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J. Phys. Chem. 199498, 1440-1443
C120+Isomers from Laser Ablation of Fullerene Films J. M. Hunter, J. L. Fye, N. M. Boivin, and M. F. Jarrold’ Department of Chemistry, Northwestern University, 21 45 Sheridan Road, Evanston, Illinois 60208 Received: April 14, 1994; In Final Form: June 8, 1994’
Laser ablation of fullerene films results in coalescence of fullerenes to form large carbon clusters. Mobility studies of size-selected Clzo+ formed by this method reveal the presence of a t least two abundant structural isomers. One of these isomers has a closed-shell fullerene structure (identical to that generated by the laser vaporization of graphite), and the other has a Cm “dimer” structure. When these clusters are collisionally annealed, around 40% of the “dimer” dissociates to give C60+ Cm and around 30% appears to anneal into the closed-shell fullerene. The balance remains unchanged up to relatively high injection energies, indicating the formation of some strongly bound “dimers”. The activation energies for the annealing and dissociation processes are low, approximately 1.5 eV.
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Several gas-phase experiments have shown that C a molecules can coalesce into larger carbon clusters. Mass spectra of C,+ clusters produced by laser desorption of Cm from fullerene films exhibit intensity maxima that occur near integer multiples of 60 atoms.lJ Based on cluster-surface collision studies in which the coalescence products (such as cll8+,CIZO+,and c194+) are unusually stable toward dissociation, it has been proposed that these large C.+ haveclosedcage fullerene struct~res.l-~ Yeretzian and co-workers have suggested that the distribution of large C,+ can be accounted for by a growth mechanism that involves coalescence of C a molecules, dissociation by loss of C2, and recapture of C2 speciesa4Laser desorption mass spectra of UVirradiated C a films show evidence for extensive aggregation to form (C&+ (n up to 2O).5 These results have been interpreted in terms of photochemically-generated cross-linked polymers consisting of covalently-bound intact fullerene C a molecule^.^^ Large (Ca),,+ clusters containing greater than 150 C a molecules bound by van der Waals interactions have been formed by inert gas condensation.10 In addition, under single-collision conditions, fusion of C6o+ and C a has been demonstrated to occur at centerof-mass energies of less than 200 eV.11 However, there currently exists no direct experimental information about the structure of coalesced carbon clusters, and the mechanism of laser-induced fullerene coalescence is not known. The structures and energeticsof several Cl20isomers have been investigated by a variety of quantum chemical methods. At the Hartree-Fock self-consistent-fieldlevel, the lowest energy closedcage fullerene isomer has Td symmetry, while isomers with DM and DSd symmetry lie slightly (2-4 eV) higher in energy.12 Nonfullerene geometries of Cl20such as c 6 0 “dimers” have also been studied, and both semiempirical (MNDO)and density functional theory calculations suggest that addition reactions of C a fullerenes should yield stable bound dimers.l3J4 According to these theoretical studies, the photochemically allowed [2 21 cycloaddition of two Ca molecules is only slightly endothermic and the [2 + 21 cycloaddition product is the lowest energy dimer isomer. In this Letter we report injected ion drift tube studies of the C120+isomers generat4 by laser ablation of fullerene films. A detailed description of the injected ion drift tube apparatus has been given previously.I”l* Fullerenes were produced by the contact arc method, and the resulting soot was extracted with t01uene.l~ The fullerene extract was then dissolved in CS2 and deposited onto a copper rod. Thin fullerene films were ablated in a laser vaporization source operating with a continuous flow of He carrier gas. The output of the XeCl excimer laser (308
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*Abstract published in Advance ACS Abstracts. July 15, 1994.
0022-3654/94/2098-1440%04.50/0
nm) used for ablation was attenuated to optimizethe Cl20+signal. Under these conditions, the laser fluence was estimated to be 1 J/cm2, roughly 3 orders of magnitude above that required for direct desorption of C a without coalescence. Because of the low abundance of cluster ions produced directly by laser ablation, neutral clusters were ionized by an electron beam before the expansion region of the source. After quadrupole mass selection, short pulses of ions were injected into a drift tube containing approximately 5 Torr of He buffer gas. The arrival time of the ions at the detector is then recorded using a multichannel scaler. The drift time distribution is obtained by subtracting the flight time of the ions through the rest of the instrument (drift tube to detector) from the time scale of the arrival time distribution. It provides information about the geometries and relative abundances of the isomers that are present. von Helden et al. were the first to use mobility measurements to probe the geometries of carbon cluster ions.20 A second quadrupole was used to collect the mass spectrum of ions exiting the drift tube. When a low laser fluencewas used to desorb the fullerene film, the mass spectra reflect the relative abundance of the starting material on the rod. Under these conditions, the carbon cluster cation abundance distribution was similar to that observed previously for laser-desorbed toluene-extracted soot, containing predominantly c60+ and C70+ in addition to smaller amounts of larger fullerenes such as c76+, C84+,and c90+.21922 The drift time distributions of C a + and C,o+ generated under these conditions exhibit only one peak, that corresponding to the fullerene i~omer.~3-2~ With increasing laser fluence, ions resulting from coalescence appear. When conditions were optimized to produce the maximum Cl20+ abundance, the intensity of the Cl20+peak was typically 3 orders of magnitude less than that of Ca+. A broad distribution of carbon clusters was generated under these conditions, and clusters having an odd number of atoms were also observed,with an intensity of about an order of magnitude less than those containing an even number of atoms. In Figure 1 the drift time distribution of Cl20+generated by laser vaporizationof graphite is compared to that of C120+formed by laser ablation of a fullerene film. The broad peak at long times (-2500 ps) in Figure l a is due to polycyclic ring isomers.26 These isomers are not present in the drift time distribution of CI20+ generated from fullerene films. The sharp peak at approximately 1215 ps is attributed to a closed-cage fullerene isomer and has the same mobility in both cases. The second sharppeak,at 1430ps,in thedistributionof Figure Ibisattributed to an isomer with a C a “dimer” structure. Assuming that this “dimer” consists of two intact C a molecules linked together, the intermolecular distance between the C a molecules (estimated 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 7441
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Time, microseconds Figure 1. Drift time distributions collected at an injection energy of 50 eV for C120+generated by laser vaporization of (a) graphite and (b) fullerene films. The vertical scale for the 2000-3500-ps region has been expanded to show the polycyclic ring isomers produced in the laser vaporization of graphite.
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(or both) of these structures. The tubelike Dw isomer has a slightly longer drift time than the more spheroidal Td and Du structures. The identity of the Cm "dimer" peak was also investigated by calculating the drift times of various possible minimum-energy (MNDO-optimized) geometries of Cm Cm cycloaddition products as well as several "peanut" or "dumbbell" structures.~3J4 The "peanut" isomers, first proposed by Strout et a].," consist of two Cm fullerenes which have fused into a structure that has a neck containing seven- or eight-membered rings. The 5-6 stick isomer arises from a C2 linkage between Cm molecules. As shown in Figure 2, the calculated drift times of the "dimer" structures lie close to the measured drift time of the non-fullerene isomer. The "dimer" peakcorresponds most closely (within 2%) to either the [2 + 21 cycloaddition product or the 5-6 stick structure, whereas the mobilities of the "peanut" structures and the thermally allowed [2 41 cycloadditionproduct fall in between the two peaks. Drift time distributionswere also recorded for some other cluster sizes(C,+,n= 116,118,and 119). Fortheeven-numberedclusters these distributions were qualitatively similar to those for Cl20+, although the amount of "dimer" present was around 50% smaller than for ClzO+.For C119+ only the "dimer" isomer was present in the drift time distribution. To examinethe annealing behavior of the Clm+"dimer" isomer, drift time distributions were measured as a function of the energy with which the clusters are injected into the drift tube. As the injection energy is increased, the cluster ions undergo a rapid transient heating and cooling cycle as they enter the drift tube. This process can result in isomerization or fragmentation of the clusters. Figure 3 shows drift time distributions recorded for C120+atseveraldifferentinjectionenergies. At the lowest injection energy the relative abundance of the "dimer" isomer roughly equals that of the fullerene. As the injection energy is raised, the relative abundance of the "dimer" decreases, and the relative abundance of the fullerene increases. In addition to these annealing processes, some of the "dimer" dissociates. The only product observed was Cm+, which presumably results from dissociation of the "dimer" into Cm+ Cm. No products resulting from C2 loss were observed at injection energies of up to 400 eV. Figure 4 shows the relative abundances of the fullerene isomer, the "dimer" isomer, and other isomers (isomers with drift times greater than 1500 ps) plotted against the injection energy. The fraction of clusters which survive (do not dissociate) is also plotted in this figure. Around 30% of the "dimer" isomer appears to
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Time, microseconds Figure 2. Drift time distribution for C120+compared to calculated drift times of closed-shellfullerenes,peanuts, and dimer structures. The lines denote the calculated drift times for the structures with the symmetry indicated. The drift times were calculated from the atomic coordinates of MNDO-optimized geometries (see text). from the mobility) is approximately 9.8 A. The amount of C2m+ present under typical experimental conditionswas negligible, and therefore the possibility that C2a2+ isomers contribute to the distribution can be eliminated. The relative abundances of the two components present in the drift time distribution for Cl20+ generated by laser ablation of the fullerene film was slightly sensitive to the laser fluence and the length of time that the film was irradiated. The drift times of three closed-cage "fullerene" CIZO isomers were calculated from MNDO-optimized atomic coordinates (provided by Scuseria and co-workers)-12 A simple hard-sphere collision model was employed in these calculations,27details of which are given elsewhere.28 The sum of the hard-sphere radii of He and C, rHc rc, used in these calculations was adjusted to 3.2 A in order to fit the experimentally determined drift time of cl20+to that calculated for the isomer with Td symmetry. Figure 2 shows how the calculated drift times of these proposed structures correlate to the measured drift time distribution. Our experimentcannot resolve the Td from the Du fullerenegeometry, and thus the "fullerene" peak could possibly correspond to either
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Time, microseconds Figure 3. Drift time distributions of Clm+recorded at injection energies of 50, 100, 150, and 225 eV.
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1442 The Journal of Physical Chemistry, Vol. 98, No. 31, 1994
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260 300 400 500 Injection Energy, eV Figure 4. Plot of the relative abundance of C120+isomers as a function of injection energy. The symbols are the experimentaldata, and the lines
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are the results of simulations to estimate activation energies for the annealing and dissociation processes. anneal into the fullerene, while approximately 40%dissociates. Because there is a relatively large difference between the mass of the parent and product in the dissociationof Cl20+to give Ca+, mass discrimination could influence the measurement of the amount of the “dimer”that dissociates. If the detection efficiency for Ca+ is smaller than for C120+,the fraction of the dimer that dissociates will be underestimated (and the fraction that anneals will be overestimated). It is difficult to directly probe the mass discrimination in these experiments, but there has been no indication of significant discrimination in our other studies. As can be seen from the results presented in Figure 4, the injection energy thresholds for dissociation and annealing of the “dimer” are relatively sharp, and these two processes appear to occur at approximately the same injection energy. The injection energy thresholds can be analyzed to obtain an estimate of the activation energies associated with these processes. The methods employed have been described in detail previo~sly.Is~2~ The lines shown in Figure 4 are the result of simulations to estimate the activation energies. From these simulations theactivation energies were estimated to be approximately 1.4 eV for annealing and 1.5 eV for dissociation. (The uncertainty in the estimated activation energies is f 0 . 2 eV and so the difference is not significant.) It is apparent from the results presented in Figures 2 and 3 that not all of the C120+ “dimer“ anneals or dissociates. At injection energies above 225 eV there is no significant change in the amount of dissociation or annealing, but a substantial fraction of the “dimer”, around 30% remains and persists to relatively high injection energies. At the higher injection energies the feature due to the “dimer” broadens, and a tail develops at longer drift times. Clearly, those “dimer” isomers which persist to the higher injection energies must have geometries that do not easily anneal into the fullerene or dissociate. The relatively low dissociation energy for the “dimer”, 1.5 eV, and the fact that the products are C a + C a suggest that at least some of the ”dimer” consists of two loosely bound Ca units. One obvious possibility is an ion-induced dipole complex ( C ~ . - C ~ + ) . Using the measured polarizability30 of C a , the ion-induced dipole interaction at a center-to-center distance of 9.8 A (as determined from the mobility of the “dimer”) is 0.8 eV. This can be used as a rough estimate of the binding energy of the ion-induced dipole complex. Including repulsive interactions will decrease this estimate, and including resonance stabilization (delocalization of the charge) will increase it. However, it seems likely that the ion-induced dipole interaction alone is insufficient to account for thestabilityof the“dimer”. ExposureofCafiimstoUVradiation has been shown to result in C a polymers6 It has been suggested that the C a molecules in these polymers are linked by [2 + 21 cycloaddition Furthermore, recent studies of the
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thermal decomposition of polymerized films indicate that the activation energy for thermal decomposition of (Ca),, to (CW),,-~ is 1.25 eV.31 Thus, it seems likely that the “dimer” isomers that dissociate consist of two C a units linked by [2 + 21 cycloaddition, although ion-induced dipole interactions probably contribute to the stability of this “dimer”. A mechanism for annealingof the [2 21 cycloadditionproduct through a “peanut” intermediateto the fullerene has been proposed by Scuseria et al.32 Although the calculated relative stabilities indicate that conversion of the dimer to the fullerene is energetically favorable, the activation energy associated with this structural transformation has not been calculated. Annealing of the dimer to the fullerene geometry is estimated to be exothermic by around 20 eV, and the dissociation energy for loss of C2 from C120 fullerene has been estimated to be 8 eV. However, no C2 loss products are expected to result from the annealing process. This is because loss of C2 (if it occurred) would arise from a statistical unimolecular reaction. The exothermicity of the annealing process will be distributed over the 120 atom fullerene, and it will therefore be insufficient to drive dissociation before the clusters are cooled by collisions with the buffer gas. So it appears plausible that the “dimer” isomers that anneal into an intact fullerene could have the [ 2 21 geometry and that annealing of this isomer competes with dissociation. However, we cannot rule out the possibility that the “dimer” isomers that anneal have a different “dimer” geometry than those that dissociate. A significant fraction of the “dimer” isomers persist to injection energies well above the thresholds for annealing and dissociation. These strongly bound “dimers” clearly must have geometries that differ from those that anneal into the fullerene or dissociate. In summary, mobility measurements have been used to probe the geometries and annealing of Cl20+ ions generated by laser desorption from fullerene films. Two abundant isomers are observed in the drift time distributions: the closed-shellfullerene and a “dimer”. When collisionally excited, some of the “dimer” dissociates, and some appears to anneal into the fullerene. The activation energies for these processes are low, and both processes could result from a dimer generated by a [2 + 21 cycloaddition reaction. A substantial fraction of the dimer does not anneal or dissociate, indicating that another, more stable “dimer” isomer exists which does not readily convert into the fullerene. These results indicate that coalescence of fullerenes can lead to the formation of a stable covalently linked C a “dimer” as well as large closed-cage fullerenes.
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Acknowledgment. We gratefully acknowledge the support of this work by the National Science Foundation (Grant CHE9306900). We are also grateful to Prof. G. Scuseria and his collaborators for providing atomic coordinates for the fullerene, “dimer”, and “peanut” isomers of C120. References and Notes (1) Beck, R. D.; Weis, P.;Briuchle, G.; Kappes, M.M.J . Chem. Phys. 1994,100, 262. ( 2 ) Yeretzian, C.; Hansen, K.; Dicdcrich, F.; Whetten, R. L. Nature 1992.359.44. (3) Yeretzian, C.; Hansen, K.;Diederich, F.; Whetten, R. L. Z . Phys. D 1993, 26, S300. (4) Hansen, K.;Yeretzian. C.; Whetten, R. L. Chem. Phys. Lett. 1994, 218. 462. (5) Cornett, D. S.;Amster, I. J.; Duncan, M.A.; Rao, A. M.;Eklund, P. C. J. Phys. Chem. 1993,97. 5036. (6) Rao, A. M.;Zhou, P.; Wang, K.-A.; Hager, G. T.; Holden, J. M.;
Wang, Y.; Lee, W.-T.; Bi, X.-X.; Eklund, P.C.; Cornett, D. S.;Duncan, M. A.; Amster, I. J. Science 1993, 259, 955. (7) Zhou, P.;Dong, 2.-H.; Rao, A. M.;Eklund, P.C. Chem. Phys. Left.
1993, 211, 337. (8) Wang,Y.;Holden,J.M.;Dong,Z.-H;Bi,X.-X.;Eklund,P.C.Chem. Phvs. Lett. 1993. 211. 341. (9) Akselrod, L.; Byrne, H. J.; Thomsen, C.; Roth, S.Chem. Phys. Lett. 1993, 215, 131.
Letters (10) Martin, T. P.;Niher, U.; Schaber, H.; Zimmerman, U. Phys. Rev. Lett. 1993, 70, 3079. (11) Campbell, E. E. B.; Schyja, V.; Ehlich, R.; Hertel, I. V. Phys. Reu. Lett. 1993, 70, 263. (12) Murry, R. L.; Colt, J. R.; Scuseria, G. E. J . Phys. Chem. 1993, 97, 4954. (13) Strout, D. L.; Murry, R. L.; Xu,C.; Ekkhoff, W. C.; mom, G. K.; Scuseria, G. E. Chem. Phys. Lett. 1993, 214, 576. (14) Matsuzawa, N.; Masafumi, A.; Dixon, D. A.; Fitzgerald, G. J . Phys. Chem. 1994, 98, 2555. (15) Jarrold, M. F.; Honea, E. C. J . Phys. Chem. 1991, 95, 9181. (16) Jarrold, M. F.; Constant, V. A. Phys. Reu. Lett. 1991, 67, 2994. (17) Jarrold, M. F.; Honea, E. C J . Am. Chem. Soc. 1992, 114, 459. (18) Jarrold, M. F.; Bower, J. E. J . Chem. Phys. 1992, 96, 9180. (19) Kritschmer, W.; Lamb, L. D.; Fostiroupolos, K.; Huffman, D. R. Nature 1990, 347, 354. (20) von Helden, G.; Hsu, M.-T.; Kemper, P. R.; Bowers, M. T. J . Chem. Phys. 1991, 95, 3835. (21) Creasy, W. R.; Zimmerman, J. A.; Ruoff, R. S . J . Phys. Chem. 1993, 97, 973.
The Journal of Physical Chemistry, Vol. 98, No. 31, 1994 7443 (22) Ulmer, G.; Campbell, E. E. B.; Kllhnle, R.; Busmann, H.-G.; Hertel, I. V. Chem. Phys. Lett. 1991,182, 114. (23) von Helden, G.; Hsu, M.-T.; Gotts, N.; Bowers, M.T. J . Phys. Chem. 1993, 97, 8182. (24) Hunter, J. M.; Fye, J. L.; Jarrold, M. F. Science 1993, 260, 784. (25) Hunter, J. M.; Fye, J. L.; Jarrold, M. F. J. Phys. Chem. 1993, 97, 3460. (26) Hunter, J. M.; Fye, J. L.; Jarrold, M. F. To be published. (27) Mason, E. A.; McDaniel, E. W. Transport Properties of Ions in Gases; Wiley: New York, 1988. (28) Jarrold, M. F.; Bower, J. E. J. Chem. Phys. 1993, 98, 2399. (29) Hunter, J. M.; Fye, J. L.; Roskamp, E.J.; Jarrold, M. F. J . Phys. Chem. 1994, 98, 1810. (30) Maltsev, V. A.; Nerushev, 0. A,; Novopashin, S. A,; Selivanov, B. A. Chem. Phys. Lett. 1993, 212,480. (31) Wang, Y.; Holden, J. M.; Bi, X.-X.; Eklund, P.C. Chem. Phys. Lett. 1994, 217, 413. (32) Scuseria, G. Private communication.
