Laboratory study of the formation of fullerene (from smaller to larger, C 44 to C 70 )/anthracene cluster cations in the gas phase
aa r X i v : . [ phy s i c s . a t m - c l u s ] J un Research in Astron. Astrophys. Vol.0 (20xx) No.0, 000–000 (L A TEX: ms-RAA-2020-0011.tex; printed on July 1, 2020; 0:46) R esearchin A stronomyand A strophysics Laboratory study of the formation of fullerene (from smaller tolarger, C to C )/anthracene cluster cations in the gas phase Deping Zhang , , Yuanyuan Yang , , , Xiaoyi Hu , , and Junfeng Zhen , CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy,University of Science and Technology of China, Hefei 230026, China; [email protected];[email protected] School of Astronomy and Space Science, University of Science and Technology of China, Hefei230026, China CAS Center for Excellence in Quantum Information and Quantum Physics, Hefei NationalLaboratory for Physical Sciences at the Microscale, and Department of Chemical Physics,University of Science and Technology of China, Hefei 230026, China
Received 20xx month day; accepted 20xx month day
Abstract
The formation and evolution mechanism of fullerenes in the planetary nebulaor in the interstellar medium are still not understood. Here we present the study on thecluster formation and the relative reactivity of fullerene cations (from smaller to larger,C to C ) with anthracene molecule (C H ). The experiment is performed in the ap-paratus that combines a quadrupole ion trap with a time-of-flight mass spectrometer. Byusing a 355 nm laser beam to irradiate the trapped fullerenes cations (C or C ),smaller fullerene cations C (60 − n )+ , n=1-8 or C (70 − m )+ , m=1-11 are generated, re-spectively. Then reacting with anthracene molecules, series of fullerene/anthracene clus-ter cations are newly formed (e.g., (C H )C (60 − n )+ , n=1-8 and (C H )C (70 − m )+ ,m=1-11), and slight difference of the reactivity within the smaller fullerene cations areobserved. Nevertheless, smaller fullerenes show obviously higher reactivity when com-paring to fullerene C and C .A successive loss of C fragments mechanism is suggested to account for the forma-tion of smaller fullerene cations, which then undergo addition reaction with anthracenemolecules to form the fullerene-anthracene cluster cations. It is found that the higher laserenergy and longer irradiation time are key factors that affect the formation of smallerfullerene cations. This may indicate that in the strong radiation field environment (suchas photon-dominated regions) in space, fullerenes are expected to follow the top-downevolution route, and then form small grain dust (e.g., clusters) through collision reactionwith co-existing molecules, here, smaller PAHs. Key words: astrochemistry — methods: laboratory — ultraviolet: ISM — ISM:molecules — molecular processes
Polycyclic aromatic hydrocarbons (PAHs) are well recognized as an essential component of the inter-stellar medium (ISM) and may account for ≤
15% of the interstellar carbons. They are observed viathe infrared (IR) emission bands at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 µ m widespread throughout the Zhang et al. 2020
Universe (Allamandola et al., 1989; Genzel et al., 1998; Sellgren, 1984; Tielens, 2008, 2013; Li, 2020).A huge effort was undertaken in the past decades to identify the carriers of those IR emission features,however, no specific PAH responsible for the IR emission features has been identified. By contrast,another important type of carbonaceous species, fullerenes (C and C ) have been unambiguously ob-served in planetary nebula Tc1 via their IR emission spectra (Cami et al., 2010). After that, C has alsobeen detected in reflection nebulae(Sellgren et al., 2010; Peeters et al., 2012; Boersma et al., 2012), pro-toplanetary nebulae(Zhang & Kwok, 2011), R Coronae Borealis stars(Garc´ıa-Hern´andez et al., 2011),the peculiar binary XX Oph(Evans et al., 2012), young stellar objects(Roberts et al., 2012) and diffuseclouds(Bern´e et al., 2017). Recently, the proposal of C as the carrier of two DIBs (9577 ˚A and9632 ˚A) was confirmed by the laboratory spectrum recorded in the gas phase (Campbell et al., 2015),which also revealed some weaker absorption features. The weak C features were subsequently de-tected in astronomical spectra (Walker et al., 2016; Cordiner et al., 2019).Since the first discovery of C in the gas phase by Kroto et al. (1985), the fullerene moleculeshave been the topic of extensive laboratory studies (see e.g. B¨ohme (2011, 2016); Linnartz et al. (2020)and references therein), which provide important knowledge on the possible formation and evolutionroutes of fullerenes in the Universe. For example, the laboratory work showed that C can be formedstarting form the carbon-rich seeded gas via a bottom-up formation route (see e.g. J¨ager et al. (2008)).On the other hand, the laboratory experiment revealed that C can be generated from UV radiationinduced photochemical evolution of large PAHs (Zhen et al., 2014). This may be a possible top-downroute formation of C in the interstellar environment. The very recent laboratory work suggested thatC can undergo facile formation from shock heating and ion bombardment of circumstellar SiC grains(Bernal et al., 2019). Garc´ıa-Hern´andez et al. (2013) summarized and discussed the possible formationmechanisms of fullerene in evolved stars and in ISM in their IR spectroscopic study of C /anthraceneadducts.Fullerenes and their ions are highly active due to the unsaturated features, and thus they can eas-ily react with other molecules (Petrie et al., 1992; Murata et al., 2001; B¨ohme, 2016; Omont, 2016).Fullerene/PAHs adducts are one family of the fullerene reaction products and are of astrochemical in-terest. It is known that fullerene/PAHs adducts are generated via the Diels − Alder cycloaddition reac-tions (Briggs & Miller, 2006; Petrie & Bohme, 2000; Cataldo et al., 2014; Zhen et al., 2019c). On theother hand, anthracene (C H ) is among the simple PAH molecules, whose reaction with C has re-ceived considerable attentions. For example, Cataldo et al. (2014) reported the sonochemical synthesisof monoanthracene adduct (C H )C and bis-anthracene adduct (C H ) C with the precursors ofC and anthracene dissolved in benzene. The same group also studied the IR spectra of these adductsby using Fourier transform IR spectroscopy. More importantly, it is found that the IR spectra of theC /anthracene adducts are similar to those of C and other unidentified IR emission bands recordedby astronomical observations (Garc´ıa-Hern´andez et al., 2013). Motivated by Garc´ıa-Hern´andez et al.(2013)’s work, the formation and photochemistry of (C and C )/anthracene cluster cations in thegas phase were investigated in this lab (Zhen et al., 2019c). The fullerene/anthracene cluster cations areformed from C / / or C / / and neutral anthracene molecules via ion-molecule reactions.Upon irradiation by a 355 nm laser beam, the cluster cations dissociated into fullerene cations and neu-tral anthracene molecules. Besides, the experimental results showed that C and C have lowerreactivity compared to their neighbor fullerene cations (C / and C ).It should be mentioned that Candian et al. (2019) reported a theoretical study of the stability and IRspectra of neutral and ionized fullerenes with a coverage from C to C . By comparing the theoreticalIR spectra to the observed emission spectra of several planetary nebulae, the authors suggested the possi-ble presence of smaller cages (44, 50 and 56 carbon atoms) in the astronomical objects. In this contribu-tion, we present the laboratory study on the formation of cluster cations between fullerene cations (C to C ) and anthracene molecules, and investigate the reactivity of these fullerenes, especially the onescontaining 44, 50 and 56 carbon atoms. In order to generate the interested fullerene cations, higher laserenergy and longer irradiation time are used compared to our recent work (Zhen et al., 2019c). The ex-perimental details are given in the following section. The results and discussions, and astronomicalimplications are presented in section 3 and section 4, respectively. Conclusions follow in section 5. ormation of fullerene/anthracene cluster cations 3 The experiment was performed using the quadrupole ion trap and time-of-flight (QIT-TOF) mass spec-trometry setup, which has been described in detail elsewhere (Zhen et al., 2019a,c). Briefly, the gasphase fullerene molecules (C or C ) were prepared by heating their powder samples at a temper-ature of ∼
613 K and then ionized by electrons ( ∼
82 eV) produced in an electron gun (Jordan, C-950). The interested fullerene cations are selected by using an ion gate and a quadrupole mass filter(Ardara, Quad-925mm-01) and then guided into the quadrupole ion trap (Jordan, C-1251). Anotheroven (used at room temperature) mounted under the quadrupole ion trap was used to vaporize anthracenepowder. The gas phase anthracene molecules effused continuously towards the center of the ion trap.Through the ion-molecule reactions between fullerene cations and anthracene molecules in the ion trap,fullerene/anthracene cluster cations were produced. The third harmonic output (355 nm) of a Nd:YAGlaser (Spectra-Physics, INDI) was used to irradiate the fullerene cations and fullerene/anthracene clustercations and induce the photochemistry process. At a appropriate timing, the ions were extracted fromthe ion trap and detected by a reflection TOF mass spectrometer (Jordan, D-850).In order to produce smaller fullerenes, we optimized the experimental conditions and found thatthe laser energy and the irradiation time are critical to the production of smaller fullerene cations. Bymonitoring the intensity of new generated fragment ions in the mass spectrum, it is found that the optimalconditions are with the laser energy of ∼
30 mJ/pulse and irradiation time of 1.6 s in each measuredperiod.The simple PAH, anthracene (C H ), was used as the reactant to examine the reactivity offullerene cations based on the following considerations. The fullerene/anthracene adducts are amongthe simple and typical fullerene/PAHs adducts. Therefore, the study of formation and photochemicalprocesses of fullerene/anthracene adducts can provide a guidance for other fullerene/PAHs clusters.Furthermore, the anthracene molecule allows us to make a reasonable comparison with the previouswork (Zhen et al., 2019c) where the same molecule is used as the reactant. At last, due to its relativelyhigh vapor pressure at room temperature, the anthracene molecule is suitable for our current experimen-tal setup. The mass spectrum of the fullerene (C )/anthracene cluster cations recorded without laser irradiationis shown in Fig. 1 (top red trace). Fullerene cations (C / / ) and series of fullerene/anthracenecluster cations ((C H ) n C / / , n=1-3) were produced in the experiment. The fullerene cationswere generated by electron bombardment of neutral C molecules, and the cluster cations were formedvia ion-molecule reactions between C / / and anthracene molecules in the ion trap. Fig. 1 (mid-dle blue trace) shows the recorded mass spectrum of trapped fullerene/anthracene cluster cations afterlaser irradiation with energy of 26 mJ/pulse and irradiation time of 1.6 s (i.e., typically ∼
16 pulses). Itcan be seen that series of fullerene cations (C (60 − n )+ , n=0-8) and fullerene/anthracene cluster cations((C H )C (60 − n )+ , n=0-8 and (C H ) C (60 − n )+ , n=0-6) are formed in the ion trap. The bottomtrace of Fig. 1 displays the differential spectrum to reflect the intensity changes of these cations formedunder the conditions of laser-off and laser-on. It can be seen that the intensity of larger mass clustercations ((C H ) / C / / , (C H )C ) and fullerene cation (C ) decreases after laser irra-diation, while the intensity of other cations increases.To clearly show the intensity changes, a zoom-in of the middle and bottom traces of Fig. 1 isdisplayed in Fig. 2. As the number of carbon atoms decreases, the intensities for fullerene cations,their mono-anthracene adducts and bis-anthracene adducts become weak gradually. Interestingly, theintensity of (C H )C is weaker not only than its neighbor ((C H )C ), but also than othersmaller mono-adducts. Likewise, the intensity of (C H ) C is weaker than the other smaller bis-adducts.In recent study of Zhen et al. (2019c), laser energy of 1.3 mJ/pulse and irradiation time of 0.5 s wereused to irradiate the trapped ions. The fullerene cations, C , C and C , and their correspond- Zhang et al. 2020
480 576 672 768 864 960 1056 1152 1248 1344 1440 15360.01.53.04.56.07.59.0 m/z I n t e n s it y ( a r b . un it s . ) Without laserWith laser, 355nm, 26 mJ
Differential spectrum C n+ (C H )C n+ (C H ) C n+ (C H ) C n+ Fig. 1: Mass spectrum of fullerene (C )/anthracene cluster cations recorded without laser irradiation(top red trace) and with laser irradiation (middle blue trace). We used 355 nm laser with energy of26 mJ/pulse and irradiation time amounting to 1.6 s. The assignments of mass spectral peaks are shown.The differential spectrum between the blue trace (laser on) and the red trace (laser off) is also shown inthe bottom trace with black color.ing fullerene/anthracene cluster cations were recorded in the mass spectra (Fig. 2 in Ref. Zhen et al.(2019c)). By contrast, more smaller fullerene cations (C (60 − n )+ , n=3-8) and fullerene/anthracene clus-ter cations ((C H )C (60 − n )+ , n=3-8 and (C H ) C (60 − n )+ , n=3-6) were newly observed in cur-rent mass spectra (Fig. 1). Note that the laser wavelength (355 nm) used in these two studies was thesame. The newly observed smaller fullerene cations and their fullerene/anthracene cluster cations weredue to the higher energy (26 mJ/pulse) and longer irradiation time (1.6 s) used in present experiment.The observed smaller fullerene cations in present study (Fig. 1) all contain even numbers of car-bon atoms, indicating the successive C loss in the formation process, which has been known asthe main evolution way followed by fullerene cations after absorption of photons (Lifshitz, 2000;Zhen et al., 2014). Combining this consideration with previous studies (Zhen et al., 2014, 2019b,c),the following photochemistry mechanism (path 1-4) is suggested. After absorption of UV photons,fullerene/anthracene cluster cations dissociated into fullerene cations and anthracene molecules (path 1).And then, the fullerene cations including free ones and ones resulted from the dissociation underwentsuccessive C loss and formed smaller fullerene cations (path 2). After that, the smaller fullerene cations ormation of fullerene/anthracene cluster cations 5
480 576 672 768 864 960 10560.00.81.62.43.24.04.8
With laser, 355nm, 26 mJDifferential spectrum m / z I n t e n s it y ( a r b . un it s . ) C n+ (C H )C n+ (C H ) C n+ Fig. 2: The zoom-in mass spectrum of fullerene (C )/anthracene cluster cations with laser irradiationand the differential spectrum in the range of m/z=480 − [(C H ) (1 − C / / ] + h ν −→ (C H ) (1 − + [C / / ] + (1) [C / / ] + h ν −→ nC + [C (60 − ] + (2) [C (60 − ] + + C H −→ [(C H )C (60 − ] + (3) [(C H )C (60 − ] + + C H −→ [(C H ) C (60 − ] + (4) In addition to C , C was also used as the precursor to produce the smaller fullerene cationsand examine their reactivity with anthracene. The experiment conditions were same as those used forthe study of C except for the laser energy of 30 mJ/pulse. The recorded mass spectra are depicted inFig. 3. The top red trace shows the mass spectrum of the C /anthracene cluster cations recorded withoutlaser irradiation. Fullerene cations (C / ) and their anthracene cluster cations ((C H ) m C / ,m=1,2 were observed. After irradiation of these trapped ions by the 355 nm laser beam with energyof 30 mJ/pulse and irradiation time of 1.6 s, the recorded mass spectrum is displayed as the middleblue trace in Fig. 3. Series of fullerene cations (C (70 − m )+ , m=2-11), their mono-anthracene adducts((C H )C (70 − m )+ , m=2-11 and bis-anthracene adducts (C H ) C (70 − m )+ , m=2-9) were newlyformed comparing to the top trace without laser irradiation. In order to clearly show the changes inthe mass spectra recorded without and with laser irradiation, the differential spectrum is derived byextraction the top trace (without laser beam) from the middle trace (with laser beam). The resultantspectrum is shown as the bottom trace in Fig. 3. The peaks with positive intensity indicate that thecations were formed after laser irradiation, such as C (70 − m )+ , m=2-11.Fig. 4 displays the zoom-in of the middle and bottom traces of Fig. 3. It can be seen that the in-tensities for fullerene cations, their mono-anthracene adducts and bis-anthracene adducts became weak Zhang et al. 2020
70 70 70706662585450 7066625854706662585450480 576 672 768 864 960 1056 1152 1248 1344 1440 1536 I n t e n s it y ( a r b . un it s . ) m/z Without laserWith laser, 355nm, 30 mJ
Differential spectrum
68 68 68 C n+ (C H )C n+ (C H ) C n+ Fig. 3: Mass spectrum of fullerene (C )/anthracene cluster cations recorded without laser irradiation(top red trace) and with laser irradiation (middle blue trace). We used 355 nm laser with energy of30 mJ/pulse and irradiation time amounting to 1.6 s. The assignments of mass spectral peaks are shown.The differential spectrum between the blue trace (laser on) and the red trace (laser off) is also shown inthe bottom trace with black color.gradually as the carbon numbers get smaller. The intensity of (C H )C is weaker than its neigh-bors, (C H )C and (C H )C . Likewise, the intensity of (C H ) C is weaker than itsneighbors, (C H ) C and (C H ) C .The increments in laser energy and irradiation time on the C /anthracene system resulted in theproduction of more smaller fullerene/anthracene cluster cations (see Fig. 3), which is similar as theC /anthracene system (Fig. 1). In the previous work (Zhen et al., 2019c), laser energy of 0.9 mJ/pulseand irradiation time of 0.5 s were used to irradiate the trapped C /anthracene cluster cations. Thefullerene cations, C and C , and their corresponding fullerene/anthracene cluster cations wererecorded in the mass spectra (Fig. 3 in Ref. Zhen et al. (2019c)). Compared to those results, more smallerfullerene cations (C (70 − m )+ , m=2-11), their mono-anthracene adducts ((C H )C (70 − m )+ , m=2-11)and bis-anthracene adducts ((C H ) C (70 − m )+ , m=2-9) were newly observed in current mass spectra(Fig. 3). Note that the laser wavelength (355 nm) used in these two studies is same. The newly observedsmaller fullerene cations and their fullerene/anthracene cluster cations can be attributed to the higherenergy (30 mJ/pulse) and longer irradiation time (1.6 s ) used in present experiment. ormation of fullerene/anthracene cluster cations 7
576 672 768 864 960 1056 1152 12480.00.51.01.52.02.53.0 m/z I n t e n s it y ( a r b . un it s . ) With laser, 355 nm, 30 mJDifferential spectrum C n+ (C H )C n+ (C H ) C n+ Fig. 4: The zoom-in mass spectrum of fullerene (C )/anthracene cluster cations with laser irradiationand the differential spectrum in the range of m/z=552 − /anthracene system, the generated smaller fullerene cationsand their anthracene adducts all contain even numbers of carbon atoms. It is suggested that the succes-sive C loss is dominated the evolution process. After absorption of UV photons, fullerene/anthracenecluster cations ((C H ) m C / , m=1,2) dissociated into fullerene cations and anthracene (path 5).Then, the fullerene cations (including the free ones) underwent photo-fragmentation process (successiveC loss) and produced serials of smaller fullerene cations (path 6). Subsequently, these smaller fullerenecations reacted with anthracene molecules to form fullerene/anthracene cluster cations again (path 7 and8). The photochemical pathways for the C / /anthracene cluster cations are summarized as below: [(C H ) (1 − C / ] + h ν −→ (C H ) (1 − + [C / ] + (5) [C / ] + h ν −→ mC + [C (70 − ] + (6) [C (70 − ] + + C H −→ [(C H )C (70 − ] + (7) [(C H )C (70 − ] + + C H −→ [(C H ) C (70 − ] + (8)The relative reactivity of fullerene cations can be derived from the intensity variations of fullerenecations and their corresponding fullerene/anthracene cluster cations in the recorded mass spectra. Toillustrate this, Fig. 5(A) depicts the intensity ratios for (anthracene)fullerene cluster cations to fullerenecations recorded in the C /anthracene system and C /anthracene system individually. Fig. 5(B) dis-plays the intensity ratios for (anthracene) fullerene cluster cations to (anthracene)fullerene clustercations recorded in the C /anthracene system and C /anthracene system individually. The peak in-tensity in the recorded mass spectra (Fig. 2 and 4) were used in the calculation. The intensity ratiocalculated in this way is a reflection of the proportion that fullerene cations or (anthracene)fullerenecluster cations have reacted with free anthracene molecules. In other words, the larger the intensity ra-tio is, the higher reactivity the corresponding fullerene cation has. As shown in Fig. 5(A), the intensityratios for C /anthracene system are in a similar range with a central value of ≈ /anthracene system are in a similar range with a central value of ≈ Zhang et al. 2020
42 44 46 48 50 52 54 56 58 60 62 64 66 68 700.00.20.40.60.81.01.2 ( C14H10 ) Cn+/ Cn+ in C60/C14H10 system ( C14H10 ) Cn+/ Cn+ in C70/C14H10 system I n t e n s it y R a ti o n (fullerene C-number) n (fullerene C-number)A
42 44 46 48 50 52 54 56 58 60 62 64 66 68 700.00.20.40.60.81.01.2 ( C14H10 ) Cn+/ ( C14H10 ) Cn+in C60/C14H10 system ( C14H10 ) Cn+/ ( C14H10 ) Cn+ in C70/C14H10 system B Fig. 5: Panel (A):the intensity ratio of formed (anthracene)fullerene cluster cations to fullerenecations in the irradiated spectrum: red line is for the C /anthracene system; blue line is for theC /anthracene system; Panel (B):the intensity ratio of formed (anthracene) fullerene cluster cationsto (anthracene)fullerene cations in the irradiated spectrum: red line is for the C /anthracene system;blue line is for the C /anthracene system.whose value is about 0.1. Similar trends are found in Fig. 5(B), the intensity ratio for (C H )C isof ≈ and (C H )C illustrates that they have lower reactivity in addition reaction withanthracene molecules compared to the other fullerene cations. These results confirm the conclusion thatC has a lower reactivity reported in Ref. Zhen et al. (2019c) where the comparison is limited toC , C and C ions.Despite it is not as apparent as that of C cation, there exists a slight variation of intensity ratio forother fullerene cations. As shown in Fig. 5(A), the intensity ratio for C appears as a local minimumin both C /anthracene and C /anthracene systems. Similar thing happens to C , C and probablyC . Considering that the mass spectra (Fig. 2 and 4) were recorded under an optimized and stablecondition and that 200 average times were taken for each spectrum, the experimental factors (such aslaser energy) that caused the intensity ratio variation should have been avoided. Moreover, the varia-tion trends of intensity ratios are observed to be consistent on both C /anthracene and C /anthracenesystems (Fig. 5(A)). If the slight variation is not caused by the experiment system errors, it indicatesthat, within the smaller fullerene cations (44-58 carbon atoms), C , C , C and probably C have relatively lower reactivity towards anthracene molecules. It should be mentioned that previousstudies (Manolopoulos et al., 1991; Zimmerman et al., 1991; Rohlfing et al., 1984) have shown that theC , C and C are more stable than their neighbors. The recent theoretical work (Candian et al.,2019) addressed the stability of different isomers of C , C and C . In their study, the stabil-ity is characterized in terms of the standard enthalpy of formation per CC bond, the HOMO − LUMOgap, and the energy required to eliminate a C fragment. These studies (Manolopoulos et al., 1991;Zimmerman et al., 1991; Rohlfing et al., 1984; Candian et al., 2019) may explain the slight variationobserved in present work. The formation and evolution of neutral and charge state fullerenes in astronomical environments are ofconsiderable interest, especially after the detection of C and C in the young planetary nebula Tc 1 viatheir IR emissions (Cami et al., 2010). To the best of our knowledge, it is still an open issue so far. In thiswork, we present the laboratory study of the formation and relative reactivity of fullerenes cations (C ormation of fullerene/anthracene cluster cations 9 to C ). It is found that the higher laser energy and longer irradiation time are key factors to producesmaller fullerene cations. The increments of laser energy and irradiation time essentially increase photonnumber density. After absorption of UV photons, the fullerene cations undergo successive C fragmentloss dissociation which results in formation of smaller fullerene cations. It indicates that under the strongUV radiation environment like photon-dominated regions, the fullerene cations are probably driven todissociate to smaller ones or even damage totally. That is to say the top-down evolution of fullerenecations is expected to be dominated in such region. However, if the fullerene molecules are on thesurface of dust grains, they may survive from the strong irradiation. The case in nebula Tc 1 may bea good example. By considering the temperature difference between the observed IR spectra and gas-phase environment in the nebula Tc 1, Cami et al. (2010) concluded that C and C are in directcontact with solid materials. The solid materials probably shield much UV radiation for the attachedfullerenes. As a contrast, the gaseous fullerenes are exposed in the strong radiation environment, andare probably driven to smaller ones or damage. This may be a possible explanation for that no gaseousC and C are observed in Tc 1.The second question addressed by this work is the relative reactivity of fullerene cations (C (60 − n )+ ,n=0-8). The anthracene molecule is used as the reactant. It is found that the smaller fullerene cationsshow significantly high reactivity when compared to C . In the harsh ISM environment, anthracenemolecules are not expected to survive. The most abundant interstellar PAHs are large condense ones(Ricca et al., 2012) which are highly reactive as anthracene. As is well known, the Diels–Alder ad-duction is one of the routine ways when fullerene react with PAHs. In view of this point, other PAHsare expected to react with smaller fullerene cations in an efficient way like anthracene molecules. Ifthe smaller fullerene and PAHs co-exist in the same astronomical region, they may react and formfullerene/PAHs adducts. These adducts may accumulate and contribute somewhat to the unidentified IRemission features. It is promising when noting the fact that C /anthracene adducts were shown to havestrikingly similar spectral features to those from C (and C ) fullerenes and other unidentified infraredemission features (Garc´ıa-Hern´andez et al., 2013). The formation and relative reactivity of fullerene cations (from smaller to larger, C to C ) arestudied in this work. It is found that the higher laser energy and longer irradiation time are key factorsto produce the smaller fullerene cations such as C (60 − n )+ , n = 1-8. When reacting with anthracenemolecules, slightly different reactivity may exist within the smaller fullerene cations, if the experimen-tal system errors are ruled out. Nevertheless, the smaller fullerene cations have significantly higherreactivity compared to C . A successive loss of C fragments mechanism is suggested to accountfor the formation of smaller fullerene cations, which then undergo addition reaction with anthracenemolecules. These results obtained here provide a growth route towards fullerene (from smaller to largerin size) derivatives based on PAH-related molecules in a bottom-up growth process and an insight fortheir photo-evolution behavior in the ISM. The results also suggest, when conditions are favorable,fullerene derivatives can form efficiently. ACKNOWLEDGEMENTS
This work is supported by the Fundamental Research Funds for the Central Universities, the NationalScience Foundation of China (NSFC, Grant No. 11743004). We thank the referee for the very construc-tive and detailed comments which help improve this work a lot.
References
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