Electronic absorption spectra of linear C 6,C 8 and cyclic C 10,C 12 in neon matrices

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1 JOURNAL OF CHEMICAL PHYSICS VOLUME 111, NUMBER OCTOBER 1999 Electronic absorption spectra of linear C 6,C 8 and cyclic C 10,C 12 in neon matrices Michel Grutter, Muriel Wyss, Evgueni Riaplov, and John P. Maier a) Institut für Physikalische Chemie, Universität Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland Sigrid D. Peyerimhoff and Michael Hanrath Institut für Physikalische und Theoretische Chemie, Universität Bonn, Wegelerstrasse 12, D Bonn, Germany Received 21 May 1999; accepted 27 July 1999 The electronic absorption spectra of the even-numbered carbon molecules C 6 C 14 have been measured in neon matrices. Bare carbon anions were produced in a cesium sputter source, mass selected, codeposited with neon at 6 K, and neutralized. The spectra show, apart from the known 1 3 u X 3 transition of linear C 6,C 8, and C 10 in the visible, absorption bands in the UV region. The spectral data when considered in conjunction with ab initio calculations show that the linear forms of C 6 and C 8 have the next strong 2 3 u X 3 transition with band maximum near 238 and 277 nm, respectively, whereas the band systems of C 10,C 12, and C 14 at 316, 332, and 347 nm are due to the monocyclic species American Institute of Physics. S INTRODUCTION a Electronic mail: maier@ubaclu.unibas.ch There has been substantial effort in understanding the structure, spectroscopy, and properties of carbon molecules in the past few decades. 1,2 Among the structures proposed for clusters smaller than the ball-shaped C 60 are chains, monocyclic rings, polycyclic rings, and fullerenes. Theoretical studies have estimated the cyclic form of the even-numbered molecules, starting at n 4, to be lower in energy while for the odd-numbered C n, the linear chains with n 9 were found to be more stable. 3,4 Many calculations which have followed agree that ring structures for n 10 are favored, and that the production of a particular isomer could depend on the experimental conditions. 5 8 Experimentally, much more is known on the linear carbon chains than on the cyclic structures. The linear isomers have been subject to electron-spin resonance 9 and infrared absorption studies, both in rare gas matrices. The electronic absorption spectra of the C 2n (n 5) and C 2n 1 (n 7) 17 carbon chains have been identified in neon matrices using a mass selected technique. The detection of the odd-numbered chains has been extended to C 21 from an experiment where laser vaporized graphite was trapped in neon with subsequent annealing. 18 In the gas phase rotationally resolved infrared spectra of the linear C 3 C 7,C 9, and C 13 19,20 have been reported but so far the cyclic forms could not be detected. Recently, infrared spectra of carbon vapor trapped in argon matrices led to identification of the cyclic C 6 (D 3h ), 21 and C 8 (D 4h ). 22 Photoelectron spectra of carbon anions indicate, from the discernible vibrational patterns and mass distribution, a transition from the linear to a monocyclic structure at C ,24 A variety of structures for carbon clusters have also been established from ion mobility measurements. 25 These studies demonstrated that the cations C 7 to C 10 coexist as linear and cyclic isomers and that above C 10 the linear chains are no longer present. However, the linear isomer is observed up to C 30 for the anions although for n 15, the monocyclic form predominates. 26 In this contribution, ab initio calculations on the excited states of the D h, D 3h, and D 6h structures of C 6, as well as the observed electronic absorption bands in the UV region are reported, leading to the identification of cyclic C 10,C 12, and C 14 and further transitions of linear C 6 and C 8. EXPERIMENT The apparatus used for the deposition of mass selected ions in neon matrices has been described. 27 Carbon anions produced in a cesium sputter source were extracted with ev kinetic energy, mass selected with a quadrupole mass filter, and codeposited in neon at 6 K. 28 The mass resolution was set to 3 u to reach higher ion currents during the deposition. Ion currents of 60, 14, 1.3, 0.9, and 0.1 na could be obtained for C 2n (n 3 7), respectively. During the 2 h neon deposition, the rhodium coated sapphire substrate was exposed to light from a medium pressure mercury 5.4 ev or a xenon arc 6.2 ev lamp in order to photodetach the electrons and produce the neutral molecules. The electronic absorption spectra of the trapped species in the nm region were measured by passing the dispersed light through the 200 m thin matrix parallel to the substrate. THEORY Ab initio configuration interaction CI calculations were carried out for C 6 with the developed DIESEL-CI program direct intern extern separated individually selected CI 29 based on configuration selection and energy extrapolation procedures. The geometries are taken from Ref. 30 for the linear /99/111(16)/7397/5/$ American Institute of Physics

2 7398 J. Chem. Phys., Vol. 111, No. 16, 22 October 1999 Grutter et al. D h (C 1 C 2 C 3 C 3 C 2 C 1,C 1 C Å, C 2 C Å, and C 3 C Å), and benzene-like D 6h structure (CC Å, 60 ); the parameters for the D 3h ring form (CC Å and C 1 C 6 C ) are those in another study. 31 The AO basis set is of double-zeta plus polarization quality; it consists of the 9s5p/5s3p basis for carbon 32 augmented with one d-polarization function exponent The basis functions in the CI are the SCF orbitals of the corresponding ground states. In each case the lowest 6 MOs corresponding to the carbon 1s shells are kept doubly occupied in the CI procedure, the complementary highest MOs were deleted so that 24 electrons were allowed to be distributed in the CI among 102 MOs. The CI calculations were carried out for convenience in the abelian subgroups D 2h instead of D h,d 6h ) and C 2v instead of D 3h ). The reference sets, from which single and double-excitations are generated, were in the order between 20 and 60 configurations. The set of generated configurations configuration state functions CSFs was in the order of 2 million 25 million ; the configuration selection threshold was generally 10 7 hartree so that about 10% 20% of the CSFs were considered explicitly for the CI wavefunction and energy while the contribution of the remaining CSFs to the energy was accounted for by perturbation procedure. The effect of yet higher excitations was estimated via a Davidson-type procedure 33 E 1 c 2 0 /c 2 0 E MRD-CI E ref CI, where c 2 0 is the contribution of the reference configurations to the total CI wavefunction and the subscripts MRD-CI and ref CI correspond to the CI of the reference and multireference single and double excitation CI space. This energy is used in what follows. RESULTS AND DISCUSSION Structures and excited states of C 6 The electronic configurations of the C 6 ground states are: linear D h : 3 1 u 4 6 u u 2 ; u 0, cyclic D 3h : 1 A 1 1a 2 2 1a 2 2 4a 1 2 5e 4 1e 4 ; 5a 1 0 6e 0 2e 0, cyclic D 6h : 1 A 1g b 2u 2 a 2u 2 a 1g 2 e 1g 4 e 1u, 4 ; b 1u, 0 e 2g, 0 e 2u 0, where the e in the D 3h structure and those not specified in the D 6h structure are -type orbitals. The 6 u and 7 orbitals are almost degenerate, as are the 5e and 1e, as well as the e 1g and e 1u orbitals. In the linear C 6 chain transitions to 3 u and 3 u are dipole-allowed. In the cyclic D 3h (D 6h ) arrangement accessible states are 1 E and 1 A 2 ( 1 E 1u, 1 A 2u ). The calculated energies for the lowest states of these symmetries are given in Fig. 1. In the linear molecule the lowest lying electronic states result from the 7 2 u ( 3 u ) and 6 u 2 u ( 3 ) excitations. The transition to this first 3 u state is very weak, with calculated oscillator strength f The next states of 3 u, 3 u, and 3 u symmetry arise from the FIG. 1. Ab initio calculated excited electronic states of linear and cyclic C 6 to which transitions from the respective ground states are allowed. (w weak, s strong, vw very weak, vs very strong). Dashed lines indicate larger error limits. 1 2 u excitation. The calculations place the 1 3 u state with admixture of 2 u 2 configurations 2.65 ev above the ground state. This value is 0.23 ev higher than the observed energy in a neon matrix. 15 However, the calculated value corresponds to a vertical transition. The geometry of the upper state is expected to show an elongated C 1 C 3 mostly terminal bond. In an attempt to optimize the 1 3 u state geometry a complete active space calculation correlating 4 electrons in 8 orbitals CAS 4,8 found an energy lowering of 0.15 ev for this state relative to the ground state geometry. Indeed, the measured spectrum shows vibrational structure with a maximum at 2.69 ev 461 nm Fig. 2. The calculated oscillator strength is in accord with the observed strong transition. FIG. 2. The 1 3 u X 3 transition of C 6 bottom, C 8 top trace, and the newly observed transitions in the UV and. The spectra were obtained after mass selected codeposition of the anions with excess of neon at 6 K, and concomitant exposure to UV light from a medium pressure mercury lamp.

3 J. Chem. Phys., Vol. 111, No. 16, 22 October 1999 Spectra of linear C 6,C 8 and cyclic C 10,C Higher states of 3 u symmetry, to which transitions are in principle allowed, arise from the 6 u u and 6 u 2 excitations. Because the first of these results mainly from a double-excitation with 6 u 2 admixture, the corresponding oscillator strength is essentially zero. The electronic configuration 6 u 2 2 u 2 gives rise to four 3 u and one 3 u states. The 3 u state is calculated at 4.25 ev, the 3 u states between 3.5 and 5 ev. These calculations are fairly complex since the dominant part of the wavefunctions already has four open shells (2 u separated into 2 ux and 2 uy ); the appropriate reference set requires six open shells which gives rise to ten open-shell configurations in the MRD-CI space. Hence convergence in the CI states is more difficult than in states with a simpler wavefunction expansion. The same holds for the calculated oscillator strength, which is low for 3 3 u but somewhat dependent on computational details for the 4 3 u and 5 3 u state with the larger value ( f 0.03) for the 5 3 u. The uncertainty in the description of the 6 3 u is large and therefore its position in Fig. 1 is shown by a dashed line. The second state of 3 u symmetry has a complementary makeup as 1 3 u and the 2 3 u X 1 transition is calculated to be the strongest of all those investigated for C 6. The calculations place this state at 5.7 ev, again with a greater uncertainty than 1 3 u since it is a high root in the secular equation which has 3 u and 3 u species together in the lower D 2h symmetry employed. The spectrum of the cyclic isomers is much less complicated. In the D 3h form the first excitations are from 5e 5a 1 ( 1 E ) and 1e 5a 1 ( 1 E ). The first transition is placed at 3.15 ev and is found to be stronger ( f 0.07) than the first 1 3 u X 1 transition in linear C 6. The state 1 1 E is at 3.25 ev, but transitions to this state are not allowed. The second allowed transition with similar oscillator strength f 0.07) to the 2 1 E state arising from the 5e 6e process, is calculated to occur around 5.8 ev. The first allowed transition to the 1 A 2 state is found at even higher energy. The energetically higher-lying symmetric C 6 ring structure (D 6h symmetry is predicted to have its first allowed transition to the 1 1 E 1u state (e 1u e 2g ) at 4.95 ev with an oscillator strength of Transitions to the first excited states 1 E 2g (e 1u b 1u ) and 1 B 2u (e 1u e 2g ) are not allowed. The second allowed transition (e 1g e 1u b 2 1u ) to the 1 1 A 2u state at 5.6 ev is very weak because of its double-excitation nature. Electronic spectra of C 6 and C 8 When mass selected C 6 and C 8 anions were deposited with excess of neon at 6 K and concomitant UV irradiation, the already identified 1 3 u X 3 transitions of linear C 6 and C 8 with origin bands at and nm, 15 respectively, could be observed. The known electronic transitions of the anions 34 were not present in the spectra because of the photobleaching process. In addition to the known transition of the linear molecules, the UV absorption bands seen in Fig. 2 were detected see also Table I. These absorptions increase in intensity upon irradiation and are strongest when the matrices are exposed to UV light during the matrix TABLE I. Observed band maxima 0.2 nm for C 2n (n 3 7) with proposed assignments. nm cm 1 cm 1 Assignment C 6 D h 3 u X 3 a v 3 ( ) v u X C 8 b D h v 4 ( ) v v 3 ( ) v 3 v u X C 10 b D h v 5 ( ) v u X v 5 ( ) C 10 D 10h 1 E 1u X 1 A 1g v(a g ) v(374) C 12 D 6h 1 1 E 1u X 1 A 1g v(a g ) v(a g ) C 12 D 6h 2 1 E 1u X 1 A 1g v(a g ) v(300) v(374) v(a g ) v(1882) v(300) v(1882) v(1882) v(300) v(1882) 2v(300) C 14 D 14h 1 E 1u X 1 A 1g a Could also be a vibrationally induced transition; see text. b Probably similar to C 6. growth. This indicates that the absorbers are not ionic and originate from the indicated bare carbon molecules. The absorption spectra of C 6 and C 8 show an intense broad system and a weaker structured one to slightly lower energy. The absorption measurements below 240 nm are not reliable due to increased scatter of light by the matrix. The band profile of the higher energy transition of C 6 is distorted and the intensity does not fall off as fast towards the blue as the spectrum shows. The broad and structured band systems arise from the same absorber because their relative intensity remains the same under various experimental conditions. A similar pattern has been seen in the electronic absorption spectra of the C 5,C 7, and C 9 species; an intense broad allowed UV transition with a weaker structured system at lower energy. 17 The current calculations on the excited states of linear C 6 (D h ) show a second state of 2 3 u symmetry with a large oscillator strength 5.7 ev above the ground state. This is in accord with the higher energy transition of C 6 observed below 240 nm 5.2 ev. The equivalent absorption system is more clearly seen for C 8 with a band maximum at 277 nm. Of the two ring structures for C 6, the one with D 3h symmetry

4 7400 J. Chem. Phys., Vol. 111, No. 16, 22 October 1999 Grutter et al. present in the matrices rather than the cyclic forms because these molecules are produced from the anionic precursors, which are predominantly linear. 23,26,37 The weaker more structured band observed at nm 4.97 ev for C 6 and shifted by 54 nm to the red for C 8 could be attributed to a transition to the 5 3 u state ( excitation on the basis of the calculations; its intensity is predicted to be significant Fig. 1. There are numerous states in the area between 5 and 6 ev and thus vibronic mixing of a state as the 2 3 u one could in addition lead to intensity borrowing in the energy range around 250 nm. FIG. 3. The electronic absorption spectra observed after mass selected codeposition of C 10,C 12, and C 14 with neon and concomitant UV irradiation. Traces of the absorption bands of the anion chains, which were not photobleached entirely, are marked with an asterisk. has been determined to be the most stable. 3,7,35,36 The present calculations on C 6 (D 3h ) predict a strong transition near 400 nm, which is not seen in the experimental spectrum. It is also apparent from Fig. 2 that the intensity of the UV systems in C 6 and C 8 are similar to that of the 1 3 u X 3 transition in the visible of the linear chains. 15 Thus, the higher energy band is assigned to the 2 3 u X 3 transition of linear C 6 and C 8. The linear C 6 and C 8 are FIG. 4. Electronic absorption spectra in the UV region observed after mass selected deposition of the respective anions and neon with concomitant UV irradiation toform6kmatrices. Electronic spectra of C 10,C 12, and C 14 The electronic absorption spectra presented in Fig. 3 have been obtained after mass selected depositions of C 10, C 12, and C 14 with concomitant UV irradiation. Traces of the C 10 and C 12 chains, marked with asterisks, could be identified from their C 2 X 2 and 2 2 X 2 electronic transitions. 16 These anions could not be neutralized completely, as was the case for the smaller anions, because of the larger electron detachment energies. As it can be seen from the expanded spectral region in Fig. 3, weak bands due to the 1 3 u X 3 transition of linear C 10 with origin at nm Ref. 16 could also be detected. A considerably stronger absorption band at 316 nm dominates the spectrum of C 10. This band system shifts progressively to shorter wavelengths for C 12 and C 14 Table I and it originates from a nonionic species because it grows upon irradiation. Figure 4 shows the absorption bands of C 6 to C 14 in the UV. The band systems assigned to the linear forms of C 6, C 8, and C 10 weak are marked with and. The band positions in these transitions are consistent with the 54 nm red shift observed for the smaller chains and are assigned accordingly to the 2 3 u X 3 and 3 u ( ) transitions of the C 10 chain. These UV band systems, as well as the 1 3 u X 3 transition of linear C 10 in the visible, are considerably weaker than the prominent absorption band system in Fig. 4. These absorptions observed for C 10, C 12, and C 14 are very likely to belong to isomers with nonlinear structures as indicated by the large drop in the intensity of the 1 3 u X 3 transition of the carbon chains in going from C 8 to C 10 Figs. 2 and 3 ; in fact this transition for linear C 12 is no longer observed. Furthermore, the band shapes are different from those of the electronic transitions of the linear C 6 and C 8 in the UV. Ab initio calculations on the structures of C 10 have found the D 5h and D 10h ring structures to be much more stable than the linear isomer. 5,7,36 In the SCF calculation, 5 as well as in the density functional theory study, 7 the D 5h structure is found lower in energy than the D 10h one. However, in both articles the authors state that due to a large correlation effect, the D 10h structure may in fact be the ground state geometry. A fused ring, naphthalene-like, structure is not considered because the poor overlap of the in-plane orbitals and the ring strain places it several ev energetically higher than the monocyclic geometries. 4,6,36 Furthermore, according to a variety of theoretical methods, the larger rings in this intermediate size range also have monocyclic ground state structures. 4,5,7,8 Rings with (4n 2) electrons manifest ad-

5 J. Chem. Phys., Vol. 111, No. 16, 22 October 1999 Spectra of linear C 6,C 8 and cyclic C 10,C ditional stability according to Huckel s aromaticity rule and have cumulenic-type bonds while the C 4n (n 2 4) rings have preferably D 2nh ground state structures with polyacetylenic bonds. 7,8 Extrapolating the results obtained from the calculations on cyclic C 6, one would expect the C 10 (D 5h ) ring structure to have, besides a UV band, a strong transition at around 450 nm considering a 50 nm shift from the transition predicted for the C 6 ring (D 3h ). This reasoning comes from the particle in a one-dimensional box, or on a ring model, where the shift in the electronic transition between linear C 6 and C 8 should be approximately the same as that between cyclic C 6 and C 10. As this transition is not observed in the C 10 absorption spectrum, the D 3h isomer is excluded. However, the higher symmetry (D 10h ) isomer of C 10, in analogy to cyclic C 6 (D 6h ), should have its first allowed transition around 300 nm Fig. 1. This is consistent with the observed band of C 10 at 316 nm, which is consequently assigned to the 1 E 1u X 1 A 1g transition of the cyclic C 10 species with D 10h symmetry. In the spectrum of C 12 a band system at 720 nm, in addition to the UV absorption band, is observed. The absorption system of the linear chain is now absent. By similar arguments as used for C 10, extrapolation of the excited state calculations on C 6 would lead to two allowed transitions in the measured spectral range for the C 12 (D 6h ) but only one for the D 12h isomer. Thus, the observed band systems at 720 and 332 nm are assigned to the 1 1 E 1u X 1 A 1g and 2 1 E 1u X 1 A 1g transitions of cyclic C 12 (D 6h ). The visible band system shows the excitation of two vibrational modes with frequencies of 1910 and 2070 cm 1 in the upper electronic state. The band system at 332 nm shows vibrational progressions at 300 and 1880 cm 1. The higher frequency modes in both electronic transitions are typical CwC stretching vibrations and are consistent with a D 6h structure of C 12 with acetylenic-type bonding as predicted by theory. 7 In the spectrum of C 14 a band near 347 nm is apparent. The signal is weak because the anion precursor is not produced efficiently in the sputter source. Monocyclic C 14 is the next member in the 4n 2 series and in analogy to C 10, the observed band is tentatively assigned to an electronic transition of the symmetric D 14h structure. CONCLUSIONS The absorption spectra of mass selected C 2n (n 3 7) have been obtained in neon matrices, and calculations on the excited electronic states of the D h, D 3h, and D 6h structures of C 6 have been carried out. These data locate a second intense transition of 2 3 u X 3 symmetry and a somewhat weaker 3 u X 3, or vibronically induced, transition in the UV region for the linear C 6 and C 8 chains. The intense absorption bands observed for C 10,C 12, and C 14 in this region are assigned to transitions of the monocyclic structures based on extrapolation of the excited state calculations on the C 6 isomers. The detection of the cyclic carbon species only for C n n 10 does not imply that the cyclic forms of C 6 and C 8 do not exist, but is a consequence that they were produced from the anions. The latter are predominantly linear for C n (n 10), while the cyclic forms coexist with the linear ones for n 10, as ion mobility studies have shown. 26 ACKNOWLEDGMENT This work is part of project No of the Swiss National Science Foundation. 1 W. Weltner, Jr. and R. J. Van Zee, Chem. Rev. 89, A. Van Orden and R. J. Saykally, Chem. Rev. 98, K. Raghavachari, R. A. Whiteside, and J. A. Pople, J. Chem. 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