p. f. bemath- laboratory astrophysics and molecular astronomy of pure carbon molecules

8/2/2019 P. F. Bemath- Laboratory Astrophysics and Molecular Astronomy of Pure Carbon Molecules

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A&. Space Rcs. Vol. 15, No. 3, pp. (3)15-(3)23, 1995Copyright 1994 COSPARPrinted in Great Britain. All rights reserved.0273-I 177/9s $7.00 + 0.00

LABORATORY ASTROPH YSICS ANDMOLECULAR ASTRONOMY OF PU RECARBON MOLECULESP. F. BemathDepurtments of Chemistry and Physics, University of Waterloo, Wuterloo,Onturio, Canada N2L 3GI and Department of Chemistry, University ofArizona. Tucson, AZ 85721, U.S.A.

ABSTRACTThe pure carbon molecules C,, are currently of great experimental and theoretical interest. Our workin this area begins with detection of the Sic molecule, which is isovalent with q. New infraredelectronic transitions of C, and C, were discovered by emission spectroscopy of hydrocarbon dicharges.The C, and C, molecules were found by infrared vibration-rotation spectroscopy of the prototypicalobscured carbon star, IRC+10216. C, and C, were searched for in the same source, btit not found.The laboratory infrared emission spectrum of C, was recorded to aid in a search for C,, inextraterrestrial sources.

INTRODUCTIONPure carbon molecules are currently of great interest in many areas including chemistry, physics andastronomy. Pure carbon molecules can be in the form of chains, rings, sheets or spheroidal fullerenes/l/. In fact the recent discovery of C, and related molecules has created a new subfield in science.The infrared part of the electromagnetic spectrum is perhaps the most useful region for the study ofpure carbon molecules. Many of these molecules are highly symmetric and, therefore, lack dipolemoments necessary for pure rotational spectroscopy. The electronic spectra of the larger pure carbonmolecules also tend to be broad and relatively featureless. The infrared spectra in contrast are unique,sharp and very diagnostic.The infrared region of the electromagnetic spectrum has also recently joined the microwave and opticalregions as one of the primary winddws for astronomical observations. A similar revolution hasoccurred in laboratory spectroscopy, driven by new detectors, new materials, infrared lasers and Fouriertransform spectrometers. In support of infrared astronomy, new laboratory measurements arc required,particularly for pure carbon molecules.Our work begins with discovery of the Sic moIecule /21 by Fourier transform infrared spectroscopyof an electronic transition. Since Sic is isovalent with C,, we began to look for new infraredtransitions of C,. Two new infrared electronic transitions of C, were discovered /3,4/. A new triplettransition of C, was also located in the infrared region /5/. In parallel with this work, C, and C, weredetected /6,7/ in the obscured carbon star IRC+10216 by vibration-rotation spectroscopy. A laboratoryinfrared emission spectrum of C,, was also recorded /8/ to provide gas-phase band positions for asearch for C, in space.

INFRARED ELECTRONIC SPECTROSCOPYSurprisingly many molecules have electronic transitions at infrared wavelengths. Closed-shell, stable

(3115


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(3117

,I +3c 9 C2

Ahv,=3590 cm- 3 +- u

Phil lipsSystem I

Voo=8268 cm-IIIII

Xc,+-

IIBalli k - Ramsay

System IISo= P632cd

IU3rlFig. 2. Energy level diagram for the low-lying states of the C, molecule.


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(318 P.F.Bernathmolecules typically have ultraviolet electronic transitions. However, many transient species such asfree radicals, high temperature molecules and ions have infrared electronic transitions. Perhaps the bestknown astronomical examples are CN and q. The red system of CN (A*lYl-X*x+) nd the Phillipssystem of C, (A&-XC,> have been detected in many astronomical sources including comets,interstellar clouds, the Sun and stellar atmospheres. Although C, has a venerable spectroscopic history,its spectra remain surprisingly incomplete.Our work on C, began with the observation of the dC+-bll infrared electronic transition of theisovalent molecule Sic /2/. Sic and C, have similar energy level diagrams, with numerous low-lyingelectronic states (Figure 1).Sic is a very elusive molecule, although C, and Si, are well-known species. Many efforts had beenmade to detect Sic in the laboratory but, until our work, without success. The Sic molecule is alsopredicted by various models to be abundant in dense interstellar clouds and circumstellar atmospheresf21.The experimental problem is that when Sic solid is heated, Sic is only a trace constituent of the vapor.We circumvented this problem by sputtering Sic in a hollow cathode lamp. A mixture of Cu and Sicpowder was pressed to make a composite wall hollow cathode. The hollow cathode was operated at200 mA current with 1.5 torr of Ne gas. The near infrared emission (1800-9000 cm-) was monitoredwith the McMath Fourier transform spectrometer of the National Solar Observatory at Kin Peak.Although the main infrared molecular emission was due to q, some lines near 6100 cm- wereidentified as part of the dZ-bII transition of Sic. Our identifications were supported by ab initiocalculations carried out by D. McLean /2/.Soon after our initial work the Sic molecule was discovered in the circumstellar envelope ofIRC+10216 by radio astronomers /9/. In support of this identification the pure rotational spectrum wasrecorded by C. Gottlieb and co-workers /9/. These observations were very close to our predictions.The accurate microwave transitions, in turn. allowed us to assign another infrared electronic transition,A3C--X311,at 4500 cm- (2.2 microns) /lo/ (Figure 1).D. McLean pointed out to us that the transition corresponding to the dC+-brl transition in Sic wasunknown for q. This was very surprising since C, is such a well-known molecule. A search in aninfrared emission spectrum of a microwave discharge of CH, in He turned up the new transitionimmediately /3/. In C, (Figure 2) the triplet manifold lies slightly higher in energy than in Sic (Figurel), causing the names of the states to change. The ground state of C, is thus XC, and the firstexcited state is the a311U tate but otherwise the energy level patterns are identical. The B*Cs+-AllI,,transition of C, was found near 6900 cm- (1.45 microns) while the BAs-AllI, transition was near3580 cm-* (2.78 microns) /3/. At the same time the Phillips system (AIl,-XE,) was reanalysed toimprove the spectroscopic constants /4/.The new C2 transitions were also found in a microwave discharge of allene and argon. The newinfrared transitions, along with the Phillips and Ballii-Ramsay systems of C,, provide a rich andcomplex pattern of lines covering the entire near infrared region of the spectrum.The new transitions of q, BC,+-A& and BA,-A&, are moderately strong transitions, observedas readily as the well-known Phillips system. They seem to have been overlooked by spectroscopistsand astronomers alike. Since the Phillips system of C, is often observed (along with the Swan andBallii-Ramsay transitions), the B-A and B-A transitions should also be observable in sources such ascomets and stellar atmospheres.Polyatomic molecules such as C, can also have strong infrared electronic transitions. The history ofC, goes back more than one century to the observation of unidentified bands at 4050 A in a comet by


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Astrophysics and Astronomy of Pure Carbon Molecules (?)19

2n U6483 cm-(1.5@)

24676 cm-405OA Comet System17072 cm- (Neon Math)

(5856h

Fig. 3. The energy level diagram of the low-lying states of the C, molecule.


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Huggins in 1882 /ll/. The 4050 8, cometary system was conclusively identified as the AIH,-%Xsftransition of C, (Figure 3) by Douglas in 1951 /12/.Unlike the isovalent triangular molecule SiCJ13,14/ C, is a linear symmetric molecule. However,the bending potential of C, is very flat with a 63 cm- bending mode of vibration /15,16/.The C, molecule also has an infrared electronic transition as illustrated in Figure 3. The b3H -a3Htransition was detected first in absorption from the me&stable g3H, state by diode laser spectr&copGof electrical discharges of various hydrocarbons /5/. The new triplet transition was also seen inemission from the b311, state by Fourier transform spectroscopy /5/. The new C, transition was foundin the same emission spectra used for the C, work.The only previous information about a triplet state of C, was the report by Weltner and co-workersof long lived phosphorescence (20 msec) from C, isolated in argon matrices. They detected the Z311,-RZ, emission near 5900 8, /17/. The long lifetime of the me&table a3H, state permits infraredabsorption spectroscopy out of this state. The assignment of our new triplet-triplet transition was alsoconsistent with ab initio calculations /5/.Like q, C, occurs prominently in the emission spectra of comets /l l/. The observation of the b3H -Fi311, ransition of C, near 1.54 microns in comets is likely. In general infrared electronic transitio&are stronger (larger Einstein A values) than infrared vibration-rotation transitions. This favors thedetection of infrared electronic transitions.

VIBRATIONAL SPECTRA OF PURE CARBON MOLECULESC, is easily detected in comets by the observation of 4050 8, emission of the All-I,-%cg transition.However, the detection of C, in carbon stars and the interstellar medium is inhibited by the lack of UVflux at 4050 A. These astronomical sources are cool and dusty.C, has no dipole moment so it cannot be seen with radio telescopes. There are three infrared modesof vibration, vr, the symmetric stretch at 1224.5 cm-t, /18/ v2, the bending mode at 63.1 cm-l, /15,16/and v3 the antisymmetric stretching mode at 2040 cm- /17/. However, v, is Raman active, notinfrared active, and v2 occurs in the far infrared region which is opaque from the ground.(Astronomical observations of v2 are possible only from satellites or from the flying KuiperAstronomical Observatory.) The antisymmetric stretching mode, v3. is ideal for monitoring C, becauseit has a remarkably strong transition dipole moment (0.44 D /19/) and occurs in a spectral region wherethe atmosphere is relatively transparent.The infrared source IRC+10216 is the brightest source in the sky at 5 microns. This object is a coolcarbon star (-2000K) with a dusty expanding envelope full of many molecules /20/. A high resolution(0.014 cm-) spectrum of IRC+10216 was recorded with the coude Fourier transform spectrometer ofthe 4 meter telescope of Kitt Peak National Observatory /6/. The spectrum was limited to 1975 to2050 cm- by a cold band pass filter.Strong, sharp absorption features near 2040 cm- were assigned to the antisymmetric mode, v3, of C,/6/. Although no laboratory spectrum was available when the work was carried out, the v3 mode couldbe identified with the help of an infrared matrix spectrum /17/ and the ground state constants availablefrom the All&-%Xs electronic transitions /15/. At the same time as our astronomical paper waspublished, the laboratory spectrum of v3 was published by Hirota and co-workers /21/.The C molecules were found to have a rotational temperature of about 40 K and a column density of1x1013molecules/cm /6/. These facts are consistent with the production of C, in the outer regionsof the circumstellar envelope. The C, molecules could be made by the photochemical degradation oflarger parent molecules or grains by the interstellar ultraviolet flux. C, molecules could also be formed


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Astrophysics and Astronomy of Pure Carbon Molecules (WIby the condensation of C atoms in the gas phase or on grains.The detection of strong C, lines in IRC+10216 inspired us to search for C!,. At the time of our search,the only laboratory spectrum securely attributed to C, was the strong vg mode at 2164 cm- in an argonmatrix /22/. Our astronomical search was successful /7/ and guided by our observations, two laboratorygroups /23,24/ confirmed our detection.The C, molecules were found in a spectrum obtained at 0.01 cm- resolution in a 218 minuteintegration on IRC+10216. A cold blocking filter limited the spectral region to 2115 to 2195 cm-region and the moon was used to provide a reference telluric spectrum.The C, molecules have a column density of 1~10~ molecules/cm2, about an order of magnitude lessabundant than C, /7/. It is interesting to note that the detection limits in infrared astronomy nowcompare favorably with those achieved in radio astronomy.The rotational temperature of C, is about 40 K in IRC+10216. Like C,, C, could be one of thebuilding blocks of carbon chemistry or one of the photofragments of carbonaceous materials. It is mostlikely that C, is formed by photochemical processes in the outer part of the envelope. The ultravioletflux present in the interstellar medium can only penetrate into the outer portions of the dusty shell ofIRC+10216.After our success with C, and C,, the next two carbon chain species C, and C, were also searchedfor in IRC+10216. Infrared laboratory measurements of these molecules are available from theBerkeley group of Heath, Saykally and co-workers /25,26/. Even with these accurate laboratory linepositions, we were unable to find C, and C,.The even members of the carbon chain family C,, C, and C, have triplet ground states (R3C,-) andare less stable than the odd members (C,, C,, C, and C,) /27,28/. The strongest infrared transition ofC, at 1549 cm- is also blocked by telluric water vapor absorption.The presence of larger pure carbon molecules in astronomical sources such as IRC+10216 is alsopossible. In fact the original measurements on C,, by mass spectrometry in 1985 were motivated byastronomical considerations /l/. The recent isolation of macroscopic samples of C!,, and C,, /30/ werealso inspired by the possibility of astronomical detection.The C,, molecule has such high symmetry (Ih, icosahedral) that only four of the vibrational modes areinfrared active. These four modes, at 528 cm-, 577 cm-t, 1183 cm-, and 1429 cm- were reportedin the original paper of KMschmer, Lamb, Fostiropoulos and Huffman in 1990 /30/. Of these modesonly the 1183 cm- mode is suitable for ground based atronomical observation through the 10 micronwindow. Unfortunately these numbers apply to a solid sample of C,,, while C,, is expected to occuras a gas-phase molecule in space.In order to provide gas-phase values for the infrared modes of vibration of C,,, emission spectra wererecorded at Kitt Peak. A sample of C,, was heated to 900C in a tube furnace and the hot molecularemission was monitored by high resolution Fourier transform spectroscopy. There was no sign ofrotational structure but the band positions and widths could be determined /8/. For example, the modeat 1183 cm- was found at 1169.1 cm- at 800C in the gas phase /8/.Armed with the gas-phase band position of C,, at 8.554 microns, a search for C,, in the sourceIRC+10216 was undertaken in collaboration with K. Hinkle. The spectra of IRC+10216 do not showthis band. The presence of large amounts of hydrogen may prevent the formation of C,, in this source.

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(3)22 P. F. BernathREFERENCES1. H.W. Kroto, Angew. Chem. Int. Ed. 31, 111 (1992).2. P.F. Bemath, S.A. Rogers, L.C. OBrien, CR. Brazier and A.D. McLean, Phys. Rev. Lett.

60, 197 (1988).3. M. Douay, R. Nietmann and P.F. Bemath, J. Mol. Spectrosc. 13 1, 261 (1988).4. M. Douay, R. Nietrnann and P.F. Bemath, J. Mol. Spectrosc. 131, 250 (1988).5. H. Sasada. T. Amano, C. Jarman and P.F. Bemath, J. Chem . Phy s. 94, 2401 (1991).6. K. Hinkle, J.J. Keady and P.F. Bemath, Science, 241, 1319 (1988).7. P.F. Bemath, K.H. Hinkle and J.J. Keady, Science, 244, 562 (1989).8. C.I. Frum, R. Engleman, Jr., H.G. Hedderich, P.F. Bemath, L.D. Lamb and D.R. Huffman,

Chem. Phys. Lett. 176, 505 (1991).9. J. Cemicharo. CA. Gottlieb, M. Guelin, P. Thaddeus and J.M. Vrtilek, Ast rophys. .I. Lett .

341, L25 (1989).10. C.R. Brazier, L.C. OBrien and P.F. Bemath, J. Chem . Phy s. 91, 7384 (1989).11. W. Huggins, Proc. R. Sot. London, 33, 1 (1882).12. A.E. Douglas, Astrophys. .I. 114, 466 (1951).13. P. Thaddeus, S.E. Cummins and R.A. Linke, Ast rophy s. .I. 283, IA 5 (1984).14. D.L. Michalopoulos, M.E. Geusic, P.R.R. Langridge-Smith and R.E. Smalley, J. Chem.

Phys. 80, 3556 (1984).1.5. L. Gausset, G. Herzberg, A. Lagerqvist and B. Rosen, Astrophys. .I. 142, 45 (1964).16. C.A. Schtnuttenmaer, R.C. Cohen, N. Pugliano, J.R. Heath, A.L. Cooksy, K.L. Busarowand R.J. Saykally, Science, 249, 897 (1990).17. W. Weltner, Jr., and D. McDonald, Jr., J. Chem . Phys. 40, 1305 (1964).18. A.J. Merer, Can. J. Phy s. 45, 4103 (1967).19. W.P. Kraemer, P.R. Bunker and M.Yoshirnine, J. Mol. Spect rosc. 107, 191 (1984).20. E. Herb& Angew . Chem . Int . Ed. 29, 595 (1990).21. K. Matsurnura, H. Kanamori, K. Kawaguchi and E. Hirota, J. Chem. Phys. 89,3491(1988).22. M. VaJa, T.M. Chandrasekhar, J. Szczepanski, R. Van Zee, and W. Welmer, Jr., .I. Chem.

Phys. 90, 595 (1989).23. J. Heath, A. Cooksy, M. Gruebele, C. Schmuttenmaer and R. Saykally, Science, 244,564(1989).


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Astrophysics and Astronomy of Pure Carbon M olecules W 2324. N. Moazzen-Abmadi, A.R.W. McKellar and T. Amano, J. Chem. Phys. 91, 2140 (1989).25. J.R. Heath and R.J. Saykally, J. Chem. Phys. 94, 1724 (1991).26. J.R. Heath and R.J. Saykally, J. Chem. Phys. 93, 8392 (1990).27. W. Weltner and R.J. Van Zee, Chem. Rev. 89, 1713 (1989).28. J.R. Heath and R.J. Saykally, J. Chem. Phys. 94, 3271 (1991).29. L.N. Shen and W.R.M. Graham, J. Chem. Phys. 91, 5115 (1989).30. W. Kr&&uner, L.D. Lamb, K. Fostiropoulos and D.R. Huffman, Nature, 347, 354 (1990).

p. f. bemath- laboratory astrophysics and molecular astronomy of pure carbon molecules

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