Faraday Discuss. 1998 109 121»135 New carbon chains in the laboratory and in interstellar space P. Thaddeus M. C. McCarthy M. J. Travers C. A. Gottlieb and W. Chen Center for Astrophysics and Division of Applied Sciences Harvard University Cambridge MA 02138 USA Twenty seven new carbon-chain molecules have been detected over the past two years with a Fourier-transform microwave (FTM) spectrometer 13 of which are reported here for the –rst time. Of the 27 11 are closed shell polyynes 9 are free radicals 2 are cumulene carbenes and 5 are carbenes formed by substituting carbon chains for one of the hydrogen atoms of the tional transitions [including hyper–ne structure (hfs) for the radicals] of the three member carbene ring C3H2 . All the astronomically interesting rotaentire set have either been measured to high precision or are readily calculated to comparable accuracy from the spectroscopic constants derived from the laboratory data.On the basis of this data 4 of the chains (C H 7H C8H 2C6 HC11N) and have already been detected in astronomical sources and with large radio telescopes under construction or the discovery of better astronomical sources it is possible that nearly all can be found. Astronomical detection is aided by the apparent high polarity of all unsymmetrical chains. The sensitivity of the present liquid-nitrogen-cooled spectrometer is far from fundamental limits ; an increase by one to two orders or magnitude is possible with liquid helium cooling and other re–nements and better precursor gases may be found.Many of the chains here probably have lowlying isomers and ionized and radical variations which may be detected by the present techniques. Carbon chains are hard to stretch but easy to bend centrifugal distortion is well described by a classical model according to which all chains distort under rotation like classical thin rods with the same Youngœs modulus E\1.7]1013 dyn cm~2 larger than that of diamond; the longest two chains we have detected HC15N H17N and C have low frequency bending vibrations which lie within the range of existing high altitude radio telescopes. Finally it is pointed out that the density of the chains at the limit of detection in our spectrometer ca. 108 cm~3 is high by the standards of modern laser spectroscopy so that optical detection of many should be possible.At least 114 molecules have now been identi–ed in the interstellar gas or in circumstellar shells. Many of the smaller ones are inorganic compounds such as H2 H OH and NH 2O 3 but in space as on Earth Nature employs the carbon bond when she chooses to assemble large structures all but 3 of the 64 interstellar molecules with more than three atoms are organic in the strict sense of the term. The largest is the linear carbon chain HC11N with a molecular weight of 147 twice that of the simplest amino acid glycine. Most of the smaller polyatomic molecules are also carbon chains the great majority acetylenic in structure with alternating single and triple CC bonds but a few cumulenic with successive CC double bonds.It is astonishing that highly unsaturated carbon of this kind should be so conspicuous in regions where hydrogen by a very large factor is the most abundant chemically active element. With a Fourier-transform microwave (FTM) spectrometer,1 we have recently been able to identify in the laboratory a large number of carbon chain molecules of astro- 121 New carbon chains in laboratory and interstellar space 122 physical interest. The total found over the last two years stands at 27 11 closed-shell polyynes (three larger than HC N) 9 radicals and 7 carbenes. Three of the carbenes were proposed as possible astronomical molecules at the 1992 Faraday Symposium on 11 Chemistry in the Interstellar Medium,2 and –ve are unusual structures containing threemember carbon rings.Fig. 1 shows the entire set and contains references for those already published ; nearly one half are new and are reported here for the –rst time. All or nearly all of the 27 probably exist in astronomical sources at some level of abundance and the four marked in Fig. 1 with asterisks have already been detected with Fig. 1 Twenty seven new carbon chains. References to published papers are given after or below the chemical formula with astronomical detections underlined. 123 P. T haddeus et al. Fig. 1 Continued radio telescopes either in the rich molecular cloud TMC-1 or in the circumstellar shell of the nearby carbon star IRC]10216 the two best carbon-chain sources so far discovered. Seven of the 13 molecules not previously reported contain nitrogen the two isoand and the two ring chains HC 4NC HC6NC 4N H6N.and 3C12N cyanopolyynes HC and H the three methylcyanopolyynes H3C8N H3C10N C The ease with which the isocyanopolyynes were made suggests that it might be possible to insert nitrogen more or less at random into the chains and ring-chains in Fig. 1 to produce a large number of 124 New carbon chains in laboratory and interstellar space isomers but we have deferred a study of this problem until ab initio calculations are available to guide an experimental search. Insertion of nitrogen is one way in which hypothetical non-polar monocyclic rings of carbon20 might be made sufficiently polar to detect by means of FTM spectroscopy. By analogy with SiC4 ,21 there may be chains and ring-chains containing silicon (or other elements) which might be detectable by the present techniques.Because of limited space it is not possible to give here a detailed spectroscopic description of the unpublished new chains or much in the way of experimental details. We will instead make some general observations about the whole set and will discuss the prospects for further laboratory and astronomical detections and identi–cations. T Spectroscopic characterization The radio spectroscopy of all the molecules in Fig. 1 is complete in the sense that all the astronomically interesting rotational transitions have either been measured to high precision or are readily calculated to comparable accuracy from the spectroscopic constants derived from the laboratory data.In addition to the rotational constants these include in all cases the leading one or two centrifugal distortion constants and for the carbon-chain radicals like C8H the several constants which determine the hfs and lambda doubling caused by the unpaired electron. 2% ladder when n is even and the 2% ladder when 3@2 C is odd. As a result A cannot be determined on the basis of data from our FTM 1@2 7H C8H and however we were able to detect both rotational is ca. 100 K and rot The short carbon chain radicals CCH and C4H have 2& electronic ground states but all longer ones including those here have 2% states with inverted –ne structure and a negative –ne structure constant A when the number of carbon atoms n is even but normal –ne structure and A positive when n is odd.At the low rotational temperature C of our molecular beam only the lowest lying of the two –ne structure ladders in the C ground state is populated the nC spectrometer alone. For C ladders in our free-space millimetre-wave spectrometer where since A is readily found when the rotational constants in both ladders are known [via the relation B1@2 3@2\B0(1^B0/A)] we were able to obtain accurate values of A for these two chain radicals. All the transitions of the new chains which are at all likely to be detected in space in the foreseeable future are now known to at least 1 km sv1 in equivalent radial velocity (i.e. 3 ppm) and often much better than that since lines have generally been measured and –t to 0.3 ppm or better.A detailed account of the data and the spectroscopic analysis for the molecules already reported can be found in the references cited in Fig. 1. For the new molecules here a full account of experimental and spectroscopic details will be given elsewhere. 2Cn a remarkably high polarity for exhibit a Dipole moments Unsymmetrical carbon chains like those here are extremely polar molecules which is one of the reasons why they are readily detected in space and in the laboratory. Dipole moments have not been measured for any of the chains in Fig. 1 but ab initio calculations which are probably accurate to a few percent have been done for all the cumulene carbenes,22,23 the shorter acetylenic radicals,24 and the shorter cyanopolyynes.23 In all three families the dipole moment is found to increase steadily with chain are the most polar with a moment of 6 D for 2C5 H2C10 rising to over 10 D for the still undetected length.10 The cumulene carbenes H H hydrocarbons.The acetylenic radicals HC and the cyanopolyynes HC n nN similar increase in polarity with length but are less polar by ca. 20% and 40% respec-125 P. T haddeus et al. tively. It will be interesting to see from further ab initio calculations if this monotonic increase in polarity with length extends to chains as long as HC17N. Sensitivity far from fundamental limits Our FTM spectrometer currently works in the microwave band from ca. 6»26 GHz. A pulsed supersonic molecular beam of an organic vapour such as acetylene diacetylene or cyanoacetylene heavily diluted in either Ne or A is produced by a commercial solenoid valve (Fig.2). As the beam expands through one of the mirrors of the large confocal Fabry»Perot cavity typically a 1000 V discharge is applied synchronously with the gas pulse which is ca. 400 ls long. At a pressure behind the pulsed valve of 2 atm the discharge is stable during the gas pulse and the rotational temperature of all molecules including any made in the discharge has dropped to only a few K by the time the molecules have moved only a few cm downstream from the nozzle. This low rotational temperature with the concomitant reduction of the rotational partition function by two orders of magnitude for a linear molecule is one of the main reasons the present technique is so sensitive for carbon chains.As the molecules traverse the high-Q Fabry»Perot cavity they are irradiated by a short (1 ms) microwave pulse resonant with one of their rotational transitions to which Fig. 2 Schematic diagram of the FTM spectrometer 126 New carbon chains in laboratory and interstellar space one of the Fabry»Perot modes has been mechanically tuned. The subsequent freeinduction decay is detected with a sensitive microwave receiver which like the mirrors of the resonator has been cooled to the temperature of liquid nitrogen to reduce thermal and ampli–er noise. The Fourier transform of the decaying free induction yields the power spectrum shown in Fig. 3 for one of the rotational transitions of C7H2 . The line shape is double peaked the result of the Doppler shift of the two travelling waves that compose the standing Fabry»Perot mode with respect to the typically Mach 2 molecular beam.This peculiar line shape however is only a minor inconvenience; in practice the resolution of line structure (e.g. hfs or K-doubling as shown in Fig. 4) is usually limited by the small width of the two Doppler components whose fractional width *l/l may be as small as 7]10~7 (0.2 km s~1). Fig. 3 Lower rotational levels of C7H2 showing the transitions detected. The double-peaked line pro–le is instrumental in origin the Doppler splitting that results when the Mach 2 axial molecular beam interacts with the standing wave in the confocal Fabry»Perot cavity (see text). The spectrum shown above is the result of a 2 min integration.P. T haddeus et al. (T Fig. 4 Typical rotational transitions of the two new methylpolyynes showing the resolution of K-structure. The K\1 ladder although lying ca. 8 K above the K\0 ladder is metastable owing to spin symmetry and therefore well populated in the rotationally cold molecular beam rot\1»3 K). The largest member of each family of chains in Fig. 1 is close to the limit of detectability of the present liquid-nitrogen-cooled FTM spectrometer but the sensitivity of this instrument is far from fundamental limits. Liquid-helium cooling of the optics and the –rst stage of receiver ampli–cation might improve the sensitivity by nearly an order of magnitude and a further improvement by a factor of ca. –ve might be gained for molecules with 10 or more heavy atoms by lowering the frequency of operation to the low-GHz range where lines are strongest at the rotational temperature of 1»3 K characteristic of our supersonic molecular beam.While technically challenging both improvements are quite feasible particularly if a closed resonant cavity is used instead of the large open Fabry»Perot resonator of our present device. Another factor favours the 127 128 New carbon chains in laboratory and interstellar space Fig. 5 Relative intensities of the strongest rotational lines of the cyanopolyynes (Ö) and relative abundances (L) from the FTM spectrometer as a function of the number of carbon atoms in the chain. Error bars are estimated 2p uncertainties. The indicated detection limit is that achieved in an integration of ca.3 h. detection of large molecules with our discharge source the line intensity and abundance decrement of the cyanopolyynes markedly —atten beyond about nine carbon atoms as Fig. 5 shows; HC17N for example turned out to be much easier to detect than was expected by extrapolation from the short members of the series.18 If a similar eÜect exists for the other types of chains in Fig. 1 and liquid-helium cooling can be exploited much larger chains than those here may be within reach. Production of carbon chains All the carbon chains in Fig. 1 were produced when simpler unsaturated molecules such as acetylene and diacetylene heavily diluted in an inert gas were subject to a small dc discharge in the throat of the supersonic nozzle of our molecular beam spectrometer.The choice of the precursor and inert gases the precise dilution factor and the parameters of the electrical discharge are all highly empirical guided largely by trial and error since no adequate theoretical model (or understanding) of the synthetic process exists. In the past acetylene and allene were shown in this laboratory and others25 to be a good source of carbon chains and the C3H2 ring in the large glow discharges used in millimetre-wave free-space spectrometers and these were employed in the present FTM instrument with some success. Better results were achieved however with diacetylene which to avoid explosions was synthesized in small quantities and immediately diluted in Ne or A.Cyanoacetylene was also synthesized and proved to be a good source of nitrogen-containing chains. Further details on the particular sources used for each chain can be found in the articles cited in Fig. 1 or for the previously unreported molecules will appear elsewhere. The point which deserves emphasis here is that our production techniques are still primitive largely hit or miss and with systematic investigation may be subject to considerable improvement. Only a few have been tried among the large number of possible production schemes and it is likely that ones we have not yet employed such as laser ablation or discharges through even more unstable precursor gases will yield signi–- cantly higher concentrations of carbon chains than any of the methods so far adopted.A systematic theoretical investigation of the problem is needed but we have no illusions that that would be easy or would quickly yield results to guide experiment. 129 P. T haddeus et al. Astronomical detections were identi–ed in astronomical sources before being detected on Earth but with the Many of the –rst carbon chains (e.g. the sequence of free radicals CCH CCCH… … …C6H) work here the laboratory spectroscopy is now well ahead of the radio astronomy allowing new astronomical molecules to be found without searches in frequency that with large radio telescopes can be prohibitive in time and cost. Any carbon chain which can be observed in space can probably be detected in the laboratory with the present techniques or extensions of these which are planned.All or most of the chains in Fig. 1 are candidates for astronomical detection because at least one shorter member of each type has already been detected in at least one source and as mentioned four of the present chains were quickly found once laboratory frequencies were in hand. With existing telescopes detecting the larger ones will be difficult or impossible in even the best presently known astronomical sources (integrations of roughly 30 h per line were required to detect HC with modest signal-to-noise14) but larger and better telescopes (e.g. the Green Bank 100 m telescope and the resurfaced 11N Arecibo 305 m telescope) will soon be available to work in the band from 1»10 GHz where the larger molecules in Fig. 1 are expected to have their strongest astronomical lines.There is moreover the possibility that signi–cantly better astronomical sources of large molecules will be found. Molecule-rich circumstellar shells of stars comparable in distance to IRC]10216 are rare and it is quite likely that this object is the best of its kind. TMC-1 is an entirely diÜerent matter; it is merely one rather nondescript molecular clump or ìcoreœ among many in the extensive collection of dark nebulae in Taurus covering some 150 square degrees which at 140 pc is one of the closest regions of molecular gas and star formation to the sun. Very few of the many other molecular cores in Taurus have been studied in any molecule heavier than HC3N and there are comparably extensive molecular clouds elsewhere in the sky which have been even less studied although not much further away than those in Taurus e.g.the well known molecular complexes associated with the dark nebulae in Auriga Perseus Aquila and Ophiuchus. More distant molecular clouds are almost entirely terra incognita with respect to large molecules. Carbon monoxide the most readily observed interstellar molecule has been detected in over one-half of the 3600 square degrees of the sky that lies within 5° of the Galactic plane,26 but almost none of the hundreds or thousands of dense generally distant molecular clouds which lie in this wide band have been studied in any molecule larger than HC3N. Once a few good sources were found the radio astronomers simply returned to these locations again and again in their search for still larger molecules unwilling to devote scarce telescope time to slow and frustrating attempts to –nd still better locations.A sensitive new 115 GHz CO survey of the Taurus Dark Clouds is currently being done by Megeath with the CfA 1.2 m telescope at full angular resolution (8.7@) to update the original low-resolution (30@) survey done with this instrument over a decade ago.27 In an attempt to –nd a better source of large molecules than TMC-1 the most promising CO cores will be studied at higher resolution with larger instruments –rst in HC3N then in longer carbon chains. Finally the most promising locations may be observed with the Arecibo telescope which because of its very large collecting area and high resolution at frequencies below 7 GHz may be the most powerful instrument with which to detect the longer carbon chains in Fig.1. Isomers With increasing number of atoms the number of low lying isomers of a molecule which one might hope to detect in the laboratory and in space tends to increase very rapidly. 130 New carbon chains in laboratory and interstellar space Fig. 6 Some possible low-lying isomers of C7H2 ; –lled circles are C open circles H. Energies and dipole moments are from an ab initio calculation.25 The C ring-chain has been calculated ab initio to be the most stable isomer with that 7H2 elemental formula but –ve others have been calculated to lie within 1 eV,28 and there are still others which may be comparably stable or only slightly less so (Fig. 6).Such calculations to our knowledge have not been done for C9H2 or the longer carbon chains in Fig. 1 but by analogy with C7H2 these must all possess a large number of low-lying isomers many of which are probably quite stable and candidates for laboratory and astronomical detection. The existence of fairly energetic molecular isomers in space in regions where the kinetic and rotational temperature is only ca. 10 K is striking evidence of how far the chemical processes there depart from thermal equilibrium. HNC for example is more energetic than HCN by 0.6 eV and in thermal equilibrium at the temperature of a typical cold molecular cloud say 15 K the abundance of HNC relative to HCN is entirely negligible exp(E/kT )B10~200. The observed ratio sometimes in fact approaches unity because the ion»molecule reactions which make both isomers couple the chemistry to the very high eÜective temperature reservoir of the cosmic rays which permeate the interstellar gas.Ionized and radical ring-chains main source of the widely distributed cyclic C The C3H3 ` ion is familiar in the laboratory as the mass 39 peak in mass spectrometers and it is almost certainly present in the interstellar gas where it is thought to be the 3H2 carbene via dissociative recombi-131 P. T haddeus et al. Fig. 7 Hypothetical radical and ionic ring-chains nation. Owing to its planarity and D3h symmetry however it possesses no electric dipole moment and no microwave spectrum (although C3H2D` has a weak rotational spectrum which we can probably detect in the laboratory).On substituting carbon chains for one of the H atoms however the symmetry is broken and one obtains as shown in Fig. 7 a family of ring-chain ions analogous to the carbene ring-chains of Fig. 1. Because these too are likely to be highly polar and because positive ions are readily formed in the interstellar gas this so-far-unobserved sequence is of considerable astronomical interest. Laboratory detection is again a prerequisite for an astronomical search but there does not appear to be any fundamental reason why this cannot be accomplished with an FTM instrument such as ours. known astronomical molecule which in the laboratory produces very strong lines in The same substitution as Fig. 7 also shows can be done with the radical c-C3H a our spectrometer.By analogy with c-C3H such radical ring-chains are expected to be planar with 2B electronic ground states and the unpaired electron in each case is expected to produce well resolved –ne and hyper–ne structure. The –rst member of this 2 hypothetical sequence C5H has been calculated by Stanton and Crawford (personal communication) to lie only ca. 5 kcal above the linear isomer a molecule which has been found in space and which also yields very strong lines in our spectrometer. Carbon chains as classical elastic rods Centrifugal distortion in long linear molecules like those here has not been systematically studied in the past. It turns out to be remarkably simple and readily understood in terms of a simple semi-classical theory.In Fig. 8 the ratio of the centrifugal distortion constant to the rotational constant is shown on a log»log plot as a function of chain length for a number of the molecules in Fig. 1. The –t in Fig. 8 is a one parameter linear –t only the vertical displacement of the line has been varied to best match the data; the slope has been set at exactly [4. To understand this simple L~4 dependence on length let us suppose that a carbon chain is a uniform thin elastic rod of cross-sectional area p and density per unit length o with a Youngœs modulus E that is independent of length and independent of the type of chain. Without centrifugal distortion the moment of inertia is I\oL 3/12 and it is 132 New carbon chains in laboratory and interstellar space Fig. 8 D/B for a number of carbon chains as a function of chain length L .(Ö) Radicals (HCn) ()) cumulenes and (H2Cn) (>) cyanopolyynes (HC2n`1N). readily shown from the theory of elasticity29 that the fractional increase in I produced by centrifugal distortion is *I/I\(144/5Epo)J2L~4. To –rst order this will lower the energy of rotation J2/2I by an amount (864/5Epo2)J4L~7. Again from the theory of elasticity,29 it is readily calculated that exactly one-half of this energy is stored in the elastic deformation of the rod. When both angular momentum and energy are conserved e.g. if the molecule is held rigid spun up and then allowed to centrifugally distort the rest is lost as vibration. Taking the quantum limit J2](h/2n)2J(J]1) it is then found that (1) E\(3h2/2n2oL 3)J(J]1)[(27h4/5n4Epo2L 7)J2(J]1)2 which if written in terms of the rotational and centrifugal distortion constants E\BJ(J]1)[DJ2(J]1)2 immediately yields (2) D/B\(18h2/5n2Epo)L ~4 exhibiting the observed dependence on the inverse fourth power of the length.It is remarkable that this expression applies over such a large range of length and with a single value of the Youngœs modulus E holds for all the chains we have studied. One might expect it to hold only in the limit of long chains or one might expect that the stiÜness would depend on the type of chain i.e. the various end groups and valence structures in Fig. 1 but that does not appear to be the case to any appreciable extent. Instead to an accuracy of a few per cent the model seems to hold to the very shortest chains studied and to –t all types of chain equally well.A full quantum mechanical treatment would presumably show that D/B depends on the full spectrum of vibrational frequencies which undoubtedly depends on chain type but evidently not strongly enough to make much of a diÜerence. The practical value of eqn. (1) is considerable when it comes to identifying new chains. It immediately allows a bona –de chain to be distinguished from weakly bonded complexes which are readily formed and observed in molecular beams such as ours (e.g. HCCCCH… … …H2O) and by making a speci–c prediction of the centrifugal distortion 133 P. T haddeus et al. once the rotational constant is determined it simpli–es signi–cantly the process of identi –cation.It is interesting to see just how stiÜ carbon chains are relative to familiar substances. To do that we must assign to them a cross sectional area p. We adopt the reasonable value of 5.6]10~16 cm2 obtained on the assumption of a hypothetical bulk density equal to that of graphite (unfortunately not a number likely to be measured because of the tendency of carbon chains to explosive polymerization !). Assuming an average bond length of 1.3 ” we then obtain from the empirical –t log(D/B)\[3.98[4 log L for the Youngœs modulus E\1.7]1013 dyn cm~2 which is ca. 8 times that of steel and 1.5 times that of diamond. Carbon chains are evidently stiÜ indeed. Low-frequency bends at radio frequencies 15N H3N should be 16 times less than that ofCwhich is at 220 and should lie at ca.11 cm~1 or 330 GHz. Long carbon chains may be difficult to stretch but they are easy to bend and the lowest frequency bends of the two longest chains in Fig. 1 HC15N HC17N and are expected to lie in the submillimetre band where they are within range of existing high-frequency radio telescopes. A fairly good estimate of the frequencies is provided by a simple classical calculation. The bending modes of a thin elastic rod scale in frequency as the inverse square of the rodœs length,29 so one expects the more ì classical œ of the transverse vibrations of a long chain the lowest-frequency bend especially to scale with length in approximately that way. The low-frequency bends measured for HCN30 and HC3N,31 and those calculated ab initio for HC5 and N HC7 N,32,33 –t this scaling law to ca.15%. Extended to the chains here such scaling predicts that the frequency of the lowestfrequency bend of HC cm~1. It should therefore lie at ca. 14 cm~1 or 420 GHz. Similarly that of HC should be 20 times less than that of HC 17N Both these frequencies lie comfortably within the range of existing radio telescopes 3N 2O O2 line at 325 GHz and an even sharper line at 425 GHz but for example the Caltech submillimetre telescope and the UK James Clark Maxwell telescope on Mauna Kea where the atmosphere is fairly transparent at both frequencies (except for a sharp H these contaminate only a fraction of the range of uncertainty of either bend and so may prove no hindrance to astronomical detection).Precise laboratory measurement of these vibrational transitions is a prerequisite for an astronomical search because the estimated frequencies are probably uncertain to ca. 15%. The transition dipole moments for these low frequency bends are even more uncertain than the frequencies. Estimates by Andrew Cooksy diÜer by an order of magnitude from ca. 1.3 to 0.1 D the uncertainty resulting from the largely unknown distribution of the dipole moment along the long conjugated chain. Ab initio calculations of the structures of HC15N HC17N and might yield considerably better transition moments and it would be desirable to have these in hand before attempting to observe the undoubtedly weak bends in astronomical sources. Weakly bonded molecules The FTM technique applied to supersonic molecular beams has been a very powerful tool for the study of van der Waals complexes which are readily formed at the characteristically low kinetic and rotational temperature of the beam.We have identi–ed one such complex without yet arriving at a certain identi–cation. It may be a stable chain such as diacetylene weakly attached to a second. There are probably many others which we could detect but which have not been pursued because of the limited astrophysical interest in such systems. No van der Waals complex has been detected in space and it is difficult in the almost total absence of three-body collisions there to imagine how they might be formed at a rate adequate to maintain a detectable population. 134 New carbon chains in laboratory and interstellar space Optical detection Carbon chains because of their conjugated electrons are similar to organic dye molecules and similarly they are expected to possess strong electronic optical transitions which move to the red as the chain length is increased.All or nearly all of the carbon chains in Fig. 1 therefore are likely to possess intense optical transitions but with the exception of C8H C10H and (J. Maier personal communication) none of these has been observed in the gas phase. At the densities which we have been able to produce almost all are probably detectable with modern laser techniques laser-induced —uorescence (LIF) spectroscopy for example for those transitions which —uoresce and resonanceenhanced multiphoton ionization (REMPI) spectroscopy for those which do not.Carbon chains such as HC17N at the limit of detection of the present FTM spectrometer (at an integration time of ca. 1 h) have a density of ca. 108 cm~3 in our molecular beam which is well above the threshold of detection by either laser technique. Aside from the interest to radio astronomy a major motivation of the present work has been to learn how to produce and to identify large carbon chains preparatory to optical experiments designed identify the carriers of the diÜuse interstellar bands. Note added in proof Five additional carbon chains have now been detected. 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