c.i. frum et al- the infrared emission spectrum of gas-phase c60 (buckminsterfullerene)

8/2/2019 C.I. Frum et al- The infrared emission spectrum of gas-phase C60 (buckminsterfullerene)

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Volume 176. number 6 CHEMICAL PHYSICS LETTERS I February 1991

The infrared emission spectrumof gas-phase CbO buckminsterfullerene)C.I. Frum, Rolf Engleman Jr., Hartmut G. Hedderich, Peter F. Bernath Department of Chemistry,niversity ofArizona, Tucson, AZ 85721, US ALowell D. Lamb and Donald R. HuffmanDepartment ofphysics. Universqv ofAnzona, Tucson, AZ 85721, USA

Received 24 October 1990; in final form 31 October 1990

The gas-phase infrared spectrum of C,, has been observed in emission with the National Solar Observatory Fourier transformspectrometer at Kitt Peak. Bands attributable to the CGOmolecule are found at 527. I, 570.3, 1169.and 1406.9 cm-. Additionalemission features are tentatively assigned to CT0or combination bands of&. A new, strong emission is observed at 1010.2 cm-belonging to an unknown molecule. None of these features can be associated with any of the strong emission bands observed sofar in astronomical sources.

Since the original report on the existence of buck-minsterfullerene (C,,) [ l] there has been consid-erable interest in the spectroscopy of it and relatedall-carbon molecules. Such molecules may be abun-dant in astronomical environments and may havedesirable practical applications as, for example,coatings and catalysts. Until the recent developmentof a simple method to prepare gram quantities ofreasonably pure Cho [ 21 there were only two exper-imental reports of Ceo optical spectra [ 3,4]. Elec-tronic spectra in the ultraviolet have been obtainedby several groups [2-51 and a theoretical predictionis available [ 61. Photoelectron spectra have also beenobtained [ 7-91. Vibrational Raman spectra of pu-rified films of C,,, and CT0 were recently measured[ lo]. Infrared vibrational spectra have been re-ported from solid films [2,4], pressed KBr pellets[ 111, and in an argon matrix [ 1 1,but all at low res-olution. A number of theoretical calculations of theinfrared spectrum of CbOhave been made [ 12-l 71,However, gas-phase spectra of the free Ceo moleculeare more useful for comparison with astronomicalspectra. Closely related calculations on the spectra ofatoms trapped within a Ceo cage have also been made

Camille and Henry Dreyfus Teacher-Scholar.

with the possibility of explaining the diffuse inter-stellar lines [ 18,191.

The gas-phase infrared spectrum of CbO in theground electronic state was recorded with the Na-tional Solar Observatory Fourier transform spec-trometer (FTS) associated with the McMath solartelescope at Kitt Peak. The unapodized resolutionwas 0.005 cm- with liquid-helium-cooled As: Si de-tectors and a KC1 beam splitter. The spectral band-pass was limited to 500- 1400 cm- with an InSb fil-ter or 500-2800 cm- with an InAs filter. Infraredradiation from a SIC glower was passed through theabsorption cell and focused on the 8 mm entranceaperture of the FIX. The entire path between glowerand entrance aperture was purged with dry nitrogen,and the FTS was operated in vacuum. For the emis-sion spectra, the glower was simply switched off. Formost spectra, three scans were co-added for 15 minof integration.

Approximately one gram of crystalline C6,, wasproduced using the technique described earlier byKrtitschmer et al. [2 1. It was expected that CT0 ful-lerene would be the main impurity, but no analysiswas made prior to our experiment. Later, however,we learned that our sample contained considerable

504 0009-2614/91/$ 03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)


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Volume 176,number 6 CHEMICALPHYSICSLETTERS I February 1991amounts of CT0 (l&15%) as measured using sizeexclusion chromatography.

Initially, we recorded the infrared absorptionspectrum of solid C,, deposited as a thin film ontoa silicon substrate by vacuum sublimation. Thebrownish film was estimated to be a few micronsthick. The measured peak absorptions agree very wellwith the values reported previously [2 1.Table 1 liststhe bands attributable to the solid CGO ilm. As an-ticipated, no structure in this condensed-phase spec-trum was observed.

Gas-phase Cho was produced in an alumina tubefurnace described earlier in a study of infrared emis-sion spectrum of SiS [ 201. The furnace consisted ofan alumina tube (0.9 m long and 50 mm diameter)to which a gas inlet, a pumping port and Kl3r win-dows were attached with 0-ring-sealed fittings.About 1 g of the C60/C70mixture was placed in themiddle of a quartz tube (0.8 m long and 36 mm outerdiameter, 32 mm inner diameter) which was used asa liner in the alumina tube. The furnace was slowlyheated to 200C with a slow flow of argon at 2 Torrto drive off water and hydrocarbons. The system wasthen pressurized with pure argon at 5 Torr and sealed.

A series of spectra was taken as the furnace tem-perature was raised at a rate of about 2C/min. Thepressure in the cell gradually increased to 1 I Torr asthe temperature reached SOOC,when a rapid changein pressure to about 15 Torr was observed. New ab-sorption features appeared and their intensities in-creased as the temperature was raised. No absorp-tion features corresponding to the solid C60spectrumwere observed. The positions of the four broad ab-sorption bands are 817.4, 1034.0,1091.1 and 1264.1cm-. At high resolution these bands did not show

Table 1The infrared vibrational frequencies of C6e(in cm-)

structure, indicating the presence of a large absorb-ing molecule perhaps a plastic material formed atthe same time as C,,, from graphite. At about 750Cthe glower was switched off and infrared emissionfrom the cell was observed. Several emission spectrawere recorded as the cell was heated to a maximumtemperature of about, 950C. At this temperature,the intensity of the CbOemission features started todecrease dramatically, and did not reintensify as thefurnace was cooled.

After cooling the furnace to near ambient tem-peratures and removing the quartz tube, it was foundthat the toluene-soluble crystalline material was de-posited in rings near the ends of the tube, corre-sponding to the lower temperature zones of the fur-nace. In the center, were the original C,, sample wasplaced, a substantial pile of a black powder wasfound. This proved to be toluene-insolubleand anX-my powder analysis indicated that this powder wasamorphous carbon. This suggests that Cho may re-vert to a soot-like carbon near 950C. Note that smallamounts of air leaked into the cell which could alsocontribute to the transformation of the initial ma-terial. The presence of impurities is confirmed bystrong emission features due to CO, and CH+ Thisobserved transformation may be of significance if CbOemission or absorption can be observed in astro-nomical sources, since this would set an upper limitto the local temperature.

The emission spectrum of C60/C70 mixture isshown in figs. la and lb. The IR bands attributed toCb,, are shown with the horizontal scaleexpanded bya factor of 6 in fig. 2. The band frequencies and theirfull-width at half maximum (fwhm ) are reported intable 1. Also a comparison with the solid-state spec-

Mode Gas phasethis work Solid statethis work ref. [2]Argon matrixref. [ I I ]

1 527.1 ( I1 ) =) 527.0 528 530.1(I) n)2 570.3 (13) b) 576.6 577 579.3 (2)3 1169.1 (13) 1182.7 1183 1184.8(2)4 1406.9 (12) 1429 1431.9 (4)*) The full-width at half maximum is reported in parentheses.) This band shows some structure (fig. 2) reminiscent ofa P (563.1 cm-), Q (570.3 cm-) and R (575.3 cm-) contour.

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Volume 176,number 6 CHEMICALPHYSICSLETTERSa

1 February 199 1

I,/,,LLLIIi._..l~~ .1/. .I..Sii...600 soa 700 900 900 1 0 0 0 1 1 0 0 120ocm*

b

cm-1 I.300 1 4 0 0 1500 1600 1100 1666 1966 2660

Fig. 1. Low dispersion emission spectra of Cw/C70mixture. (a) The 500 -1300 cm- region limited by an InSb tilter. (b) The 12%2000cm- region using an InAs filter. Note the presence in the spectra of other emission features.

tntm is shown in this table. Sm all blue shifts of about20 cm- are not inconsistent with matrix shifts forother molecules.

There is little evidence of rotational structure. Theonly ChOeature wh ich shows a structure rem iniscentof P, Q, and R branches is the band at 570.3 cm-Thisvibrational band has a maximum at 57 0.3 cm-and lower intensity shoulders at 563.1 and 575.3cm- However, this structure could be due to theoverlap of other un know n infrared ban ds. The solidphase spectrum of this band is also asymmetric.

Besides the very sma ll rotational constant (B=0.0028 cm -I), several factors can contribute to thecomplexity of the spectrum. At high temperatures,not only are ho t bands quite likely to be imp ortant,but rotational quantum numbers approaching 1000are predicted. W hile this d iscussionhas assum ed thatthe species responsible s C6 ,,, he presence of CT 0mayalso contribute overlapping bands. In ad dition, C6 ,,is primarily composed of 2C 6o (51% ) and 2C 593C(34%) isotopom ers, assum ing that % has a naturalabund ance of l.lW. T he 12C s,3C isotopomer hasmuch lower symmetry (C,) than *CbO , n ad dition506

to being a slightly asymm etric top, and thus m orecomplex spectral structures migh t be expected.The peak po sitions of the infrared ban ds of C69 e-ported in table 1 will differ s lightly from the actua lgas-pha se band origins. Because of vibrational an-harmonicity the hot b ands w ill contribute more in-tensity to the red side of the band origins for the hotCeoband contours. The extents of the red shifts aredifficult to calculate since the vibrational anhar-monicities are unknown. Experimentally the posi-tion of the band at 1169.1 cm- shifts by about 2cm- when the temperature is changed by 200C.This sug gests hat the actual band o rigin may be asmuch as 10 cm- to the blue, close to the Ar matrixvalue of 1184 .8 cm-

Surprisingly, he w idths of the infrared bands arechanged very little by the presence of hot bands.Weeksand Harter [ 171 calculate that the four strongband s of C ,, will have full-widths at h alf maximumof 5.5, 7.5, 6.7, and 10.7 cm- at 293 Knot includ-ing hot band contributions. At our temperature of1065K the expected bandw idthsare 10.5, 14.3, 12.8,and 10.3 cm- compared to the observed widths of


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Volume 176,number 6 CHEMICALPHYSICSLETTERS I February 1991

,%a0 1100 ~115 1150 m-1Fig. 2. The infrared emission bands attributed to C,. The hori-zontal scale was expanded by a factor of six relative to fig. 1. Notethe structure associated with the band at 570.0cm-.

11, 13, 13, and 12 cm- for modes 1-4 respectively,assuming that the band widths scale as the squareroot of the ratio of the temperatures. The presenceof 174 slightly anharmonic modes in C& drains thepopulation from the ground vibrational level whichshifts the band positions but leaves the widths of thecontours unchanged. In fact the shift and width ofthe bands can be used as a thermometer for labo-ratory and astronomical sources.Numerous additional weak emission features, forexample, at 638.3, 670.9, 951.3 and 1539.3 cm-,probably attributable to CYo or some IR allowedcombination bands of CsOare also found. While im-purities cannot be ruled out, these features are alsopresent in the solid-state spectrum of Kr%tschmer etal. [2 1.As expected, these frequencies are shifted tothe red in the gas phase spectrum. A strong emissionband of unknown identity appeared around 1010.2cm- (with a fwhm of 32 cm-) at elevated tem-

peratures. The intensity of this band decreased as thefurnace temperature approached the maximum andcompletely disappeared as the cell cooled down.

Finally, none of these gas phase infrared bandscorrespond to any of the strong emission bands ab-served so far in astronomical sources. However, webelieve that our measurements will facilitate thesearch for C& in space.

We wish to thank the National Optical AstronomyObservatories, operated by the Association of Uni-versities for Research in Astronomy Inc. under con-tract with the National Science Foundation, for al-lowing us to use the McMath Fourier transformspectrometer. We also thank J. Wagner and G. Laddfor assistance in obtaining the spectra. We would alsolike to thank Dr. Hall, Dr. Padias, Dr. Tun-Fun Wayfrom the Department of Chemistry for their help inthe production and the analysis of the CeOsample.This research was supported by the AstronauticsLaboratory, Edwards Air Force Base, CA, and theNASA Origins of the Solar System Research Program.

References[I ] H.W. Kroto, J.R. Heath, S.C. OBrien,R.F. Curl and R.E.

Smalley,Nature318 (1985) 162.[2] W. Krltschmer, L.D. Iamb, K. Fostiropoulos and D.R.Huffman, Nature 347 (1990) 354.

[3] J.R. Heath, R.F. Curl and R.E. Smalley,J. Chem. Phys. 87(1987) 4236.[4] W. Kr&chmer, K. Fostiropoulosand D.R. Huffman,Chem.Phys. Letters 170( 1990) 167.[S] H. Ajie, M.M. Alvarez, S.J. Anz, R.D. Beck, F. Diederich,K. Fostiropoulos, W. Kr;itschmer,Y. Rubin, D. Sensharmaand R.L. Whetten, J. Phys. Chem., submitted forpublication.

[6] S. Larsson, A. Volosov and A. Rotin, Chem. Phys. Letters137 (1987) 501.[7] R.F. Curl and R.E. Smalley,Science 242 (1988) 1017.[81 S.H. Yang,C.L. Pettiette, J. Conceicao,0. Cheshnovskyand

R.E. Smalley,Chem. Phys. Letters 139( 1987) 233.[9] D.L. Lichtenberger, K.W. Nebesny, C.D. Ray, D.R.Huffman and L.D. Lamb, Chem. Phys. Letters 176 (1990)

203.[lo] D.S. Bethune,G. Meijer, W.C.Tang and H.J. Rosen, Chem.Phys. Letters 174 (1990) 219.[ I I] R.E. Haufler, J. Conceicao, L.P.F. Chibante, Y. Chai, N.E.Byrne, S. Flanagan, M.M. Haley, S.C. OBrien,C. Pan, Z.Xiao, W.E. Billups, M.A. Ciufolini, R.H. Hauge, J.L.

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Volume 176,number 6 CHEMICALPHYSICSLETTERS I February 1991Margrave,L.J. Wilson, R.F. Curl and R.E. Smalley,J. Phys.Chem., accepted for publication.[121Z.C. Wu, D.4. Jelski and T.F. George, Chem. Phys. Letters137 (1987) 291.[131R.E. Stanton and M.D. Newton, J. Phys. Chem. 92 (1988)2141.[14 Z. Slanina.J.M. Rudzinski,M. Togasiand E. Osawa,J. Mol.Struct. 202 (1989) 169.[ 151 W.E. Harter and D.E. Weeks, J. Chem. Phys. 90 ( 1989)4721.

[ 161D.E. Weeks and W.G. Harter, 3. Chem. Phys. 90 ( 1989)4744.[ 171D.E. Weeks and W.G. Harter, Chem. Phys. Letters 176(1990) 209; private communication[181 .L. Ballester, P.R. Antoniewicz and R. Smoluchowski,Astrophys. J. 356 ( 1990) 507.[ 191A.H.H.Chang,WC. Ermlerand R.M. Pitzer, in preparation.

[20] C.I. Frum, R. Engleman Jr. and P.F. Bemath, J. Chem.Phys., in press.

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c.i. frum et al- the infrared emission spectrum of gas-phase c60 (buckminsterfullerene)

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