New carbon chains in the laboratory and in interstellar space

Abstract

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 first 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 three member carbene ring C₃H₂. All the astronomically interesting rotational transitions [including hyperfine structure (hfs) for the radicals] of the entire 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 (C₇H, C₈H, H₂C₆, and HC₁₁N) 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 refinements, and better precursor gases may be found. Many of the chains here probably have low-lying 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 Young's modulus: E=1.7×10¹³ dyn cm^-2, larger than that of diamond; the longest two chains we have detected, HC₁₅N and HC₁₇N, 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.