standard formula for rovibrational levels of a linear molecule (18). Three molecular pa- rameters, the vibrational band origin (V4) and the lower and upper state rotational constants (B" and B'), were determined by least-squares analysis, and the uncertainties were evaluated from the resulting covariance matrix. Statistical analysis indicated that the two rotational constants are highly correlat- ed (0.99). C3 is thought to be slightly quasi-linear (13, 14). Quasi-linearity is a result of a small barrier to linearity in the molecular bending potential (11). Molecules exhibiting this phenomenon are characterized by highly anharmonic low-frequency bending vibra- tions and strong stretch-bend interactions. Quasi-linearity manifests itself in the anti- symmetric mode of the C3 cluster in the average molecular structure becoming more compact (the rotational constant increases by 1.2%) upon excitation of the antisym- metric stretch vibration. The rotational con- stant of a normal linear molecule would be expected to decrease upon similar vibration- al excitation. C5 and C7 have low-energy bending vibrations and are also potentially quasilinear. Recent spectroscopic measure- ments on C5 indicate that its bending vibra- tions are reasonably harmonic, and no evi- dence for quasi-linearity is observed (15). The results presented here similarly do not evidence quasi-linearity in the case of C7. Upon excitation of the V4 vibration the rotational constant is found to decrease by about 0.35%, and thus the average length of the molecule increases. While this is not suggestive of quasi-linearity in C7, it does not rule it out either. The detection of the C5 and C3 clusters in the carbon star IRC+ 10216 by Bemath and co-workers (1, 19) with infrared astronomy highlights the importance of carbon clusters in astrophysical processes. The following ion-molecule reaction scheme has been pro- posed by Suzuki (20) for astrophysical pro- duction of carbon chains: C+ + Cn -> Cn+I+1 + +hv Cn+ I+ + H2-* Cn+ 1H+ + H (1) Cn,H+ H + e-* Cn+I + H Calculations done by Freed, Oka, and Su- zuki (21) compared unimolecular dissocia- tion rates versus rates for infrared emission. Cluster growth was shown to proceed at the Langevin rate for n . 4. This may explain the high C5 abundance observed in IRC+ 10216 (1). It follows from the model that C7 should also be abundant. The C7 absorption spectrum reported here was a factor of 3 to 5 less intense than the V3 spectrum observed for C5. This is expected if the oscillator strength of the V4 transition of C7 is similar to that of the V3 transition of C5. The observed decrease of absorption intensi- ty comes from an increased ground-state partition function for the larger C7 cluster and an expected slight decrease in the rela- tive density of C7 in the molecular beam (6, 22). If the interstellar C7/C5 abundance ratio is not too small, then the frequencies report- ed here will make astronomical observation of this molecule possible. This, in tum, will constitute an important step in understand- ing the nature and distribution of carbon in the interstellar medium. REFERENCES AND NOTES 1. P. F. Bemath, K. H. Hinkle, J. J. Keady, Science 244, 562 (1989). 2. Ph. Gerhardt, S. Loffler, K. H. Homann, Chem. Phys. Lett. 137, 306 (1987). 3. K. Matsumura et al., J. Chem. Phys. 89, 3491 (1988). 4. N. Moazzen-Ahmadi, A. R. W. McKellar, T. Amano, Chem. Phys. Lett. 157, 1 (1989). 5. J. R. Heath et al., Science 244, 564 (1989). 6. M. E. Geusic etal., J. Chem. Phys. 84, 2421 (1986). 7. S. H. Yang et al., Chem. Phys. Lett. 139, 233 (1987). 8. K. S. Pitzer and E. Clementi, J. Am. Chem. Soc. 81, 4477 (1959). T HHE STUDY OF CARBON CLUSTERS BY both theory and experiment has re- ceived a great deal of recent attention (1-3). These species have proven to be re- markably ubiquitous. They are observed, for example, in flames and electrical discharges, in carbon stars, and in laser-produced plas- mas of various polymers. They are implicat- ed in the process of soot formation and in the aggregation of interstellar dust grains. Many investigations have evidenced new closed structures of carbon composed of five and six membered rings. The elucidation of the roles of carbon clusters in these various contexts has been seriously impeded by the absence of spectral and structural data for them. In fact, detailed Department of Chemistry, and Materials and Chemical Sciences Division, University of California and Lawrence Berkeley Laboratory, Berkeley, CA 94720. 24 AUGUST 1990 9. K. Raghavachari and J. S. Binkley, J. Chem. Phys. 87, 2191 (1987). 10. W. Weltner, Jr., and R. J. Van Zee, Chem. Rev. 89, 1713 (1989). 11. L. Fusina and I. M. Mills, J. Mol. Spectrosc. 79, 101 (1980). 12. K. Kawaguchi et al., J. Chem. Phys. 91, 1953 (1989). 13. E. A. Rohlfing, ibid. 89, 6103 (1988). 14. C. A. Schmuttenmaer et al., Science 249, 897 (1990). 15. N. Moazzen-Ahmadi, A. R. W. McKellar, T. Amano, J. Chem. Phys. 91, 2140 (1989). 16. D. Karu et al., Appl. Opt. 29, 119 (1990). 17. K. L. Busarow et al., J. Chem. Phys. 89, 1968 (1988). 18. G. Herzberg, Infrared and Raman Spectra ofPolyatomic Molecules (Van Nostrand Reinhold, New York, 1945). 19. K. H. Hinkle, J. J. Keady, P. F. Bemath, Science 241, 1319 (1988). 20. H. Suzuki, paper presented at 13th Okazaki Confer- ence on Transient Ions and Their Roles in Interstel- lar Processes, Okazaki, Japan, 1981. 21. K. F. Freed, T. Oka, H. Suzuki, Astrophys. J. 263, 718 (1982). 22. J. Drowart et al., J. Chem. Phys. 31, 1131 (1959). 23. Supported by the Office of Naval Research (grant N0014-90-J-1368). J.R.H. thanks the Miller Foun- dation for support. A.L.C. thanks Search for Extra- terrestrial Intelligence for support. We thank J. Tartar and S. Chang for their continued support. 1 May 1990; accepted 20 June 1990 laboratory spectra have been measured only for C2, C2-, C2', C3, and C5, although both infrared and electron spin resonance data have been obtained for several other species from studies in cryogenic matrices (4). We are attempting to improve this situation. In a previous report (5) we described a general new technique for measuring carbon clus- ters with mid-infrared laser absorption spec- troscopy, and infrared (IR) spectra of the C5 cluster were presented as an initial demon- stration of the method. The detection of C5 in the carbon star IRC+ 10216 by infrared astronomy was presented at the same time by Bernath et al. (6). In this report, we describe a new experiment for measuring far-infrared (FIR) (10 to 350 cm-') spectra of carbon clusters with high precision (1 x 10-6) and high sensitivity, and present the first direct measurement of the astrophy- sically important 63 cm-' bending mode of REPORTS 897 Tunable Far-IR Laser Spectroscopy of Jet-Cooled Carbon Clusters: The v2 Bending Vibration of C3 C. A. SCHMUTITENMAER, R. C. COHEN, N. PUGLIANO, J. R. HEATH, A. L. COOKSY, K. L. BUSAROW, R. J. SAYKALLY Seven rovibrational transitions of the (01'0) *- (0000) fundamental bending band of C3 have been measured with high precision with the use of a tunable far-infrared laser spectrometer. The C3 molecules were produced by laser vaporization of a graphite rod and cooled in a supersonic expansion. The astrophysically important v2 fundamental frequency is determined to be 63.416529(40) cmn-. These measurements provide the basis for studies of C3 in the interstellar medium with far-infrared astronomy. on February 1, 2015 www.sciencemag.org Downloaded from on February 1, 2015 www.sciencemag.org Downloaded from on February 1, 2015 www.sciencemag.org Downloaded from on February 1, 2015 www.sciencemag.org Downloaded from