microwave spectra of 11 polyyne carbon … astrophysical journal supplement...

THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 129 :611È623, 2000 August2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.(

MICROWAVE SPECTRA OF 11 POLYYNE CARBON CHAINS

M. C. MCCARTHY, W. CHEN, M. J. TRAVERS, AND P. THADDEUS

Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 ; and Division of Engineering and Applied Sciences, HarvardUniversity, 29 Oxford Street, Cambridge, MA 02138Received 2000 January 21 ; accepted 2000 March 9

ABSTRACTA summary is given of the laboratory study of the rotational spectra of 11 recently detected carbon

chain molecules. ClassiÐed according to their various end groups, these are the cyanopolyynes HC15Nand the isocyanopolyynes and the methylcyanopolyynesHC17N; HC4NC HC6NC; CH3(C¹C)3CN,and and the methylpolyynesCH3(C¹C)4CN, CH3(C¹C)5CN; CH3(C¹C)4H, CH3(C¹C)5H,

and Measured line frequencies and derived spectroscopic constants areCH3(C¹C)6H, CH3(C¹C)7H.given for each. The microwave laboratory astrophysics of the entire set is now complete in the sense thatall the astronomically most interesting rotational transitions, including those with nitrogen quadrupolehyperÐne structure, have now been directly measured or can be calculated from the derived constants toa small fraction of 1 km s~1 in equivalent radial velocity. All 11 carbon chains are candidates for astron-omical discovery since they are closely related in structure and composition to ones that have alreadybeen discovered in space.Subject headings : ISM: molecules È line : identiÐcation È molecular data È molecular processes È

radio lines : ISM

1. INTRODUCTION

Highly unsaturated polyynes with alternating triple andsingle carbon-carbon bonds and cumulenes with successivedouble carbon-carbon bonds represent the dominant struc-tural theme of the nearly 100 polyatomic molecules so faridentiÐed in space, and many more can probably be foundonce rest frequencies have been measured in the laboratory.In a series of short papers we recently reported the detectionand spectroscopic characterization of the 11 new longpolyyne chains shown in Figure 1. All are calculated to behighly polar, and all are candidates for astronomical detec-tion because shorter chains of similar structure have alreadybeen identiÐed in space in sources such as Sgr B2, themolecular shell of the evolved carbon star IRC ]10216, orthe cold molecule-rich source TMC-1 in the Taurus cloudcomplex. With reÐnements in radio receivers and the avail-ability of larger and more powerful telescopes and arrays,many, if not all, may eventually be found. The purpose ofthis Supplement article is to provide in one place a conciseand useful summary of our laboratory results, includingtabulations of measured line frequencies and derived spec-troscopic constants not contained in the brief previousaccounts of the present work. Table 1 is a brief overview,giving a summary of the laboratory references, the pro-duction methods employed, and the frequency bands thathave been covered in the laboratory.

As Figure 1 shows, the 11 polyynes are similar in struc-ture, di†ering only in the end groups that terminate theircarbon chain backbones ; they are (1) cyanopolyynes, (2)isocyanopolyynes, (3) methylcyanopolyynes, and (4) methyl-polyynes. All are closed-shell molecules with fairly simplerotational spectra characterized by transitions that areseparated by harmonic intervals. In the absence of nitrogenquadrupole hyperÐne structure (hfs), the radio spectra of allcan be calculated to high precision from the standardexpression for the rotational transitions of a symmetric topmolecule :

lJ?J~1\ 2BJ[ 4DJ3 [ 2D

JKJK2 , (1)

where J is the angular momentum quantum number for theupper level of the transition, K is that for the component ofangular momentum along the symmetry axis, and B, D, and

are the rotational and two leading centrifugal distor-DJKtion constants. For the molecules here, is nonzero onlyD

JKfor the symmetric tops with methyl terminations. Thederived spectroscopic constants of each polyyne are sum-marized in Table 2. For and the two shortestHC4NCmethylcyanopolyynes, line centroids can be calculated fromthe rotational and centrifugal distortion constants in Table2 and the nitrogen hfs from standard expressions for thehyperÐne energies and intensities (Townes & Schawlow1955).

As in a previous Supplement article on carbon chains(McCarthy et al. 1997b), nothing is added here with respectto the identiÐcations of the molecules in question. That wasa crucial consideration in our original discovery papers, andno new information has come to light to cause us to ques-tion any of the identiÐcations ; the original assignments willtherefore be assumed without further discussion. We havealso omitted much in the way of experimental details,except for those reÐnements in the Fourier transformmicrowave (FTM) spectrometer (Appendix A) and the dis-charge nozzle source (Appendix B) that were important forthe present work. We conclude with some general obser-vations about the whole set of newly discovered carbonchains.

2. CYANOPOLYYNES

Cyanopolyynes are the most readily observedHC2nCNand the most numerous class of carbon chains in space.Those up to have been detected in at least oneHC11Nastronomical source (Bell et al. 1997), and rest frequenciesfor the next longer one, are available (Travers et al.HC13N,1996). With improvements in production efficiency anddetection sensitivity, we have now detected the rotationalspectra of the next two in the series, andHC15N HC17N.

The main reÐnements we have made in our productionscheme are the use of diacetylene as a precursor gas(HC4H)

611

612 MCCARTHY ET AL. Vol. 129

FIG. 1.ÈMolecular geometries of the present carbon chains, showing the characteristic alternating triple and single carbon-carbon bonds of thepolyynes.

and Ne as a bu†er gas. When cyanoacetylene is(HC3N)used in combination with diacetylene rather than acetylene,line intensities of and increase by a factor ofHC11N HC13Nabout 4, and when Ne is used instead of Ar as the bu†er gasthey increase by another factor of about 2. The conditions

for optimal production of and are similarHC15N HC17Nto those now used for and shorter cyanopolyynes : aHC13Nmixture of 0.5% of cyanoacetylene and 0.5% diacetylene inNe, a discharge in the throat of the supersonic nozzle ofabout 1900 V and 50 mA, a gas pulse of 300 ks length at a

No. 2, 2000 SPECTRA OF 11 POLYYNE CARBON CHAINS 613

TABLE 1

SUMMARY OF PRESENT POLYYNE DETECTIONS

Frequency BandMolecule Precursor Gasesa (GHz) Reference

Cyanopolyynes :HC15N . . . . . . . . . . . . . . . . . HC3N/HC4H 5È11 1HC17N . . . . . . . . . . . . . . . . . HC3N/HC4H 5È7 1

Isocyanopolyynes :CH3C2CN/HC4H,

HC3N, HC3N/HC4H,HC4NC . . . . . . . . . . . . . . . . or (CN)2/HC4H 8È20 2

CH3C2CN/HC4H,HC3N, HC3N/HC4H,

HC6NC . . . . . . . . . . . . . . . . or (CN)2/HC4H 10È18 2Methylcyanopolyynes :

CH3(C¹C)2CNb . . . . . . CH3C2CN 6È18 3CH3(C¹C)3CN . . . . . . . CH3C2CN 6È22 3CH3(C¹C)4CN . . . . . . . CH3C2CN 8È14 3

CH3C2CN/HC4HCH3(C¹C)5CN . . . . . . . or HC3N/HC4H 6È12 3

Methylpolyynes :CH3(C¹C)4H . . . . . . . . . CH3C2H/HC4H 9È16 4CH3(C¹C)5H . . . . . . . . . CH3C2H/HC4H 8È11 4CH3(C¹C)6H . . . . . . . . . CH3C4H/HC4H 5È11 5CH3(C¹C)7H . . . . . . . . . CH3C4H/HC4H 5È8 5

a All precursor gases diluted in Ne.b Previously studied by Alexander et al. 1978.REFERENCES.È(1) McCarthy et al. 1998a ; (2) Botschwina et al. 1998 ; (3) Chen et al.

1998 ; (4) Travers et al. 1998 ; (5) Chen et al. 1999.

repetition rate of 6 Hz, and a total gas pressure behind thenozzle of 2.5 atm.

2.1. HC15NEighteen lines of were measured to an accuracyHC15Nof 0.1 km s~1 between 5 and 11 GHz (Table 3). Although

is somewhat less abundant than in ourHC15N HC13Nmolecular beam (by a factor of about 3), integrations of only10È20 minutes yielded lines with high signal-to-noise ratio,as the sample line in Figure 2a demonstrates. As shown in

the energy level diagram in Figure 3, the measured linescover a fairly wide range of rotational levels, with J ] J [ 1from J \ 38 to 71. There is no evidence for quadrupole hfsfrom the 14N nucleus in our spectra, and none isHC15Nexpected : hyperÐne splittings are of order eqQ/4J2 and aretherefore less than 1 kHz for the lowest observed J tran-sitions on the assumption that eqQB [4.3 MHz, the valuefor the shorter cyanopolyynes, (La†erty & LovasHC3N1978), (Winnewisser, Creswell, & Winnewisser 1978),HC5Nand (McCarthy et al. 2000).HC7N

TABLE 2

SPECTROSCOPIC CONSTANTS (IN MHz)

Molecule B D] 106 DJK

] 103 eqQ

Cyanopolyynes :HC15N . . . . . . . . . . . . . . . . . . . . . 71.950133(6) 0.0369(9)HC17N . . . . . . . . . . . . . . . . . . . . . 50.70323(6) 0.025(7)

Isocyanopolyynes :HC4NC . . . . . . . . . . . . . . . . . . . . 1401.18227(7) 34.3(9) 0.96(2)HC6NC . . . . . . . . . . . . . . . . . . . . 582.5203(1) 5.4(3) \1.0

Methylcyanopolyynes :a . . . . . .CH3(C¹C)2CNb . . . . . . . . . . 778.03974(4) 9.2(2) 4.37(2) [4.25(3)CH3(C¹C)3CN . . . . . . . . . . . 374.72127(1) 1.61(2) 1.38(1) [4.2(1)CH3(C¹C)4CN . . . . . . . . . . . 208.73699(2) 0.422(9) 0.543(8)CH3(C¹C)5CN . . . . . . . . . . . 128.0723(2) 0.1665(6) 0.21(1)

Methylpolyynes : a . . . . . . . . . . . .CH3(C¹C)4H . . . . . . . . . . . . . 376.71252(2) 1.55(2) 1.382(9)CH3(C¹C)5H . . . . . . . . . . . . . 210.23883(3) 0.46(3) 0.566(8)CH3(C¹C)6H . . . . . . . . . . . . . 129.07609(2) 0.134(6) 0.25(1)CH3(C¹C)7H . . . . . . . . . . . . . 84.86220(3) 0.05c

NOTE.ÈUncertainties (in parentheses) are 1 p in the last signiÐcant digit.a Constants derived on the assumption that the A rotational constant is 157 GHz.b Previously studied by Alexander et al. 1978.c Scaled from CH3(C¹C)6H.

TABLE 3

MEASURED ROTATIONAL TRANSITIONS OF ANDHC15N HC17N

HC15N HC17N

Frequency O[C Frequency O[CJ@] J (MHz) (kHz) (MHz) (kHz)

38 ] 37 . . . . . . 5468.202 039 ] 38 . . . . . . 5612.102 040 ] 39 . . . . . . 5756.001 041 ] 40 . . . . . . 5899.901 042 ] 41 . . . . . . 6043.800 043 ] 42 . . . . . . 6187.700 044 ] 43 . . . . . . 6331.599 045 ] 44 . . . . . . 6475.499 046 ] 45 . . . . . . 6619.398 048 ] 47 . . . . . . 6907.196 [149 ] 48 . . . . . . 7051.096 050 ] 49 . . . . . . 7194.995 051 ] 50 . . . . . . 7338.894 055 ] 54 . . . . . . 5577.339 059 ] 58 . . . . . . 5982.960 060 ] 59 . . . . . . 6084.366 061 ] 60 . . . . . . 6185.770 [162 ] 61 . . . . . . 6287.178 163 ] 62 . . . . . . 6388.582 064 ] 63 . . . . . . 9209.578 0 6489.984 [265 ] 64 . . . . . . 9353.477 0 6591.391 [166 ] 65 . . . . . . 9497.375 0 6692.800 270 ] 69 . . . . . . 10072.968 071 ] 70 . . . . . . 10216.866 0

NOTES.ÈEstimated experimental uncertainties are 2 kHz. Thedi†erence between observed and calculated frequencies (O[C)are with respect to those calculated from the best-Ðt constants inTable 2.

TABLE 4

MEASURED ROTATIONAL TRANSITIONS OF ANDHC4NC HC6NC

HC4NC HC6NC

Frequency O[C Frequency O[CJ@] J F@] F (MHz) (kHz) (MHz) (kHz)

3 ] 2 . . . . . 4 ] 3 8407.078 03 ] 2 8407.090 02 ] 1 8407.138 0

4 ] 3 . . . . . 5 ] 4 11209.442 04 ] 3 11209.450 13 ] 2 11209.468 [2

5 ] 4 . . . . . 6 ] 5 14011.800 05 ] 4 14011.806 04 ] 3 14011.819 2

6 ] 5 . . . . . 7 ] 6 16814.153 [16 ] 5 16814.157 [15 ] 4 16814.166 1

7 ] 6 . . . . . 8 ] 7 19616.502 07 ] 6 19616.504 [16 ] 5 19616.510 0

9 ] 8 . . . . . 10485.350 010 ] 9 . . . . 11650.385 111 ] 10 . . . 12815.420 212 ] 11 . . . 13980.447 [313 ] 12 . . . 15145.480 014 ] 13 . . . 16310.508 [115 ] 14 . . . 17475.538 2

NOTES.ÈEstimated experimental uncertainties are 1 kHz. O[C thesame as in Table 3.

FIG. 2.ÈSample FTM spectra. (a) Spectrum of the J \ 45 ] 44 ofshowing the characteristic double-peaked line shape that resultsHC15Nfrom the Doppler splitting of the Mach 2 supersonic molecular beam

relative to the two traveling waves that compose the confocal mode of theFabry-Perot cavity. (b) The J \ 4 ] 3 transition of showingHC4NCresolved nitrogen quadrupole hyperÐne structure. (c) The J \ 15 ] 14transition of showing the distinctive, tightly spaced KCH3(C¹C)4Hdoublet rotational pattern of the methylsymmetric top molecules.

SPECTRA OF 11 POLYYNE CARBON CHAINS 615

FIG. 3.ÈLower rotational levels of and showing theHC15N HC17N,transitions measured in the laboratory (vertical bars).

The rotational constant and the leading centrifugal dis-tortion constant were determined by Ðtting the two freeparameters in equation (1) to the measured lines (Table 3),yielding a Ðt rms comparable to the measurement uncer-tainty of about 2 kHz. In the frequency range measured, thenext term H in the centrifugal expansion is expected tomake a negligible contribution (of order 1 Hz or less). Onlyat very high rotational transitions, where J approaches1000, will neglect of this term result in an error as large as1 km s~1 in equivalent radial velocities.

2.2. HC17NFor this cyanopolyyne, nine lines were measured in the

frequency range from 5 to 7 GHz; they are systematicallyabout 3 times weaker than those of and are fairlyHC15N

closeÈwithin about a factor of 2Èto the detection limit ofthe spectrometer, requiring detection integration times ofabout 1 hr each. Relative to the shorter cyanopolyynes,however, is surprisingly abundant in our supersonicHC17NbeamÈan order of magnitude more so than predicted byextrapolation from the shorter cyanopolyynes HC3Nthrough Its measured lines (Table 3) are well repro-HC9N.duced by the expression in equation (1), but owing to thenarrow range of transitions from J \ 55 to 66 (Fig. 3), thecentrifugal distortion constant could only be determined toabout 30%.

3. ISOCYANOPOLYYNES

Although the cyanopolyynes have been the subject ofmuch laboratory and radioastronomical study, relativelylittle is known about their isomers, the isocyanopolyynes.Until now, only the two shortest, HNC and HCCNC, havebeen detected in laboratory discharges (Saykally et al. 1976 ;

et al. 1991) and in astronomical sources (Snyder &Kru� gerBuhl 1971 ; Kawaguchi et al. 1992). The presence of theseenergetic isomers in interstellar clouds is important becauseit provides a striking example of how far chemical processesthere can depart from thermal equilibriumÈand a goodillustration as well of the crucial role ion-molecule reactionsplay in the formation of interstellar molecules (Watson1974 ; Herbst 1978). By careful adjustment of the laboratorydischarge chemistry for HCCNC, we have been able todetect the next two isocyanopolyynes andHC4NC HC6NCwith our FTM spectrometer. Astronomical detection ofthese polar isomers may help clarify the role of speciÐcion-molecule reactions in the formation of longer cyano-polyyne and isocyanopolyyne chains.

The most intense lines of and were pro-HC4NC HC6NCduced under nearly the same experimental conditions thatyielded the best cyanopolyyne spectra. As shown in Table 1,these isochains were made with three di†erent mixtures ofprecursor gases, each yielding comparably strong lines. Theisospectra at best were more than 100 times weaker thanthose of the more stable cyanopolyynes of the same size ;they are slightly more complicated than those of the longercyanopolyynes in this paper, owing to their shorter lengths,the comparatively low-J transitions studied, and theresolution of nitrogen hfs. The derived rotational, centrifu-gal distortion, and hyperÐne constants are given in Table 2.

3.1. HC4NCThe rotational transitions of one of which isHC4NC,

shown in Figure 2, have well-resolved quadrupole hfs overmuch of the frequency range accessible to our spectrometer ;the energy level diagram in Figure 4 shows the Ðve tran-sitions measured (Table 4). The quadrupole coupling con-stant of eqQ\ 0.96(2) MHz, is about 4 timesHC4NC,smaller than that of the cyanopolyynes and is of oppositesign, but it is close to that measured in HCCNC,eqQ\ 0.945(1) MHz Stahl, & Dreizler 1993).(Kru� ger,From the spectroscopic constants derived from the labor-atory data (Table 2), the radio spectrum of can beHC4NCcalculated to better than 3 ppm (1 km s~1 in equivalentradial velocity) up to 100 GHz in frequency.

3.2. HC6NCThe energy level diagram in Figure 4 also shows the

seven rotational transitions of the next longer iso-cyanopolyyne, that have been measured from 10HC6NC,

616 MCCARTHY ET AL.

FIG. 4.ÈLower rotational levels of and showing theHC4NC HC6NCtransitions measured in the laboratory (vertical bars).

to 18 GHz (Table 4). Even at our high resolution, a precisevalue for eqQ could not be determined, but the evidentbroadening of the rotational lines is consistent with a coup-ling constant eqQ\ 1.0 MHz. The lines of areHC6NCabout 20 times weaker than those of a decrementHC4NC,close to that observed for the cyanopolyyne series from

to Detection of and inHC5N HC7N. HC4NC HC6NCspace may be difficult, but searches should be aided by thehigh polarity of these chains, calculated (Botschwina et al.1998) to be [3.25 and [3.49 D, respectively.

4. METHYLCYANOPOLYYNES

The shorter methylcyanopolyynes andCH3(C¹C)CNare readily observable in the laboratoryCH3(C¹C)2CN

and have been found in the astronomical source TMC-1(Matthews & Sears 1983 ; Broten et al. 1984). Owing to thelack of laboratory data, longer chains have not yet beenfound in space, but those up to have beenCH3(C¹C)4CNincluded in models of interstellar clouds (Herbst &Leung 1989), where they are predicted to exist in appre-ciable quantities. Methylcyanopolyynes have a distinctiverotational signature at low rotational temperature, speciÐ-cally, doublets produced by the closely spaced rotationaltransitions from the K \ 0 and K \ 1 levels (see Fig. 2c).The ortho-para nuclear spin statistics of these symmetrictops with three equivalent methyl group protons forbidradiative transitions between levels of di†erent spin sym-metry. As a result, the K \ 1 para levels, which lie over 7 Kabove the K \ 0 ortho levels, are comparably populated to

TABLE 5

MEASURED ROTATIONAL TRANSITIONS OF CH3(C¹C)2CN

Frequency O[CJ@ ] J K F@] F (MHz) (kHz)

4 ] 3 . . . . . . . . 1 4] 3 6224.175 10 3] 2 6224.223 [1

4 ] 3 6224.315 [15 ] 4 6224.348 0

5 ] 4 . . . . . . . . 1 5] 4 7780.296 00 4] 3 7780.341 [1

5 ] 4 7780.394 16 ] 5 7780.414 [1

6 ] 5 . . . . . . . . 1 6] 5 9336.385 [10 5] 4 9336.437 0

6 ] 5 9336.468 [17 ] 6 9336.485 0

7 ] 6 . . . . . . . . 1 7] 6 10892.464 06 ] 5 10892.476 28 ] 7 10892.500 [2

0 6] 5 10892.522 17 ] 6 10892.544 08 ] 7 10892.558 2

8 ] 7 . . . . . . . . 1 8] 7 12448.535 17 ] 6 12448.540 19 ] 8 12448.560 [2

0 7] 6 12448.601 08 ] 7 12448.617 09 ] 8 12448.627 0

9 ] 8 . . . . . . . . 1 9] 8 14004.601 08 ] 7 14004.605 2

10 ] 9 14004.621 00 8] 7 14004.677 1

9 ] 8 14004.686 [310 ] 9 14004.697 1

10 ] 9 . . . . . . . 1 10] 9 15560.665 19 ] 8 15560.665 0

11 ] 10 15560.678 [20 9] 8 15560.749 1

10 ] 9 15560.757 [111 ] 10 15560.766 1

11 ] 10 . . . . . . 1 10] 9 17116.724 [111 ] 10 17116.724 012 ] 11 17116.737 1

0 10] 9 17116.816 011 ] 10 17116.825 012 ] 11 17116.832 0

NOTES.ÈEstimated experimental uncertainties are 1 kHz.O[C the same as in Table 3.

the ortho levels in our rotationally cold K) molec-(Trot \ 2.5ular beam. Because very sharp lines only 5 kHz wide areroutinely achieved in our experiments, it has also been pos-sible to resolve the nitrogen hfs in the lowest rotationaltransitions of (Fig. 5) and determineCH3(C¹C)2CN(Table 5) the quadrupole coupling constant eqQ. Thederived rotational and centrifugal distortion constants arein good agreement with those previously reported by Alex-ander et al. (1978).

The three new methylcyanopolyynes were produced by alow-current 1300 V discharge synchronized with a 440 kslong gas pulse at a stagnation pressure of 2 atm. The gasmixture consisted of dilute (0.5%) methylcyanoacetylene inNe for the chains up to and a mixture ofCH3(C¹C)4CNmethylcyanoacetylene and diacetylene (0.5% each) in Ne for

For all three, somewhat weaker signalsCH3(C¹C)5CN.

TABLE 6

MEASURED ROTATIONAL TRANSITIONS OF CH3(C¹C)3CN

Frequency O[CJ@] J K F@] F (MHz) (kHz)

9 ] 8 . . . . . . . . 1 9] 8 6744.944 08 ] 7 6744.946 [1

10 ] 9 6744.965 00 8 ] 7 6744.966 0

9 ] 8 6744.978 010 ] 9 6744.986 0

10 ] 9 . . . . . . . 1 10] 9 7494.388 39 ] 8 7494.389 4

11 ] 10 7494.403 20 9 ] 8 7494.410 1

10 ] 9 7494.420 111 ] 10 7494.424 [1

11 ] 10 . . . . . . 1 10] 9 8243.823 [111 ] 10 8243.824 012 ] 11 8243.837 1

0 10] 9 8243.851 011 ] 10 8243.859 [112 ] 11 8243.865 0

12 ] 11 . . . . . . 1 11] 10 8993.259 [312 ] 11 8993.261 [113 ] 12 8993.273 0

0 11] 10 8993.293 112 ] 11 8993.298 [113 ] 12 8993.304 0

13 ] 12 . . . . . . 1 12] 11 9742.697 [213 ] 12 9742.699 [114 ] 13 9742.708 0

0 12] 11 9742.734 113 ] 12 9742.737 [214 ] 13 9742.743 0

14 ] 13 . . . . . . 1 13] 12 10492.135 [114 ] 13 10492.137 015 ] 14 10492.143 0

0 13] 12 10492.174 114 ] 13 10492.176 [115 ] 14 10492.182 1

15 ] 14 . . . . . . 1 14] 13 11241.573 115 ] 14 11241.573 016 ] 15 11241.578 [1

0 14] 13 11241.612 [115 ] 14 11241.619 216 ] 15 11241.619 [1

16 ] 15 . . . . . . 1 15] 14 11991.008 116 ] 15 11991.008 017 ] 16 11991.012 [1

0 15] 14 11991.051 016 ] 15 11991.056 217 ] 16 11991.056 [1

17 ] 16 . . . . . . 1 16] 15 12740.443 117 ] 16 12740.443 018 ] 17 12740.446 [1

0 16] 15 12740.489 117 ] 16 12740.493 218 ] 17 12740.493 [1

18 ] 17 . . . . . . 1 17] 16 13489.877 118 ] 17 13489.877 019 ] 18 13489.877 [4

0 17] 16 13489.928 218 ] 17 13489.928 019 ] 18 13489.928 [2

19 ] 18 . . . . . . 1 18] 17 14239.311 119 ] 18 14239.311 020 ] 19 14239.311 [3

0 18] 17 14239.364 2

TABLE 6ÈContinued

Frequency O[CJ@ ] J K F@] F (MHz) (kHz)

19 ] 18 14239.364 020 ] 19 14239.364 [2

20 ] 19 . . . . . . 1 19] 18 14988.744 120 ] 19 14988.744 021 ] 20 14988.744 [3

0 19] 18 14988.799 220 ] 19 14988.799 021 ] 20 14988.799 [2

21 ] 20 . . . . . . 1 20] 19 15738.175 121 ] 20 15738.175 022 ] 21 15738.175 [2

0 20] 19 15738.234 221 ] 20 15738.234 022 ] 21 15738.234 [2

22 ] 21 . . . . . . 1 21] 20 16487.606 122 ] 21 16487.606 023 ] 22 16487.606 [3

0 21] 20 16487.668 222 ] 21 16487.668 023 ] 22 16487.668 [1

23 ] 22 . . . . . . 1 22] 21 17237.036 123 ] 22 17237.036 024 ] 23 17237.036 [2

0 22] 21 17237.100 223 ] 22 17237.100 024 ] 23 17237.100 [1

24 ] 23 . . . . . . 1 23] 22 17986.465 124 ] 23 17986.465 025 ] 24 17986.465 [2

0 23] 22 17986.532 224 ] 23 17986.532 125 ] 24 17986.532 [1

25 ] 24 . . . . . . 1 24] 23 18735.893 125 ] 24 18735.893 026 ] 25 18735.893 [2

0 24] 23 18735.964 225 ] 24 18735.964 126 ] 25 18735.964 0

26 ] 25 . . . . . . 1 25] 24 19485.321 126 ] 25 19485.321 027 ] 26 19485.321 [2

0 25] 24 19485.394 226 ] 25 19485.394 227 ] 26 19485.394 0

27 ] 26 . . . . . . 1 26] 25 20234.748 227 ] 26 20234.748 128 ] 27 20234.748 0

0 26] 25 20234.822 227 ] 26 20234.822 028 ] 27 20234.822 0

28 ] 27 . . . . . . 1 27] 26 20984.174 328 ] 27 20984.174 229 ] 28 20984.174 1

0 27] 26 20984.251 228 ] 27 20984.251 129 ] 28 20984.251 0

29 ] 28 . . . . . . 1 28] 27 21733.596 029 ] 28 21733.596 030 ] 29 21733.596 [2

0 28] 27 21733.678 229 ] 28 21733.678 230 ] 29 21733.678 1

NOTES.ÈEstimated experimental uncertainties are 1 kHz.O[C the same as in Table 3.


FIG. 5.ÈLower rotational levels of and showing the transitions measured in the laboratory (vertical bars)CH3(C¹C)2CN CH3(C¹C)3CN

were also observed with a mixture of diacetylene and cyano-acetylene (1% each) in Ar or Ne.

4.1. CH3(C¹ C)3CNAs Figure 5 shows, 21 transitions of CH3(C¹C)3CN

were measured between 6 and 22 GHz; the measured fre-quencies are given in Table 6. Most rotational transitions of

are a complex blend of lines because theCH3(C¹C)3CNsplittings from the nitrogen hfs, the K doublets, and theinstrumental Doppler doublets are all comparable. At highJ, however, the nitrogen hfs has largely collapsed and thespectrum has simpliÐed sufficiently to allow the K-typedoublets to be assigned without ambiguity. For the lower Jlines, where assignments and line positions were more diffi-cult to determine a priori, precise frequencies for thehyperÐne-split transitions were obtained by directly Ðttingthe power spectrum in the frequency domain (Haekel &

1988) using the spectroscopic constants determinedMa� derfrom the high-frequency data and an initial estimate of thenitrogen eqQ of [4.25 MHzÈthat found for

A good Ðt with an rms of only 1 kHz wasCH3(C¹C)2CN.achieved by this procedure, allowing determination ofthe four spectroscopic constants (B, and eqQ) inD

J, D

JK,

Table 2.

and4.2. CH3(C¹C)4CN CH3(C¹C)5CNNitrogen hfs was not resolved for andCH3(C¹C)4CN

owing to the high J of the observed tran-CH3(C¹C)5CN

sitions. The characteristic K-type doublets, however, werewell resolved in all 10 transitions of the former and the 11transitions of the latter. The measured laboratory fre-quencies of both molecules between 6 and 14 GHz are givenin Table 7. Precise values for the rotational and centrifugaldistortion constants (Table 2) were derived from the labor-atory data, and the entire rotational spectra of both mol-ecules can be predicted again to better than 3 ppm over therange of interest to radio astronomers. We are unaware ofany ab initio calculations of the dipole moments for thelonger chains, but both should be fairly polar molecules ;extrapolation from the shorter ones (Bester et al. 1984 ;Arnau et al. 1990) suggests the dipole moments lie in therange 5È7 D.

With the addition of successive units to the carbonC2chain backbone, the line strengths of the longer methyl-cyanopolyynes typically decrease by a factor of about 7È10.A similar decrement in line intensity is observed onascending the series of shorter cyanopolyynes to HC9N(McCarthy et al. 1997b). Under the best conditions, lines of

were fairly easy to detect, but those ofCH3(C¹C)4CNwere close to the limit of sensitivity,CH3(C¹C)5CN

requiring cooling of the spectrometer and integrations ofabout 1 hr to achieve a signal-to-noise ratio of about 5.

5. METHYLPOLYYNES

Much less polar than the carbon chains here with termin-al CN groups, the methylpolyynes are nonetheless of


TABLE 7

MEASURED ROTATIONAL TRANSITIONS OF ANDCH3(C¹C)4CNCH3(C¹C)5CN

CH3(C¹C)4CN CH3(C¹C)5CN

Frequency O[C Frequency O[CJ@] J K (MHz) (kHz) (MHz) (kHz)

21 ] 20 . . . . . . 1 8766.915 00 8766.938 0

22 ] 21 . . . . . . 1 9184.387 20 9184.408 [1

23 ] 22 . . . . . . 1 9601.855 [10 9601.880 [1

24 ] 23 . . . . . . 1 10019.327 1 6147.447 [30 10019.353 1 6147.460 0

25 ] 24 . . . . . . 1 10436.795 [1 6403.592 [10 10436.823 0 6403.603 [1

26 ] 25 . . . . . . 1 6659.733 [30 6659.747 0

29 ] 28 . . . . . . 1 12106.671 [10 12106.706 2

30 ] 29 . . . . . . 1 12524.140 [10 12524.172 [2

31 ] 30 . . . . . . 1 12941.609 00 12941.644 1

32 ] 31 . . . . . . 1 13359.078 1 8196.587 [30 13359.111 [1 8196.606 2

33 ] 32 . . . . . . 1 13776.545 1 8452.732 00 13776.580 0 8452.747 1

34 ] 33 . . . . . . 1 8708.878 40 8708.889 0

35 ] 34 . . . . . . 1 8965.017 10 8965.033 2

36 ] 35 . . . . . . 1 9221.159 10 9221.174 1

44 ] 43 . . . . . . 1 11270.289 40 11270.306 2

45 ] 44 . . . . . . 1 11526.424 [10 11526.442 [2

46 ] 45 . . . . . . 1 11782.565 00 11782.581 [4


astronomical interest because the Ðrst two, methylacetyleneand methyldiacetylene[CH3(C¹C)H] [CH3(C¹C)2H],

have been observed in several astronomical sources (Irvineet al. 1981 ; MacLeod, Avery, & Broten 1984 ; Walmsley etal. 1984). Precise rest frequencies are available for those upto (Alexander et al. 1978). MethylpolyynesCH3(C¹C)3Hlarger than have been included in chemicalCH3(C¹C)3Hmodels of interstellar clouds (Herbst & Leung 1989), andastronomical detection of some of these is presumably onlya matter of sensitivity.

As Table 1 shows, the rotational spectra of four methyl-polyynes n \ 4È7, have now been detected.CH3(C¹C)

nH,

Those with n \ 4 and n \ 5 were detected Ðrst using a dis-charge with a dilute mixture of methylacetylene anddiacetylene in Ne (Travers et al. 1998). Typically, a dis-charge potential of 1300 V in the throat of the nozzle and agas pulse 200È350 ks long at a stagnation pressure behindthe pulsed valve of 2 atm produced the strongest lines.When methyldiacetylene was used instead of methyl-acetylene as a precursor, it was possible to extend this serieseven further, detecting the next two chains with n \ 6 andn \ 7 (Chen et al. 1999), shown in Figure 1. Like the long

TABLE 8

MEASURED ROTATIONAL TRANSITIONS OF ANDCH3(C¹C)4HCH3(C¹C)5H

CH3(C¹C)4H CH3(C¹C)5H


12 ] 11 . . . . . . 1 9041.056 [10 9041.089 [1

13 ] 12 . . . . . . 1 9794.475 [10 9794.513 1

14 ] 13 . . . . . . 1 10547.895 00 10547.933 0

15 ] 14 . . . . . . 1 11301.313 00 11301.355 0

16 ] 15 . . . . . . 1 12054.731 00 12054.776 1

19 ] 18 . . . . . . 1 14314.981 00 14315.032 [1

20 ] 19 . . . . . . 1 15068.396 00 15068.451 0

21 ] 20 . . . . . . 1 15821.810 0 8829.990 00 15821.868 0 8830.015 1

22 ] 21 . . . . . . 1 9250.464 00 9250.488 [1

23 ] 22 . . . . . . 1 9670.938 00 9670.963 [1

24 ] 23 . . . . . . 1 10091.411 00 10091.438 0

25 ] 24 . . . . . . 1 10511.883 [10 10511.913 0

26 ] 25 . . . . . . 1 10932.358 10 10932.387 0


methylcyanopolyynes, all four methylpolyynes here havereadily identiÐable microwave spectra owing to the clearlyresolved, tightly spaced doublets from the K \ 0 and K \ 1ladders. The lines of are observed with aCH3(C¹C)6Hsignal-to-noise ratio of about 25 in only 10 minutes of inte-gration, but those of are much fainterÈby aCH3(C¹C)7Hfactor of about 10, the same decrement found for othersymmetric top polyynes in our spectrometer when suc-cessive units are added to the molecular backbone.C2Detection of the fainter lines required liquid nitrogencooling.

and5.1. CH3(C¹C)4H CH3(C¹C)5HA total of eight a-type rotational transitions of

and six of (Table 8) wereCH3(C¹C)4H CH3(C¹C)5Hdetected and analyzed in terms of the standard expressionfor a symmetric top given in equation (1). In each Ðt the Arotational constant was Ðxed at 157 GHz; the rms of the Ðtsis comparable to the measurement uncertainties of about1 kHz. For both molecules the rotational and two leadingquartic centrifugal distortion constants were determined tohigh accuracy. The derived constants, tabulated in Table 2,are in excellent agreement with those predicted by extrapo-lation from the shorter members of the series.

and5.2. CH3(C¹C)6H CH3(C¹C)7HA total of 10 transitions of from J \ 22 toCH3(C¹C)6H39 in the two lowest K ladders, and eight transitions of

from J \ 32 to 44 in the K \ 0 ladderCH3(C¹C)7H(Table 9), were detected between 5 and 11 GHz. Line fre-


TABLE 9

MEASURED ROTATIONAL TRANSITIONS OF ANDCH3(C¹C)6HCH3(C¹C)7H

CH3(C¹C)6H CH3(C¹C)7H


22 ] 21 . . . . . . 1 5679.329 [20 5679.343 1

23 ] 12 . . . . . . 1 5937.481 [10 5937.495 1

24 ] 23 . . . . . . 1 6195.631 [20 6195.645 0

25 ] 24 . . . . . . 1 6453.784 00 6453.798 2

26 ] 25 . . . . . . 1 6711.934 00 6711.948 1

27 ] 26 . . . . . . 1 6970.084 [10 6970.100 2

32 ] 31 . . . . . . 1 8260.838 20 8260.850 [2 5431.170 [4

33 ] 32 . . . . . . 1 8518.987 10 8519.001 [2 5600.902 4

34 ] 33 . . . . . . 0 5770.620 [235 ] 34 . . . . . . 0 5940.341 [436 ] 37 . . . . . . 0 6110.072 338 ] 37 . . . . . . 1 9809.735 1

0 9809.753 0 6449.522 639 ] 38 . . . . . . 1 10067.884 0

0 10067.903 0 6619.235 [544 ] 43 . . . . . . 0 7467.858 2


quencies were again measured to an accuracy of 0.3 ppm orbetter for both molecules. For the threeCH3(C¹C)6H,spectroscopic constants B, and were determined,D

J, D

JKbut for only the rotational constant B (TableCH3(C¹C)7H2) could be extracted from the data because of the narrowrange of transitions measured.

6. DISCUSSION

Several general comments are suggested by the presentwork :

TABLE 10

PREDICTED CONSTANTS FOR LONGER

POLYYNES (IN MHz)

Molecule B

Cyanopolyynes :HC19N . . . . . . . . . . . . . . . . . . . . . 37.069(2)HC21N . . . . . . . . . . . . . . . . . . . . . 27.919(3)HC23N . . . . . . . . . . . . . . . . . . . . . 21.550(4)

Methylcyanopolyynes :a . . . . . .CH3(C¹C)6CN . . . . . . . . . . . 84.193(4)CH3(C¹C)7CN . . . . . . . . . . . 58.293(4)

Methylpolyynes :a . . . . . . . . . . . .CH3(C¹C)8H . . . . . . . . . . . . . 58.747(4)CH3(C¹C)9H . . . . . . . . . . . . . 42.339(4)

NOTE.ÈEstimated 1 p uncertainties aregiven in parentheses.

a Constants derived on the assumptionthat the A rotational constant is 157 GHz.

1. For the molecules here (and by extension for carbonchains generally) the laboratory astrophysics is well aheadof the radio astronomy, allowing new astronomical mol-ecules to be found without searches in frequency that withlarge telescopes can be prohibitive in time and cost. As theastronomical detection of demonstrates, anyHC11Ncarbon chain that can be detected in space can probablynow be detected with our present microwave instrumen-tation, or a slight reÐnement of it.

2. The rotational constants of the polyynes here areremarkably well predicted by a simple extrapolation inwhich a third-order polynomial in chain length is Ðtted tothe moments of inertia of the shorter chains in the sequence.The predicted constants typically agree with those mea-sured to within a few parts in 105, so that at 5È8 GHz, wheremost of these long chains have their most intense rotationaltransitions in our cold K) molecular beam, a fre-(Trot \ 2.5quency search of a few MHz or less was required for detec-tion. The close agreement of extrapolation with experimentsuggests that the moment of inertia of long carbon chains isquite insensitive to structural details such as the rotation-vibration interactions important in small molecules. Rota-tional constants of still longer polyynes can probably bepredicted with very high accuracy from extrapolation ; somepredicted constants are given in Table 10.

3. The centrifugal distortion of the polyynes is welldescribed by a simple semiclassical theory (Thaddeus et al.1998), which treats a carbon chain as a thin classical rodwith a YoungÏs modulus that is independent of chain length.As Figure 6 shows, D/B for the three types of polyyne chainshere is closely proportional to the inverse fourth power ofthe chain length L , the same relation as that found for theacetylenic free radicals and the cumulene carbenesC

nH

All carbon chains so far studied apparently distortH2Cn.

under rotation in this same simple way, regardless of the

FIG. 6.ÈLog-log plot of D/B as a function of chain length L for thecyanopolyynes (solid triangles), the methylcyanopolyynesHC2n`1N(open circles), and the methylpolyynes (solidCH3C2nCN CH3C2nCHsquares). The errors of log (D/B) are generally smaller than the size in thedata points.


FIG. 7.ÈRelative intensity of the strongest rotational lines of thecyanopolyynes (solid circles) and their relative abundance (open circles) inthe FTM spectrometer, as a function of the number of carbon atoms in thechain. The error bars are estimated 2 p uncertainties. Relative abundancesare obtained from line intensities by taking into account the dependenceon the chain length of the rotational partition functions and dipolemoments. The limit of the detection sensitivity is approximately thatachieved in a total observation time of 3 hr.

end groups that terminate the carbon chain backbone, thetype of carbon-carbon bonding, or the low-lying vibrationalstructure of the individual molecules.

4. As shown in Figure 7, the abundance of the longcarbon chains and in our laboratory dis-HC15N HC17Ncharge is only slightly less than that of the shorter membersof the sequence, implying that still longer chains are beingproduced at similar concentrations in our discharge sourceand might be detected with slight improvements in instru-mentation. In the interstellar gas, as in our molecular beam,long chains may be more abundant than extrapolation frompresent radio observations suggest for two reasons : (a) thesynthesis of carbon chains in space may be somewhatsimilar to that in the laboratory and (b) long chains as largeas those detected here may behave in the interstellar spacemore like grains than more familiar smaller molecules andmay be fairly stable against processes such as dissociativerecombination and photodissociation which rapidlydestroy small molecules in the di†use interstellar gas.

We are indebted to E. S. Palmer for technical assistancein the construction and maintenance of the FTM spectro-meter, particularly with the microwave electronics, and wethank those colleagues who made partial contributions tosome of the original papers : P. Botschwina, C. A. Gottlieb,

Heyl, S. E. Novick, J.-U. Grabow, and M. R. Munrow.A� .

APPENDIX A

THE COOLED FOURIER TRANSFORM MICROWAVE SPECTROMETER

The present Fourier transform microwave spectrometer shown in Figure 8 is an improved version of the instrumentpreviously described by McCarthy et al. (1997a, 1997b). The sensitivity has been improved by more than an order ofmagnitude, cooling of the Fabry-Perot cavity and reÐnements to the microwave receiver (Grabow et al. 2000, in preparation)accounting for at least a factor of 5 of this increase. Improvements in production efficiency with the use of larger organicprecursors such as cyanoacetylene, diacetylene, methylcyanoacetylene, and methyldiacetylene, and additional optimization ofthe geometry and electrical characteristics of the discharge nozzle (Appendix B), account for the remainder. The bettersensitivity of the microwave receiver results from (1) the use of separate antennas for excitation and detection to eliminatelossy microwave components (i.e., circulators) and unnecessary cables in the receiver front end, (2) improvement in thecoupling between the Ðrst-stage ampliÐer and the receiver antenna by mounting the ampliÐer directly behind the antennainside the vacuum chamber, (3) better coupling of the receiver antenna to the microwave cavity to obtain a higher qualityfactor Q of the cavity over much of the centimeter-wave band, and (4) cooling the mirrors of the Fabry-Perot and Ðrst-stageampliÐer to 77 K by liquid nitrogen cooling.

Good coupling of the receiver antenna to the Fabry-Perot cavity is important because line intensities are sensitive to thecavity Q. The highest unloaded Q now achieved is 2] 105, which is within a factor of 2 of the theoretical limit of about4 ] 105 set by the ohmic losses in the aluminum mirrors. When the cavity is critically coupled, the loaded is one-half theQ

Lunloaded i.e., about 105ÈsigniÐcantly higher than the loaded Q achieved before the present work. The higher cavity QQ0,produces rotational lines stronger by a factor of 5 or more and was crucial in the detection of most of the polyynes here.Operation with liquid nitrogen reduces the system noise temperature by about a factor of 4, from 800 to 190 K. Above

10 GHz di†ractive losses in the open resonator are negligible, and roughly two-thirds (110 K) of the receiver noise is from thecold ampliÐer and one-third from the 77 K mirrors. Below 10 GHz di†raction from the open resonator contributes signiÐ-cantly to the cavity Q, and the system temperature rises to about 400 K; the modest twofold decrease in the noise level oncooling then results largely from the lower noise Ðgure of the cold ampliÐer.

Cooling of the Fabry-Perot cavity and the Ðrst ampliÐer of the receiver was done fairly cheaply, without extensive redesignof the spectrometer. Each mirror is cooled separately by continuously Ñowing liquid nitrogen through a copper coil solderedto a copper disk making good thermal contact with the mirrorÏs back surface. Thin-walled stainless steel tubes connect thetwo copper coils to liquid nitrogen vacuum feedthroughs. These are coiled inside the vacuum chamber to be sufficiently


Ñexible at 77 K to allow the cavity to be tuned by adjusting the separation of the mirrors. Thermal isolation of the mirrors isachieved by suspension on epoxy strips. The pulsed discharge nozzle is kept close to room temperature by (1) mounting thenozzle assembly on TeÑon stando†s that minimize contact with the cold mirror and (2) surrounding the nozzle with a copperinsulating jacket heat sunk with copper braid to the warm walls of the vacuum chamber. Condensation of gas from thesupersonic jet does not appreciably degrade the reÑectivity of the cold mirrors.

FIG. 8.ÈSimpliÐed schematic of the 77 K FTM spectrometer showing the Fabry-Perot cavity and an expanded view of the discharge nozzle source

FIG. 9.ÈPulsed discharge nozzle source showing the supersonic molecular beam, the reaction zone, and the TeÑon housing


APPENDIX B

DISCHARGE NOZZLE FOR THE PRODUCTION OF LONG CARBON CHAINS

ReÐnements to the pulsed discharge source contributed signiÐcantly to the detection of long polyynes. As Figure 9 shows,the present nozzle geometry consists of a short TeÑon spacer directly after the nozzle, two oxygen-free high-conductivitycopper electrodes separated by a 10 mm spacer of the same dielectric, and a third 20 mm TeÑon spacer following the cathode.The crucial di†erence between this nozzle and the one used on our spectrometer to detect long-chain free radicals andcarbenes is the addition of the third spacer. This reÐnement conÐnes the discharge plasma before the free expansion andproduces much stronger lines of the longer polyynesÈpresumably because a greater number of collisions prior to expansionallows time for long chains to assemble. ConÐning the discharge plasma also appears to quench production of open-shellmolecules, collisions apparently driving the hydrocarbon chemistry to the more stable closed-shell polyynes. The polarity ofthe discharge plates, as shown in Figure 9, is normally chosen so that the cathode is the second (outer) electrode ; the strengthof rotational lines usually decreases by a factor of 2È4 when the polarity is reversed.

REFERENCESAlexander, A. J., Kroto, H. W., Maier, M., & Walton, D. R. M. 1978, J.

Mol. Spectrosc., 70, 840Arnau, A., Tunon, I., Andres, J., & Silla, E. 1990, Chem. Phys. Lett., 166, 54Bell, M. B., Feldman, P. A., Travers, M. J., McCarthy, M. C., Gottlieb,

C. A., & Thaddeus, P. 1997, ApJ, 483, L61Bester, M., Yamada, K., Winnewisser, G., Joentgen, W., Altenbach, H. J., &

Vogel, E. 1984, A&A, 137, L20Botschwina, P., Heyl, Chen, W., McCarthy, M. C., Grabow, J.-U.,A� .,

Travers, M. J., & Thaddeus, P. 1998, J. Chem. Phys., 109, 3108Broten, N. W., MacLeod, J. M., Avery, L. W., Irvine, W. M.,

B., Friberg, P., & Hjalmarson, 1984, ApJ, 276, L25Ho� glund, A� .Chen, W., Grabow, J.-U., Travers, M. J., Munrow, M. R., Novick, S. E.,

McCarthy, M. C., & Thaddeus, P. 1998, J. Mol. Spectrosc., 192, 1Chen, W., McCarthy, M. C., Novick, S. E., & Thaddeus, P. 1999, J. Mol.

Spectrosc., 196, 335Haekel, J., & Z. 1988, Z. Naturforsch., 43a, 203Ma� der,Herbst, E. 1978, ApJ, 222, 508Herbst, E., & Leung, C. M. 1989, ApJS, 69, 271Irvine, W. M., B., Friberg, P., Askne, J., & J. 1981, ApJ,Ho� glund, Ellde� r

248, L113Kawaguchi, K., Ohishi, M., Ishikawa, S., & Kaifu, N. 1992, ApJ, 386, L51

M., Dreizler, H., Preugschat, D., & Lentz, D. 1991, Angew. Chem.,Kru� ger,103, 1671

M., Stahl, W., & Dreizler, H. 1993, J. Mol. Spectrosc., 158, 298Kru� ger,La†erty, W. J., & Lovas, F. J. 1978, Phys. Chem. Ref. Data, 7, 441MacLeod, J. M., Avery, L. W., & Broten, N. W. 1984, ApJ, 282, L89

Matthews, H. E., & Sears, T. J. 1983, ApJ, 267, L53McCarthy, M. C., Grabow, J.-U., Travers, M. J., Chen, W., Gottlieb, C. A.,

& Thaddeus, P. 1998, ApJ, 494, L231McCarthy, M. C., Levine, E. S., Apponi, A. J., & Thaddeus, P. 2000, J. Mol.

Spectrosc., 203, 75McCarthy, M. C., Travers, M. J., A., Chen, W., Novick, S. E.,Kova� cs,

Gottlieb, C. A., & Thaddeus, P. 1997a, Science, 275, 518McCarthy, M. C., Travers, M. J., A., Gottlieb, C. A., & Thaddeus,Kova� cs,

P. 1997b, ApJS, 113, 105Saykally, R. J., Szanto, P. G., Anderson, T. G., & Woods, R. C. 1976, ApJ,

204, L143Snyder, L. E., & Buhl, D. 1971, BAAS, 3, 388Thaddeus, P., McCarthy, M. C., Travers, M. J., Gottlieb, C. A., & Chen, W.

1998, Faraday Discuss., 109, 121Townes, C. H., & Schawlow, A. L. 1955, Microwave Spectroscopy (New

York : Dover), 499Travers, M. J., Chen, W., Grabow, J.-U., McCarthy, M. C., & Thaddeus, P.

1998, J. Mol. Spectrosc., 192, 12Travers, M. J., McCarthy, M. C., Kalmus, P., Gottlieb, C. A., & Thaddeus,

P. 1996, ApJ, 472, L61Walmsley, C. M., Jewell, P. R., Snyder, L. E., & Winnewisser, G. 1984,

A&A, 134, L11Watson, W. D. 1974, ApJ, 188, 35Winnewisser, G., Creswell, R. A., & Winnewisser, M. 1978, Z. Naturforsch.,

33a, 1169

microwave spectra of 11 polyyne carbon … astrophysical journal supplement...

Documents