular clouds of the ISM are barrierless and rapid neutral-neutral reactions involving ethynyl radicals (C 2H) with 1,3-butadiene (C 4H 6;H 2CCHCHCH 2) (Scheme 8-1). Recent experimental studies 120, have provided evidence that formation of monocyclic aromatic molecules from acyclic precursors is possible through exoergic and fast, barrierless reactions as a result of a single collision. These studies are based on both experimental and computational data of bimolecular neutral-neutral reactions of ethynyl and dicarbon (C 2) with vinylacetylene (C 4H 4), 1,3-butadiene, and isoprene (2-methyl-1,3-butadiene; C 5H 8). These studies, all conducted by the groups of Kaiser and Mebel ( ), provided some of the 25

26 first evidence that the formation of benzene rings or other monocyclic aromatic molecules in the gas phase is possible via bimolecular cyclization reactions involving two neutral reactants. 72 These reactions are shown in Scheme 8. Scheme 8 Monocyclic aromatic molecules formed by exoergic and barrierless bimolecular reactions. 1.) Reaction of ethynyl (C 2H) with 1,3-butadiene (C 4H 6) towards benzene ) Reaction of dicarbon (C 2) with 1,3-butadiene towards phenyl (C 6H 5) ) Reaction of ethynyl with vinylacetylene (C 4H 4) towards o-benzyne (C 6H 4) ) Reaction of ethynyl and 2-methyl-1,3- butadiene (C 5H 8) towards toluene (C 6H 5CH 3) ) Reaction of dicarbon with 2-methyl-1,3-butadiene towards benzyl (C 6H 5CH 2 ). Based on ref Benzene is abundant in molecular clouds so it can either be formed in the cold ISM of 10 K via this mechanism, or transported there from stellar ejecta. From this, successive ring formation can take place, as is described in the next section Successive ring formation In 2012 it was shown by the groups of Tielens, Kaiser and Mebel that formation of naphthalene is possible via a barrierless and exoergic reaction between the phenyl radical (C 6H 5) and vinylacetylene (CH 2=CH-C CH) (Scheme 9). 127 Scheme 9 Reaction between the phenyl radical and vinylacetylene. Further research showed that also naphthalene derivatives, such as hydrogenated naphthalene, mono- and di-methyl-substituted isomers, could be formed by exoergic and fast, barrierless reactions of C4 and C5 hydrocarbons vinylacetylene, 1,3-butadiene, 1-methyl-1,3-butadiene (C 5H 8), and 2-methyl-1,3-butadiene (C 5H 8) with phenyl-type radicals such as phenyl, meta-tolyl (m-c 6H 4CH 3), para-tolyl ( p-c 6H 4CH 3)]. The formation of naphthalene and its derivatives start with a practically barrierless addition of a phenyl-type radical within an initial Van der Waals complex through a submerged barrier to a vinyl-type moiety. When this vinyl-type moiety is in conjugation with a -C CH or -HC=CH 2 group in the hydrocarbon reactant (vinylacetylene, 1,3-butadiene, 1-methyl-1,3- butadiene (C 5H 8), or 2-methyl-1,3-butadiene (C 5H 8)), a resonantly stabilized free radical (RSFR) intermediate can be formed. The RSFR intermediates then rearrange and form a cyclic intermediate. Atomic hydrogen loss then causes this cyclic intermediate to aromatize and form a PAH. A condition for this pathway is the sufficiently strong, attractive, long-range interactions are present, because these are needed to form a Van der Waals complex. This means that phenyl type radicals only form van der Waals complexes with C4 or higher hydrocarbons. These reactions with 26

27 submerged reaction barriers can takes place in molecular clouds with high yield of PAH formation. 72 This research, performed using combined cross beam experiments and electronic structure calculations, is thus complementing the PAH-formation in high temperature environments such as stellar ejecta and gives a basis for the PAH formation in cold molecular clouds at 10 K. Figure 15 Schematic representation of the stepwise growth of polycyclic aromatic hydrocarbons in the interstellar medium via reactions of phenyl-type radicals with vinylacetylene involving low-temperature chemistry. Reproduced from ref. 72. More than two rings: The formation of naphthalene gives way for the prediction of more complex PAHs such as anthracene and phenanthrene in the ISM. It is known that in the ISM photodissociation of naphthalene can lead to 1- and 2-napthyl radicals. 127 These 1- and 2-napthyl radicals may react with vinylacetylene in a similar fashion as the previous described formation of naphthalene. This can give expansion to PAHs with three fused benzene rings anthracene and phenanthrene. Because these reactions have a much stronger bound Van der Waals complex between vinylacetylene and the naphthyl radical 131, they are also expected to have a submerged reaction barrier, and thus be de facto barrierless. It could therefore be assumed that the reaction of vinylacetylene with the phenyl radical is only a prototype of a class of vinylacetylene mediated reactions for PAH growth in the cold interstellar medium

28 The recent research into the formation of PAHs and their derivatives by bimolecular collisions of phenyl-type radicals with unsaturated C4 and C5 hydrocarbons involving an initial van der Waals complex and a submerged barrier following an exoergic reaction pathway gives an interesting new outlook on the formation of PAHs in cold molecular clouds. It certainly presents a fundamental shift from PAH formation in hot stellar ejecta through a HACA mechanism. Considering that the HACA mechanism is inefficient to form PAHs beyond acenaphthalene, 105 such an alternative mechanism is much needed. An overview of the complete formation route is given in Figure 15. First results of this relatively recent research are very promising, but further research needs to be conducted with other molecules than only naphthalene and its derivatives. Especially molecules with more aromatic rings need to be investigated. A further problem is that neither phenyl radicals nor vinylacetylene have been detected in interstellar clouds. 132,133 A reason for this could be their low dipole moments, or their high reactivity. In molecular clouds barrierless and exoergic vinylacetylene can be formed by reactions of ethynyl radical (C 2H), formed by photodissociation of acetylene (C 2H 2), with ethylene (C 2H 4). 134 The phenyl radicals can be formed in photon dominated regions through photolysis of benzene (C 6H 6). 120,135 The formation of phenyl radical cannot happen in dense clouds, where these reactions are proposed, and phenyl should thus come from ex situ, probably the edges of molecular clouds. Here atomic gas is transformed into molecular gas, and carbon drives hydrocarbon chemistry. Furthermore, UV radiation is also needed for the formation of PAHs with more two aromatic rings according to the proposed scheme because of the photodissociation of naphthalene towards 1- and 2-napthyl radicals (Figure 15). The lack of UV photons in dense molecular clouds makes this problematic in situ. Therefore only the edges of molecular clouds could be suitable for the barrierless formation routes involving two neutrals towards PAHs as described above, because here UV radiation might be high enough to photodissociate, by low enough to not lead to a strong ionization balance. 72 More investigation is needed to solve all these issues and make this a wellestablished route in the formation of PAHs Ethynyl addition Another barrierless, bottom up mechanism that could take place in molecular clouds is ethynyl addition (Scheme 10). 136 Ethynyl radicals (C 2H) can be formed by the photolysis of acetylene. This mechanism is initiated by the addition of an ethynyl radical (C 2H) to the ortho-carbon of a phenylacetylene (C 6H 5C 2H) molecule. The intermediate loses a hydrogen atom rapidly, resulting in 1,2-diethynylbenzene. This can react with a second ethynyl molecule through addition to a carbon atom of one of the ethynyl side chains. Subsequently, a ring closure takes place leading to an ethynyl-substituted naphthalene core. This core can lose a hydrogen and form ethynyl-substituted 1,2-didehydronaphthalene when single-collision conditions are present (as is the case in the ISM). Under higher pressures (such as the atmosphere of Titan), threebody collisions can also lead to stabilisation of this naphatalene-core intermediate. Ethynyl addition should be a viable alternative of the HACA mechanism for PAH formation in lowtemperature environments, since ethynyl radicals have been detected in cold dark clouds. 137 This mechanism could thus be of great importance to form PAHs in the ISM. 72,136 More research is still needed, however, since mainly computational research has been performed until now. 136,

29 Scheme 10 Ethynyl addition mechanism. Based on ref SUPERNOVAE SHOCKS IN THE ISM Another source of PAH formation are supernovae (SNe) explosions of massive stars. These send shock waves through the interstellar medium. They compress gas, form dense clouds and distort molecular clouds throughout the galaxy. 77 This causes the fragmentation of graphite grains and the formation of PAHs through a top-down mechanism. The interstellar shock waves convert the kinetic motion of the gas into thermal energy in the ISM. However, the graphite grains are relatively heavy, and thus have a certain inertia. This makes them plough through the warm gas. Furthermore, the grains are charged, which makes them gyrate around the magnetic field. When compression of the postshock gas takes place, the grains are betatron accelerated. 142 The produced high velocities subsequently induce collisions between large and small grains. These collisions either lead to cratering or destruction of the grains. The smallest fragments resulting from these collisions are likely small, two-dimensional species, taken into account the layered structure of graphite. These PAH-like species, pieces of graphite layers, are then exposed to the UV photons in the ISM and to other reactive species. These will react with the scattering fragments, and this might lead to the formation of small PAHs. The graphite sheet is thus the template for the PAH. 26 Calculations 142 have shown that shattering caused by grain-grain collisions is an important process for the evolution of the interstellar grain-size distribution, and the lifetime of a large grain is short enough to be a viable mechanism to produce very small grains, interstellar PAHs and PAH clusters. However, there is little evidence 113 for PAHs being present in regions processed by supernova shocks. This may however be because it is difficult to perceive the emission of supernova remnants (SNRs) against the galactic background emission. PAHs and dust grains heat slowly and cool efficiently, and thus give little excess emission, opposed to the rapid heating and inefficient cooling of the general gas, which is visible. 26,143 This mechanism can thus be considered a viable alternative top-down mechanism, considering the omnipresence of PAHs throughout the ISM. 29

30 30

31 4 DISCUSSION As described in the previous sections, many mechanisms for the formation of interstellar PAHs are possible, both in situ, within the ISM, and ex situ, in stellar ejecta. Because no individual PAHs have been identified and the exact PAH population is unknown, no conclusive pathway can directly be directly determined. Therefore extrapolation of terrestrial-like PAH formation needs to take place, e.g. soo t formation in flames, or new reaction pathways need to be invented. The pathways in stellar ejecta seem very plausible because of their parallels with PAH formation on earth. Conditions are more favourable for complex organic molecule formation. However, observationally, little evidence is available due to the lack of UV pumping photons in stellar spectra. 85 Furthermore, the HACA mechanism, a favourite for the explanation of high-temperature PAH formation, cannot explain ring growth beyond the second ring and is not very efficient. 105 A combination of the efficient PAC pathway and the HACA pathway could offer a solution for this problem. Also polymerisation reactions may be an interesting alternative for PAH growth, though these might require higher densities. The alternative mechanisms definitely need more research, especially in an extraterrestrial context to determine if they might be applicable to stellar ejecta. Currently, studies of the formation of PAHs in stellar ejecta are mainly theoretical and extrapolate terrestrial soot research. 67,79, The main difference between stellar ejecta and combustion processes on earth is the density, which has to be taken into account to see if the extrapolation is correct. Furthermore, the recently suggested alternative top-down mechanisms in stellar ejecta need to be seriously considered. 75 Especially in prevalent grain makers, such as C-rich AGB stars, a top-down mechanism is very plausible. Top-down formation of PAHs in the ISM by SNe shocks could also be explanatory. Also, an equilibrium between soot and PAHs might also exist, because the current theory on soot formation is through nucleation of PAHs, as is shown in Figure 16. Figure 16 Clusters formed by PAHs form the bridge between individual PAHs and amorphous carbon nanoparticles. Reproduced from ref. 26. Bottom-up mechanisms in molecular clouds, especially neutral-neutral reactions with submerged barriers, also offer an interesting complementary route to the formation of PAHs in stellar ejecta. 147 In cold dense clouds, the temperature is only a few degrees above zero and it is assumed that under such extreme conditions, predominantly reactions unhindered by barriers on their potential energy surface can take place (Figure 13b). However, recent research 148 showed that reactions with a significant barrier on their reaction path may also occur at low temperatures by tunnelling through it. This happens when a system first forms an initial, relatively weakly bound complex, which will subsequently cross a shallow potential barrier to form the products. It was observed 147 that at high temperatures a positive activation energy was observed, but when the temperature decreases, the 31

32 rate constant passes through a minimum and then increases again by almost an order of magnitude between 200 K and 23 K (Figure 13c, lower graph). This behaviour is attributed to the relationship between the formation and the redissociation of the initial complex compared with the subsequent crossing of the barrier. 149,150 In cold environments, the dissociation of the initial complex will go more slowly, so it has more chance to complete the reaction. When further studied, it was discovered that the increase of the rate constant for a cold environment can be explained by quantum tunnelling light particles, such as H atoms, have a substantial probability of penetrating beneath potential energy barriers. 148 This research offers interesting perspective on other reactions with a submerged barrier at low temperatures. To be certain that the above described reactions between phenyl radicals and vinylacetylene and its derivatives (Scheme 9) are actually a plausible formation mechanism for PAHs, it first needs to be experimentally established that formation of molecules beyond the two rings of naphthalene and its derivatives is possible (Figure 15). Additionally, observational proof should be gathered for the presence of phenyl radicals and vinylacetylene in molecular clouds. 132,133 If this all yields positive results, an interesting new mechanism for the formation of PAHs on the edges of molecular clouds has been established. Ethynyl addition (Scheme 10) is a good alternative for PAH formation in more diffuse clouds due to the photolysis of acetylene, together with ion molecule reactions. In more diffuse clouds, endothermic reactions with might also take place due to additional internal energy of excited (non-thermal) species. Outside the scope of this literature survey, but important to the PAH abundance and to be taken into account when comparing the efficiency of formation mechanisms, is the destruction of PAHs. Current models have shown that PAHs are destroyed faster in the ISM than they can be synthesised. 143,151,152 There are three causes for this destruction. The first is that theoretical studies predict lifetimes of a few hundred million years for PAHs in the diffuse ISM. This timescale is much shorter then the timescale of over which new PAH-based material from carbon-rich AGB stars can be injected into the ISM. 72,113 The theoretical studies predicting the lifetimes are based on laboratory studies of the loss of acetylene (C 2H 2) when small PAHs undergo photolysis. Secondly, SNe interstellar shock waves also cause short lifetimes for PAHs in the ISM apart from creating them in a top-down mechanism. 113 Finally, small PAHs are rapidly degraded by energetic cosmic ray bombardment. Here also timescales of a few hundred million years apply. These considerations show that the definite assignment for a formation mechanism of PAHs is not easily made. The lack of knowledge about PAH populations in the different environments of how they change makes it even more difficult to assess their formation, evolution and destruction mechanisms. This makes also the environment for the formation undecided. In my opinion, a combination of these mechanisms is the most probable explanation and might possibly give a high enough yield for the abundance of PAHs throughout the universe. An especially high yield is needed so that the formation of PAHs can be in equilibrium with its destruction. 32

33 5 SUMMARY & CONCLUSION It is generally believed that PAHs are omnipresent in the ISM, in which they play an important role. However, their formation mechanism(s) remain elusive. It is especially unclear in which pathways, both in and ex situ, are dominant for which local environments. PAHs could be transported to the ISM from stellar ejecta, or formed locally in diffuse and/or dense molecular clouds. Stellar ejecta and molecular clouds differ significantly in their physical parameters with having temperatures of 2500 K and 10 K, respectively, and densities of 10-6 atm and 150 cm -3. In stellar ejecta the physical conditions resemble combustion processes in a terrestrial context more, and thus similar reactions can possibly take place. The rate-determining step of the first aromatic ring closure is proposed to occur through recombination of two propargyl radical forming benzene (Scheme 1) and the phenyl radical by a reaction of 1-buten-3-ynyl or 1,3-butadienyl (1-C 4H 5) with acetylene (Scheme 2 and 3). The abundances of the precursors in the stellar ejecta determine which reaction will be more prevalent. Further ring growth can than take place via sequential reactions of radicals (ARs and RSFRs), such as the HACA mechanism (Scheme 4) or phenyl addition-cyclisation (Scheme 5). Alternatively polymerisation reactions can cause further ring growth (Scheme 6 and 7). Apart from these bottomup mechanisms, also a top-down mechanism can take place in stellar ejecta graphene etching on SiC grains (Figure 12). These PAH formation pathways, however, are not completely satisfactory due to their inefficiency. Alternative pathways in the ISM itself have been proposed, where because of the low temperatures no net barrier is desired. In diffuse clouds, ion-molecule reactions are possible, and on the edges of dense molecular clouds radical reactions probably dominate. The formation of the first aromatic ring could take place through ion-molecule reactions in diffuse clouds (Figure 14) and neutral-neutral reactions involving ethynyl radicals with 1,3-butadiene in dense clouds (Scheme 8). Naphthalene and its derivatives can be formed via reaction with a submerged reaction barrier of a phenyl radical and vinylacetylene, again in dense clouds (Scheme 9). It is speculated that a similar reaction of the radicals formed from naphthalene and vinylacetylene can cause subsequent ring growth, but UV photons are needed for this process (Figure 15). Alternatively, ethynyl addition can cause successive ring growth of PAHs in more diffuse clouds with UV photons (Scheme 10). Finally, a top-down mechanism is also possible in the ISM due to interstellar SNe shocks, which can cause fragmentation of interstellar graphite grains. Subsequent photoprocessing can yield PAHs. It can thus be concluded that many plausible formation mechanisms of PAHs are possible, both in the ISM as in CSEs. However, no completely satisfactory mechanism is present, and the lack of knowledge about the exact PAH population in different astronomical environments makes a conclusive determination of a formation pathway very difficult. Further experimental and observational investigations are required to give a decisive conclusion on which pathway is prevalent in the formation of PAHs in which local environment. Currently, theoretical models incorporating the formation pathways for PAHs cannot yield the abundances as derived from observations, 83 suggesting that neither the pathways in the ISM as in CSE give sufficient yield. Unless a very efficient mechanism for PAH formation not yet discovered exists, a combination of pathways in both the interstellar medium as in stellar ejecta is in my opinion the most probable explanation for the omnipresence of PAHs throughout the universe. 5.1 FUTURE RESEARCH Further research could be on the formation of PAHs through a barrierless reaction pathway with more than two rings (Figure 15). This research should also take place experimentally, as was done 33

34 with the investigation of such reactions towards naphthalene. If these experiments yield positive results, a promising new reaction pathway towards PAHs in the ISM, specifically on the edges of dense clouds, has been established. This could also be a viable mechanism in diffuse molecular clouds. Further dedicated research also needs to be performed on the mechanisms towards PAH formation in stellar ejecta, to conclude which is the most prevalent. This needs to be done under the conditions present in CSE, and not under terrestrial soot formation conditions. Due to the long reaction time scales in space, a computational study might be the most viable. Another emerging field is the formation of PAHs in the solid phase, e.g. in ices. These can be then desorbed back in the gas phase. It is known that PAHs are present in ice grain mantles, but not much research have been performed if formation also can take place here Finally, NASA s James Webb Space Telescope (JWST) (Figure 17), to be launched in 2018, might provide new observational data that can help to solve the issue of the formation mechanism. The JWST has unprecedented spatial resolution, meaning that it can track the PAH and the precursor reactants along the ISM or in CSE. It is specifically sensitive for evolution, which is very important for the determination of PAH formation mechanisms. This will be invaluable in further determining the possible reaction mechanisms, or even detecting a whole new class of reactions. Figure 17 James Webb Space Telescope Credits: NASA 34