Over 140 molecules have been identified in the interstellar gas and circumstellar shells, the largest among them a carbon chain with 13 atoms and a molecular weight of 147 (twice that of the simplest amino acid glycine). The astronomical identifications rely on very precise laboratory measurements of molecular line frequencies, often as good as one part in 107. Astronomers in the RG and AMP divisions carry out such measurements in a laboratory equipped with two unique instruments: a free space millimeter-wave absorption spectrometer and a Fourier transform microwave (FTM) spectrometer used in combination with a supersonic molecular beam. The FMT device is equipped with a sensitive microwave receiver and is fully computer controlled so that nearly round-the-clock operation is routinely achieved. This spectrometer possesses both high sensitivity—about a million molecules per quantum state per gas pulse—and high spectral resolution.
The FTM spectrometer has been used with good success in the past nine years to detect a large number of molecules of astronomical interest: more than 130 by late 2006. Ten of these species have already been detected in rich astronomical sources and many of the others are good candidates for detection because they are closely related in structure and composition to known astronomical molecules. Many are also calculated to be quite polar, which should assist radioastronomical searches.
In late 2006 the group succeeded in measuring precise rotational line frequencies for the negative molecular ion (anion) C6H– (see picture), and soon thereafter was able to identify this species in the dense interstellar cloud TMC–1—the first detection of a molecular anion in interstellar space. Widely thought to be too fragile to exist in the severe environment of space, the surprisingly large abundance measured for C6H– suggests that many other anions may be detectable both in the laboratory and in space. Indeed, since their discovery of C6H–, the group also has identified C8H– and C4H– in the lab, and astronomical searches are now under way.
Michael McCarthy (AMP),
The electrostatic potential around the anion C6H–, recently detected in the laboratory and in space by the spectroscopy group (McCarthy, Gottlieb, Gupta, & Thaddeus 2006, ApJ, 653, L141).