Fragmentation of Fullerenes to Linear Carbon Chains
aa r X i v : . [ phy s i c s . a t m - c l u s ] S e p Draft version September 4, 2017
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FRAGMENTATION OF FULLERENES TO LINEAR CARBON CHAINS.
Dmitry V. Strelnikov, Manuel Link, and Manfred M. Kappes Karlsruhe Institute of Technology (KIT), Division of Physical Chemistry of Microscopic Systems, Karlsruhe, Germany
ABSTRACTSmall cationic fullerene fragments, produced by electron impact ionization of C , were mass-selected and accumulated in cryogenic Ne matrixes. Optical absorption spectroscopy of these frag-ments with up to 18 carbon atoms revealed linear structures. Considering the recent discovery offullerenes in Space and the very strong absorptions of long linear carbon clusters both in the UV-Visand IR spectral regions, these systems are good candidates to be observed in Space. We present lab-oratory data, supported by quantum-chemical calculations and discuss the relevance of long carbonchains for astronomy. Keywords: methods: laboratory: molecular — ISM: lines and bands — ISM: molecules
Corresponding author: Dmitry V. [email protected]
Strelnikov et al. INTRODUCTIONAfter the recent discovery of C , C and C +60 in Space (Cami et al. 2010; Garc´ıa-Hern´andez et al.2010; Sellgren et al. 2010; Otsuka et al. 2013) and in particular after the unequivocal attribution ofseveral of the diffuse interstellar bands (DIBs) to NIR absorption bands of C +60 (Campbell et al. 2015;Walker et al. 2015), fullerenes have become a hot topic in the astronomical community. Researchersare now also beginning to consider other fullerene-derived or fullerene-related species, which couldbe present and observable in Space (Omont 2016). One can divide these species into four classes:(i) reaction products of fullerenes with abundant atoms, ions and molecules in Space, (ii) fullerenefragmentation products, (iii) precursors of fullerenes, and (iv) species, which could be formed togetherwith fullerenes. We have decided to explore the second class of species, concentrating in particularon fullerene fragments expected to be strongly light absorbing. Fullerenes are known to fragmentpredominantly via a sequential C -loss cascade, leading to C , C , C and smaller fullerenes downto C (Bekkerman et al. 2006). Nevertheless, it was already established in the early days of fullerenemass spectrometry that given enough excitation energy, fullerene precusors could also fragment intomuch smaller carbon cluster species (V¨olpel et al. 1993).In the present work we investigate these smaller carbon clusters (C + n , n < . Clustersin this size range have been the object of studies for many years – both from the point of viewof cluster science and also from the vantage of astronomy and diffuse interstellar bands. However,the species in question have typically been generated using “bottom-up” approaches, e.g, by ag-gregation of atomic (and small molecule) carbon vapour. The corresponding spectroscopic studiesimply that for all accessible charge states (+/0/-) and for many nuclearities, ring and chain isomerscan be formed – sometimes simultaneously. Nevertheless, spectral coverage is often incomplete andassignment (to specific structures and sizes) sometimes questionable. On the basis of ion mobilityspectrometry, it appears that carbon cluster aggregation growth regimes can become kinetically con-strained – thus giving rise to isomer distributions which are far from thermodynamic equilibriumand which vary strongly with source conditions (Fromherz et al. (2002) – C − − chains and rings; ragmentation of Fullerenes to Linear Carbon Chains. +7 − rings and chains). The mass spectra obtained upon multifragment-ing fullerenes down to small cluster sizes show abundance maxima and minima which are similarto those obtained in aggregation growth (V¨olpel et al. 1993; Cheng et al. 1996; Hunsche et al. 1996;Rohlfing et al. 1984). However, until now it has not been known which molecular structures areformed and how these structures relate to those obtained by aggregation growth. Our measurementsdescribed below indicate for the first time that the molecular structures obtained by electron-impactmultifragmentation of C include carbon chains. The state of knowledge concerning the structuresand spectroscopy of small carbon molecules C n (smaller than fullerenes) was thoroughly reviewed in1998 (Van Orden & Saykally 1998). Astronomical relevance of carbon clusters was recently reviewed(Zack & Maier 2014; Campbell & Maier 2017). Small carbon chain molecules such as C , C andC have already been unequivocally observed in circumstellar environments (Hrivnak & Kwok 1991;Hinkle et al. 1988; Bernath et al. 1989). C and C were also detected further away from stars in thediffuse clouds (Souza & Lutz 1977; Maier et al. 2001). Furthermore, linear cyanopolyynes as longas HC N (Broten et al. 1978) have been detected by radio astronomy. The idea that carbon chainmolecules may be responsible for the DIBs was initially proposed by Douglas (1977) and has beenhotly debated over the years. Our new findings provide additional support for carbon chain moleculesin Space. EXPERIMENTAL DETAILSOur experimental setup is designed to study mass-selected, ionic species, soft-landed and trappedin cryogenic matrixes (Kern et al. 2013). The apparatus allows investigation of such matrix isolatedspecies by optical absorption spectroscopy, Raman spectroscopy and Laser-Induced-Fluorescence(LIF) measurements. One of the unique features of the setup is the possibility to cover a very broadspectral range from UV to far-IR without changing the sample, which is achieved by using diamondoptics (Strelnikov et al. 2017). In the current study cationic fullerene fragments, produced by anelectron-impact (EI) ionization source and also corresponding neutral and anionic species, formedupon charge changing during deposition into the matrix, were characterized by one-photon absorptionspectroscopy.
Strelnikov et al. (cid:1)(cid:2) (cid:3)(cid:2)(cid:2) (cid:3)(cid:1)(cid:2) (cid:4)(cid:2)(cid:2) (cid:4)(cid:1)(cid:2) (cid:5)(cid:2)(cid:2) (cid:6)(cid:7)(cid:8) (cid:2)(cid:2) (cid:9) (cid:4)(cid:2) (cid:9) (cid:10)(cid:2) (cid:9) (cid:11)(cid:2) (cid:9) (cid:12)(cid:3) (cid:9) (cid:2)(cid:3) (cid:9) (cid:4)(cid:3) (cid:9) (cid:10) (cid:13) (cid:14)(cid:15) (cid:16) (cid:17) (cid:18) (cid:19)(cid:19) (cid:20)(cid:15) (cid:21)(cid:22)(cid:16) (cid:15) (cid:23) (cid:1) (cid:2)(cid:2)(cid:3) (cid:1) (cid:2)(cid:4)(cid:3) (cid:1) (cid:2)(cid:5)(cid:3) (cid:1) (cid:2)(cid:6)(cid:3) (cid:1) (cid:2)(cid:7)(cid:3) (cid:1) (cid:8)(cid:3) (cid:1) (cid:4)(cid:3) (cid:1) (cid:9)(cid:7)(cid:6)(cid:3) (cid:1) (cid:9)(cid:7)(cid:10)(cid:3) (cid:1) (cid:6)(cid:11)(cid:5)(cid:3) (cid:1) (cid:2)(cid:11)(cid:3) (cid:1) (cid:6)(cid:9)(cid:5)(cid:3) (cid:1) (cid:5)(cid:2)(cid:3) (cid:1) (cid:6)(cid:5)(cid:5)(cid:3) (cid:1) (cid:4)(cid:6)(cid:10)(cid:3)
Figure 1.
Typical cationic mass spectrum of fullerene fragmentation and ionization products by electronsat 250 eV. Note, that the mass resolution of our experimental setup (optimized for high ion throughput)does not allow differentiation of the isotopomers of C +15 and C . Ion current for m/z = 240 is 4.5 nA.
In the preliminary experiments reported here, we deposited mass selected C +15 and C +18 (together withthe corresponding fullerene multications, C and C , respectively), into neon matrixes. Despitethe fact that the ion currents for these species were quite low (0.3–1 nA), it turns out that UV-Visand IR spectra can be measured after a few hours of deposition. This means that correspondingabsorption cross sections must be very large. RESULTSFig. 1 shows a partial cation mass spectrum of fragment ions obtained upon 250 eV electron im-pact ionization of sublimed C . The spectrum resembles previously published mass spectra of carbonclusters in that it manifests local ion abundance maxima at m/z 132 and 180 amu (Cheng et al. 1996;Hunsche et al. 1996; V¨olpel et al. 1993; Rohlfing et al. 1984). These mass peaks are thought to corre-spond primarily to C +11 and C +15 respectively. However, the underlying electron impact fragmentationcascade proceeds in part via multiply charged fullerene cages. Therefore, some of the signal inparticular at m/z=180 is contributed to by multiply charged fullerene cages, e.g. C .3.1. Electronic Spectroscopy ragmentation of Fullerenes to Linear Carbon Chains. (cid:1)(cid:2)(cid:2) (cid:3)(cid:2)(cid:2) (cid:4)(cid:2)(cid:2) (cid:5)(cid:2)(cid:2) (cid:6)(cid:2)(cid:2)(cid:2) (cid:7)(cid:8)(cid:9)(cid:10)(cid:11)(cid:10)(cid:12)(cid:13)(cid:14)(cid:15)(cid:16)(cid:17)(cid:16)(cid:12)(cid:18) (cid:2) (cid:19) (cid:2)(cid:2)(cid:2) (cid:19) (cid:2)(cid:1)(cid:2) (cid:19) (cid:2)(cid:3)(cid:2) (cid:19) (cid:2)(cid:4) (cid:20) (cid:21) (cid:22) (cid:23) (cid:24) (cid:21)(cid:8)(cid:12) (cid:25) (cid:10) (cid:26) (cid:6)(cid:27) (cid:26) (cid:6)(cid:27) (cid:26) (cid:4)(cid:2)(cid:28) (cid:27)(cid:2)(cid:2)(cid:2)(cid:4)(cid:2)(cid:2)(cid:2)(cid:29)(cid:2)(cid:2)(cid:2)(cid:5)(cid:2)(cid:2)(cid:2)(cid:30)(cid:2)(cid:2)(cid:2)(cid:6)(cid:2)(cid:2)(cid:2)(cid:2) (cid:7)(cid:8)(cid:9)(cid:10)(cid:12)(cid:31)(cid:18)(cid:21)(cid:10)(cid:24)(cid:16)(cid:17)(cid:16)(cid:25)(cid:18) (cid:6) (cid:2) (cid:19) (cid:2)(cid:2)(cid:2)(cid:2) (cid:19) (cid:2)(cid:2)(cid:3)(cid:2) (cid:19) (cid:2)(cid:2)(cid:5) (cid:26) (cid:6)(cid:27) (cid:26) (cid:4)(cid:2) ! (cid:1) " (cid:8) (cid:21) (cid:11) Figure 2. a:
UV-NIR absorption spectrum of C in Ne at 5K, obtained after deposition of ca. 1000 nAminof C +15 /C at about 100 eV kinetic energy. Bands between 200 and 350 nm may be caused partly by opticalinterference. b: NIR absorption spectrum of the same sample, obtained with an FTIR spectrometer. C +15 (+ C ) deposition at an average kinetic energy of about 100 eV resulted in relatively cleanabsorption spectra (Fig. 2). From previous measurements of the Maier group (in which the clusterswere formed in a Cs-sputtering ion source and mass-selected or – alterntively – produced by laserablation of graphite without mass-selection), it is clear that the absorptions observed in the UV-Visand NIR ranges can be assigned to linear isomers of C and C − (Forney et al. 1996; Wyss et al.1999). At this point we cannot yet say where the absorptions of C +15 are, as this requires additionalmeasurements in matrixes doped by electron scavengers (to change the C + / / − charge distribution).However, TDDFT calculations predict the strongest absorption of C +15 to be in the vicinity of theabsorption of neutral C , see 3.2.C +18 deposition resulted in the spectra presented in Fig. 3. Only NIR absorptions of linear C − in Newere previously reported – in experiments without mass-selection, using an extrapolation procedurebased on measurements of shorter chains (Freivogel et al. 1995). The UV-Vis spectrum of mass-selected C is presented here for the first time.Upon going to higher deposition energies of about 200 eV, we observe (further) fragmentation ofthe mass-selected ions upon deposition (Fig. 4). Again the absorptions of these fragments can be Strelnikov et al. (cid:1)(cid:2)(cid:2) (cid:3)(cid:2)(cid:2) (cid:4)(cid:2)(cid:2) (cid:5)(cid:2)(cid:2) (cid:6)(cid:2)(cid:2) (cid:7)(cid:2)(cid:2)(cid:8)(cid:9)(cid:10)(cid:11)(cid:12)(cid:11)(cid:13)(cid:14)(cid:15)(cid:16)(cid:17)(cid:18)(cid:17)(cid:13)(cid:19) (cid:2) (cid:20) (cid:2)(cid:2)(cid:2) (cid:20) (cid:2)(cid:3)(cid:2) (cid:20) (cid:2)(cid:7) (cid:21) (cid:22) (cid:23) (cid:24) (cid:25) (cid:22)(cid:9)(cid:13) (cid:26) (cid:11) (cid:17) (cid:27) (cid:13) (cid:28) (cid:15) (cid:23) (cid:29)(cid:13)(cid:15)(cid:11)(cid:25)(cid:30)(cid:11)(cid:25)(cid:11)(cid:13)(cid:26)(cid:11) (cid:31) !(cid:31)"(cid:12)(cid:28)(cid:13)(cid:11)(cid:9)(cid:25)(cid:17) $(cid:7) (cid:12)(cid:28)(cid:13)(cid:11)(cid:9)(cid:25)(cid:17) $(cid:7)% (cid:12)(cid:28)(cid:13)(cid:11)(cid:9)(cid:25)(cid:17) $(cid:7)& $(cid:4)(cid:17) &’(cid:11)((cid:24)(cid:23)(cid:28)(cid:15)(cid:28)(cid:24)(cid:13) ) ) $(cid:7)(cid:2)(cid:18)%(cid:18)& (cid:9) (cid:22) (cid:1)(cid:2)(cid:3)(cid:3)(cid:2)(cid:3)(cid:3)(cid:3)(cid:2)(cid:2)(cid:3)(cid:3)(cid:4)(cid:3)(cid:3)(cid:3)(cid:4)(cid:2)(cid:3)(cid:3)(cid:5)(cid:3)(cid:3)(cid:3) (cid:6)(cid:7)(cid:8)(cid:9)(cid:10)(cid:11)(cid:12)(cid:13)(cid:9)(cid:14)(cid:15)(cid:16)(cid:15)(cid:17)(cid:12) (cid:18)(cid:19) (cid:3) (cid:20) (cid:3)(cid:3)(cid:19)(cid:3)(cid:3) (cid:20) (cid:3)(cid:3)(cid:21)(cid:3)(cid:3) (cid:20) (cid:3)(cid:3)(cid:22)(cid:3) (cid:3)(cid:7)(cid:1)(cid:1)(cid:3)(cid:7)**(cid:4)(cid:3)(cid:6)+(cid:4)(cid:1)*(cid:7)(cid:4)(cid:1)(cid:4)*(cid:20)+ $(cid:7) & ) , + - Figure 3. a:
UV-NIR absorption spectrum of C in Ne at 5K (green), obtained after deposition ofca. 1800 nAmin of C +18 /C at about 120 eV kinetic energy. Bands between 200 and 350 nm are causedby interferences. C absorption (gray) is shown as a reference. TDDFT calculations are shown withoutwavelength scaling. b: NIR absorption spectrum of the same sample, obtained with an FTIR spectrometer. assigned to (smaller) linear carbon chains. We base this assignment on previous data derived frommass-selected deposition of carbon clusters into Ne matrixes (Forney et al. 1996; Grutter et al. 1999;Wyss et al. 1999). We find that in our experiments the main fragmentation channel is loss of C -units. Note, that C , C and C were previously misassigned as rings (Grutter et al. 1999). Thiscan be concluded from the fragmentation pattern: linear C should also produce linear C (Fig.4).Similarly, upon depositing C +16 we observe enhanced absorption of linear C (C -loss) as well asabsorption of linear C − (Freivogel et al. 1995).3.2. Quantum Chemical Calculations
Although, small carbon clusters have been treated theoretically many times (Van Orden & Saykally(1998); Guo et al. (2012) and references therein), we present own TDDFT calculations for chain andring isomers here with an emphasis on the wavelength ranges containing the strongest electronicabsorptions. Our results are generally consistent with previous calculations at the same level oftheory where available (for the spectral range and cluster sizes of interest here). For all charge ragmentation of Fullerenes to Linear Carbon Chains. (cid:1)(cid:2)(cid:2) (cid:3)(cid:2)(cid:2) (cid:4)(cid:2)(cid:2) (cid:5)(cid:2)(cid:2) (cid:6)(cid:2)(cid:2) (cid:7)(cid:2)(cid:2) (cid:8)(cid:9)(cid:10)(cid:11)(cid:12)(cid:11)(cid:13)(cid:14)(cid:15)(cid:16)(cid:17)(cid:18)(cid:17)(cid:13)(cid:19) (cid:2) (cid:20) (cid:2)(cid:2)(cid:2) (cid:20) (cid:2)(cid:5)(cid:2) (cid:20) (cid:21)(cid:2)(cid:2) (cid:20) (cid:21)(cid:5)(cid:2) (cid:20) (cid:1)(cid:2)(cid:2) (cid:20) (cid:1)(cid:5)(cid:2) (cid:20) (cid:3)(cid:2) (cid:1)(cid:1)(cid:2) (cid:22)(cid:7)(cid:6) (cid:3)(cid:21)(cid:2) (cid:1)(cid:3) (cid:21)(cid:3)(cid:21)(cid:2) (cid:1)(cid:4) (cid:23) (cid:24) (cid:22) (cid:21)(cid:4) (cid:25) (cid:26) (cid:27) (cid:28) (cid:29) (cid:26)(cid:9)(cid:13) (cid:30) (cid:11) (cid:22)(cid:2)(cid:2)(cid:17)(cid:13)(cid:25)(cid:19)(cid:31)(cid:13)(cid:17) (cid:5) (cid:1)(cid:3)(cid:6) (cid:3)(cid:2)(cid:2)(cid:2)(cid:17)(cid:13)(cid:25)(cid:19)(cid:31)(cid:13)(cid:17) (cid:5) (cid:1)(cid:1)(cid:6) (cid:17) (cid:3) !(cid:12)(cid:28)(cid:27)(cid:27) Figure 4.
UV-Vis absorption spectra in solid neon obtained after deposition of mass-selected C +15 (red) andC +11 (blue) at 200 eV average kinetic energy. Fragmentation of parent chains can be observed. states considered, ground state geometries were optimized at the RI-DFT BP86/def2-SV(P) level oftheory in C and higher symmetries, then harmonic analysis was done to check that a real groundstate was obtained. After this, energies of allowed vertical transitions were obtained by TDDFT(Turbomole 2016). According to our TDDFT estimations, the most intense absorptions of linearC + / − / n are concentrated in comparatively narrow wavelength ranges which depend strongly on chainlength (Fig. 5a). These wavelength ranges shift to the red with increasing number of carbon atoms(Fig. 5b). The strongest absorptions of mono-cyclic carbon rings according to TDDFT are close to200 nm (Fig. 6a) – the vacuum-UV boundary. Calculated oscillator strengths are comparable tothe strongest chain absorptions – for the same cluster nuclearity. There is no experimental dataso far concerning these strongest ring absorptions. We expect that they could be recorded with a Strelnikov et al. (cid:1)(cid:2)(cid:2) (cid:1)(cid:3)(cid:2) (cid:4)(cid:2)(cid:2) (cid:4)(cid:3)(cid:2) (cid:5)(cid:2)(cid:2) (cid:5)(cid:3)(cid:2) (cid:6)(cid:2)(cid:2) (cid:6)(cid:3)(cid:2) (cid:2) (cid:7) (cid:2)(cid:2) (cid:7) (cid:1)(cid:2) (cid:7) (cid:4)(cid:2) (cid:7) (cid:5)(cid:2) (cid:7) (cid:6)(cid:2) (cid:7) (cid:3) (cid:8) (cid:9)(cid:10) (cid:7)(cid:11) (cid:12) (cid:13) (cid:14) (cid:7)(cid:11) (cid:15) (cid:11) (cid:16) (cid:17) (cid:18) (cid:14) (cid:19) (cid:1)(cid:2)(cid:20) (cid:21) (cid:11) (cid:16) (cid:22) (cid:23) (cid:24) (cid:25) (cid:22)(cid:26)(cid:27)(cid:28)(cid:17)(cid:28)(cid:20)(cid:29)(cid:13)(cid:30)(cid:11)(cid:31)(cid:11)(cid:20)(cid:21) (cid:1)(cid:3) (cid:1)(cid:3)! (cid:1)(cid:3)" (cid:1)(cid:2)(cid:2) (cid:1)(cid:3)(cid:2) (cid:4)(cid:2)(cid:2) (cid:4)(cid:3)(cid:2) (cid:5)(cid:2)(cid:2) (cid:5)(cid:3)(cid:2) (cid:6)(cid:2)(cid:2) (cid:6)(cid:3)(cid:2) (cid:3)(cid:2)(cid:2) (cid:2) (cid:7) (cid:2)(cid:2) (cid:7) (cid:1)(cid:2) (cid:7) (cid:4)(cid:2) (cid:7) (cid:5)(cid:2) (cid:7) (cid:6)(cid:2) (cid:7) (cid:3) (cid:1)(cid:2) (cid:1)(cid:5) (cid:1)(cid:3) (cid:4)(cid:2) (cid:1) (cid:8) (cid:9)(cid:10) (cid:7)(cid:11) (cid:12) (cid:13) (cid:14) (cid:7)(cid:11) (cid:15) (cid:11) (cid:16) (cid:17) (cid:18) (cid:14) (cid:19) (cid:1)(cid:2)(cid:20) (cid:21) (cid:11) (cid:16) (cid:22) (cid:23) (cid:24) (cid:25) (cid:22)(cid:26)(cid:27)(cid:28)(cid:17)(cid:28)(cid:20)(cid:29)(cid:13)(cid:30)(cid:11)(cid:31)(cid:11)(cid:20)(cid:21) (cid:26) $
Figure 5. a:
The strongest electronic absorptions of linear C and its singly charged ions according toTDDFT (BP86/def-SV(P)) prediction, unscaled. b: Predictions of the strongest electronic absorptions fora range of long neutral long carbon chains (TDDFT (BP86/def-SV(P)), unscaled). Calculated lines arebroadened by multiplication with a 10nm-FWHM Lorentzian.
VUV-spectrometer, assuming sufficient quantity of the species can be accumulated. Similar to linearstructures, the strongest absorptions of the ionized rings are predicted to be close to the absorptionsof the corresponding neutral ring species (Fig. 6b). Note, that while ring species also have DIBs-relevant absorptions in the visible or near-IR ranges they are however, much weaker.To rationalize our observation that fullerenes can be fragmented to linear chains we have also carriedout molecular dynamics (MD) simulations of C fragmentation based on the semi-empirical PM7 levelof theory (MOPAC 2016). The initial kinetic energy of C -atoms was varied from 100 eV to 500 eVand the energy dissipation time was varied from 10 fs to 20 ps. Only a few types of different structuresresult: linear chains (most abundant), monocyclic rings and chain-ring hybrid structures. Examplefigures of two typical MD-simulations can be found in the Appendix (Figs. 9,10). Qualitatively similarresults were also reported previously using lower level MD simulations (Kim & Tom´anek 1994).3.3. Vibrational Spectroscopy ragmentation of Fullerenes to Linear Carbon Chains. (cid:1)(cid:2)(cid:2) (cid:1)(cid:3)(cid:2) (cid:4)(cid:2)(cid:2) (cid:4)(cid:3)(cid:2) (cid:5)(cid:2)(cid:2) (cid:5)(cid:3)(cid:2) (cid:6)(cid:2)(cid:2) (cid:6)(cid:3)(cid:2) (cid:2) (cid:7) (cid:2)(cid:2) (cid:7) (cid:1)(cid:2) (cid:7) (cid:4)(cid:2) (cid:7) (cid:5)(cid:2) (cid:7) (cid:6)(cid:2) (cid:7) (cid:3) (cid:8) (cid:9)(cid:10) (cid:7)(cid:11) (cid:12) (cid:13) (cid:14) (cid:7)(cid:11) (cid:15) (cid:11) (cid:16) (cid:17) (cid:18) (cid:14) (cid:19) (cid:1)(cid:2)(cid:20) (cid:21) (cid:11) (cid:16) (cid:22) (cid:23) (cid:24) (cid:25) (cid:26) (cid:1)(cid:2) (cid:26) (cid:1)(cid:5) (cid:26) (cid:1) (cid:27) (cid:26) (cid:4)(cid:2) (cid:22)(cid:28)(cid:29)(cid:30)(cid:17)(cid:30)(cid:20)(cid:31)(cid:13) (cid:11)!(cid:11)(cid:20)(cid:21) (cid:1)(cid:2)(cid:2) (cid:1)(cid:3)(cid:2) (cid:4)(cid:2)(cid:2) (cid:4)(cid:3)(cid:2) (cid:5)(cid:2)(cid:2) (cid:5)(cid:3)(cid:2) (cid:6)(cid:2)(cid:2) (cid:6)(cid:3)(cid:2) (cid:2) (cid:7) (cid:2)(cid:2) (cid:7) (cid:1)(cid:2) (cid:7) (cid:4)(cid:2) (cid:7) (cid:5)(cid:2) (cid:7) (cid:6)(cid:2) (cid:7) (cid:3) (cid:26) (cid:1)(cid:6) (cid:26) (cid:1)(cid:6)" (cid:26) (cid:1)(cid:6) $(cid:28) (cid:22)(cid:28)(cid:29)(cid:30)(cid:17)(cid:30)(cid:20)(cid:31)(cid:13) (cid:11)!(cid:11)(cid:20)(cid:21) Figure 6. a:
The strongest absorptions of neutral monocyclic carbon rings as calculated by TDDFT(BP86/def-SV(P)), unscaled. b: The strongest electronic absorption of C -ring and its ions. TDDFT(BP86/def-SV(P)), unscaled. Calculated lines are broadened by multiplication with a 10nm-FWHMLorentzian. Fig. 7 shows an IR absorption spectrum, which corresponds to the UV-NIR spectrum of Fig. 2,i.e. after C +15 deposition under low energy impact conditions. Similar to the electronic absorptionspectrum, the spectrum looks quite clean with a very prominent C IR absorption. The IR absorptionof C is known from previous experiments with a mixture of neutral matrix-isolated carbon clusters(prepared by aggregation in matrix) (Strelnikov et al. 2005). There is an unidentified absorptionof a neutral carbon cluster C x , which was also present as a weak absorption line in the former IRspectra (Strelnikov et al. 2005). This absorption does not correlate with the IR absorption of neutrallinear C and remains to be identified. Fig. 8 shows an IR absorption spectrum, corresponding tothe UV-NIR measurement presented in Fig. 3, i.e. for the matrix sample prepared by depositionof C +18 /C . The absorption at 1818.5 cm − belongs to a C -derived species. Determination of itscharge state requires further experiments with electron scavengers. DISCUSSION AND OUTLOOK0
Strelnikov et al. (cid:1) (cid:2)(cid:3)(cid:3)(cid:1)(cid:4)(cid:3)(cid:3)(cid:1)(cid:5)(cid:3)(cid:3)(cid:1)(cid:6)(cid:3)(cid:3)(cid:1)(cid:7)(cid:3)(cid:3)(cid:1)(cid:8)(cid:3)(cid:3)(cid:1)(cid:9)(cid:3)(cid:3)(cid:1)(cid:10)(cid:3)(cid:3) (cid:11)(cid:12)(cid:13)(cid:14)(cid:15)(cid:16)(cid:17)(cid:18)(cid:14)(cid:19)(cid:20)(cid:21)(cid:20)(cid:22)(cid:17) (cid:23)(cid:1) (cid:3) (cid:24) (cid:3)(cid:3)(cid:3)(cid:3) (cid:24) (cid:3)(cid:3)(cid:2)(cid:3) (cid:24) (cid:3)(cid:3)(cid:5) (cid:25) (cid:18) (cid:26) (cid:27) (cid:19) (cid:18)(cid:12)(cid:15) (cid:22) (cid:14) (cid:28) (cid:1)(cid:6) (cid:29) (cid:2) (cid:30) (cid:28) (cid:31)
Figure 7.
IR absorption spectrum of C in Ne at 5K, obtained after deposition of ca. 1000 nAmin ofC +15 /C at about 100 eV kinetic energy. The line at 1293 cm − is an unidentified carbon molecule. (cid:1)(cid:2)(cid:2)(cid:3)(cid:2)(cid:2)(cid:2)(cid:3)(cid:4)(cid:2)(cid:2)(cid:3)(cid:5)(cid:2)(cid:2)(cid:3)(cid:6)(cid:2)(cid:2)(cid:3)(cid:1)(cid:2)(cid:2) (cid:7)(cid:8)(cid:9)(cid:10)(cid:11)(cid:12)(cid:13)(cid:14)(cid:10)(cid:15)(cid:16)(cid:17)(cid:16)(cid:18)(cid:13) (cid:19)(cid:3) (cid:19) (cid:2) (cid:20) (cid:2)(cid:2)(cid:4)(cid:2) (cid:20) (cid:2)(cid:2)(cid:2)(cid:2) (cid:20) (cid:2)(cid:2)(cid:4)(cid:2) (cid:20) (cid:2)(cid:2)(cid:5)(cid:2) (cid:20) (cid:2)(cid:2)(cid:6)(cid:2) (cid:20) (cid:2)(cid:2)(cid:1) (cid:21) (cid:14) (cid:22) (cid:23) (cid:15) (cid:14) (cid:8) (cid:11) (cid:18)(cid:10) (cid:24) (cid:3)(cid:1)(cid:2)(cid:17)(cid:25)(cid:17)(cid:19) (cid:26)(cid:3)(cid:1)(cid:3)(cid:1)(cid:20)(cid:27) (cid:28) (cid:4) (cid:29)(cid:28) (cid:4) (cid:29) Figure 8.
IR absorption spectrum, obtained after deposition of ca. 1800 nAmin of C +18 /C at about120 eV kinetic energy in Ne at 5K. The baseline is distorted by matrix interferences. Our measurements imply that fragmentation of fullerenes in Space by electron impact (and conceiv-ably also by interaction with cosmic rays) would lead to the formation of strongly absorbing linearcarbon chains. In contrast, absorption of a single photon with energy below 13.6 eV (hydrogen cut-off) is not expected to be relevant for carbon-chain formation as the excitation energy is insufficientfor multifragmentation. The wavelength region, within which DIBs are observed, starts at about ragmentation of Fullerenes to Linear Carbon Chains. and C containing matrixes and the associated TDDFT calculations imply that the strongestabsorptions of linear C + / − / n +1 ( n >
6) and C + / − / n ( n >
8) can in fact fall in the DIBs region. Thepositions of the gas-phase absorptions of neutral and ionized carbon chains are close to their absorp-tions in Ne-matrix. This is known from experiments of the Maier group on smaller carbon clusters(Boguslavskiy & Maier 2006; Zack & Maier 2014). At the current stage of our experiments, absoluteabsorption cross-sections are difficult to estimate, because of the overlap of C + n with C n /C n /C n .Furthermore, additional experiments are needed to distinguish absorptions of C + / − / n in differentcharge states. Nevertheless, from the fact that one can easily observe optical absorptions of C andC despite their low ion currents (below 1 nA vs. ca. 100 nA for C +60 ), one can deduce that the longcarbon chains absorb at least one order of magnitude more strongly than C +60 in the NIR. Theoreti-cal (TDDFT) oscillator strengths of the strongest C + / n linear chain absorptions approximately scaleas n/
2. For any linear C n this is already considerably more than that of the C +60 NIR absorption E g ← X A u ( f theoretical = 0 . f experimental = 0 . ± .
02 – integrated over all NIR C +60 vibronicbands (Strelnikov, D. et al. 2015; Campbell et al. 2016)). Consequently, it seems plausible that C +60
DIBs would be accompanied by measurable absorptions due to chain-like fragments. To prove this,we suggest that future gas-phase measurements (first in the laboratory and then in Space) shouldconcentrate efforts on the chain species C + / − / n +1 ( n >
6) and C + / − / n ( n > n precursors (Strelnikov et al. 2007). Such carbon cluster oxideswould therefore also be good candidates for future gas-phase measurements. CONCLUSIONFragmentation of fullerenes upon electron impact (and conceivably also by collision with otherenergetic particles) can lead to the formation of long carbon chains with up to at least 18 carbonatoms. The strongest absorptions of linear C + / − / n +1 ( n >
6) and C + / − / n ( n >
8) fall in the DIBs relevantregion. Given recent advances in the He-tagging technique (Campbell et al. 2015; Roithov´a et al.2
Strelnikov et al. ragmentation of Fullerenes to Linear Carbon Chains. APPENDIX: MD SIMULATIONS.
Figure 9.
Selected frames of the MD-simulation of C fragmentation. (MOPAC, PM7 DRC, 150 eV initialenergy, 1 ps energy dissipation half-life). The corresponding animation can be viewed online. Strelnikov et al.
Figure 10.
Selected frames of the MD-simulation of C fragmentation. (MOPAC, PM7 DRC, 200 eVinitial energy, 4 ps energy dissipation half-life). The corresponding animation can be viewed online. REFERENCES
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