M. C. McCarthy

Laboratory and Astronomical Identification of the Negative Molecular Ion C6H

The negative molecular ion C6H- has been detected in the radio band in the laboratory and has been identified in the molecular envelope of IRC +10216 and in the dense molecular cloud TMC-1. The spectroscopic constants derived from laboratory measurements of 17 rotational lines between 8 and 187 GHz are identical to those derived from the astronomical data, establishing unambiguously that C6H- is the carrier of the series of lines with rotational constant 1377 MHz first observed by K. Kawaguchi et al. in IRC +10216. The column density of C6H- toward both sources is 1%-5% that of neutral C6H. These surprisingly high abundances for a negative ion imply that if other molecular anions are similarly abundant with respect to their neutral counterparts, they may be detectable both in the laboratory at high resolution and in interstellar molecular clouds.

Laboratory and Astronomical Detection of the Negative Molecular Ion C3N

The negative molecular ion C3N− has been detected at millimeter wavelengths in a low-pressure laboratory discharge, and then with frequencies derived from the laboratory data in the molecular envelope of IRC+10216. Spectroscopic constants derived from laboratory measurements of 12 transitions between 97 and 378 GHz allow the rotational spectrum to be calculated well into the submillimeter-wave band to 0.03 km s−1 or better in equivalent radial velocity. Four transitions of C3N− were detected in IRC+10216 with the IRAM 30 m telescope at precisely the frequencies calculated from the laboratory measurements. The column density of C3N− is 0.5% that of C3N, or approximately 20 times greater than that of C4H− relative to C4H. The C3N− abundance in IRC+10216 is compared with a chemical model calculation by Petrie & Herbst. An upper limit in TMC-1 for C3N− relative to C3N (<0.8%) and a limit for C4H− relative to C4H (<0.004%) that is 5 times lower than that found in IRC+10216, were obtained from observations with the NRAO 100 m Green Bank Telescope (GBT). The fairly high concentration of C3N− achieved in the laboratory implies that other molecular anions containing the CN group may be within reach.

Detection of HC[TINF]11[/TINF]N in the Cold Dust Cloud TMC-1

Two consecutive rotational transitions of the long cyanopolyyne HC11N, J=39-38, and J=38-37, have been detected in the cold dust cloud TMC-1 at the frequencies expected from recent laboratory measurements by Travers et al. (1996), and at about the expected intensities. The astronomical lines have a mean radial velocity of 5.8(1) km/s, in good agreement with the shorter cyanopolyynes HC7N and HC9N observed in this very sharp-lined source [5.82(5) and 5.83(5) km/s, respectively]. The column density of HC11N is calculated to be 2.8x10^(11) cm^(-2). The abundance of the cyanopolyynes decreases smoothly with length to HC11N, the decrement from one to the next being about 6 for the longer carbon chains.

Detection of the carbon chain negative ion C8H- in TMC-1

The negative molecular ion C8H- has been detected in the Galactic molecular source TMC-1. Four rotational transitions have been observed in the centimeter-wave band with the NRAO 100 m Green Bank Telescope (GBT) at precisely the frequencies calculated from the recent laboratory spectroscopy of this large carbon chain anion. C8H- is about 5% as abundant as C8H, or somewhat more than C6H- relative to C6H (1.6%). Improved values of the column densities of C6H- and C6H, and an upper limit for the abundance of the smaller carbon chain C4H- of 0.014% with respect to C4H, have also been determined.

Detection of C5N− and Vibrationally Excited C6H in IRC +10216*

We report the detection in the envelope of the C-rich star IRC +10216 of four series of lines with harmonically related frequencies: B1389, B1390, B1394 and B1401. The four series must arise from linear molecules with mass and size close to those of C6H and C5N. Three of the series have half-integer rotational quantum numbers; we assign them to the 2Delta and 2Sigma vibronic states of C6H in its lowest (v_11) bending mode. The fourth series, B1389, has integer J with no evidence of fine or hyperfine structure; it has a rotational constant of 1388.860(2) MHz and a centrifugal distortion constant of 33(1) Hz; it is almost certainly the C5N- anion.

Astronomical identification of CN-, the smallest observed molecular anion

We present the first astronomical detection of a diatomic negative ion, the cyanide anion CN-, as well as quantum mechanical calculations of the excitation of this anion through collisions with para-H2. CN- is identified through the observation of the J = 2-1 and J = 3-2 rotational transitions in the C-star envelope IRC +10216 with the IRAM 30-m telescope. The U-shaped line profiles indicate that CN-, like the large anion C6H-, is formed in the outer regions of the envelope. Chemical and excitation model calculations suggest that this species forms from the reaction of large carbon anions with N atoms, rather than from the radiative attachment of an electron to CN, as is the case for large molecular anions. The unexpectedly large abundance derived for CN-, 0.25 % relative to CN, makes likely its detection in other astronomical sources. A parallel search for the small anion C2H- remains so far unconclusive, despite the previous tentative identification of the J = 1-0 rotational transition. The abundance of C2H- in IRC +10216 is found to be vanishingly small, < 0.0014 % relative to C2H.

Eight New Carbon Chain Molecules

A summary is given of the laboratory measurements of the rotational spectra of eight new carbon chains: the cyanopolyynes HC11N and HC13N; the carbon chain radicals C7H, C8H, C9H, and C11H; and the cumulene carbenes H2C5 and H2C6. Measured line frequencies and derived spectroscopic constants are listed for all eight, and rest frequencies for the astronomically important transitions that are not easily calculated are tabulated. With our laboratory measurements, four of the new carbon chains have already been detected in at least one astronomical source; the remaining four should be detectable with existing large centimeter- or millimeter-wave telescopes.

Microwave Spectra of 11 Polyyne Carbon Chains

A summary is given of the laboratory study of the rotational spectra of 11 recently detected carbon chain molecules. Classified according to their various end groups, these are the cyanopolyynes HC15N and HC17N; the isocyanopolyynes HC4NC and HC6NC; the methylcyanopolyynes CH3(C≡C)3CN, CH3(C≡C)4CN, and CH3(C≡C)5CN; and the methylpolyynes CH3(C≡C)4H, CH3(C≡C)5H, CH3(C≡C)6H, and CH3(C≡C)7H. Measured line frequencies and derived spectroscopic constants are given for each. The microwave laboratory astrophysics of the entire set is now complete in the sense that all the astronomically most interesting rotational transitions, including those with nitrogen quadrupole hyperfine structure, have now been directly measured or can be calculated from the derived constants to a small fraction of 1 km s-1 in equivalent radial velocity. All 11 carbon chains are candidates for astronomical discovery since they are closely related in structure and composition to ones that have already been discovered in space.

Carbon chains and rings in the laboratory and in space

Seventy-seven reactive organic molecules of astrophysical interest have been identified in a supersonic molecular beam, 73 in the radio band by Fourier-transform microwave spectroscopy, four in the optical by laser cavity ringdown spectroscopy. Most are linear carbon chains, but six consist of carbon chains attached to the compact, highly polar C3 ring, and two are rhomboidal cyclic configurations of SiC3. The laboratory astrophysics of the radio molecules is complete for the time being, in the sense that essentially all the rotational transitions of current interest to radio astronomy (including hyperfine structure when present) can now be calculated to a small fraction of 1 km s(-1) in equivalent radial velocity; six of the radio molecules have already been detected in space on the basis of the present data. The FTM spectrometer employed in this work is far from fundamental limits of sensitivity, so many more molecules can probably be found by refinements of present techniques. The density of reactive molecules in our supersonic beam is generally high by the standards of laser spectroscopy, and many of the radio molecules probably have detectable optical transitions which we are attempting to find, largely motivated by the long-standing problem of the diffuse interstellar bands. Our most interesting result to date is the detection of a fairly strong molecular band at 443 nm in a benzene discharge, in exact coincidence with the strongest and best known interstellar band. Isotopic shifts measured with partially and totally deuterated benzene suggest that the carrier of the laboratory band is a hydrocarbon molecule with the elemental formula CnH5, with n most likely in the range 3-6.

Structure of the CCCN and CCCCH radicals: Isotopic substitution and ab initio theory

The millimeter‐wave rotational spectra of the 13C isotopic species of the CCCCH and CCCN radicals and CCC15N were measured and the rotational, centrifugal distortion, and spin‐rotation constants determined, as previously done for the normal isotopic species [Gottlieb et al., Astrophys. J. 275, 916 (1983)]. Substitution (rs) structures were determined for both radicals. For CCCN, an equilibrium structure derived by converting the experimental rotational constants to equilibrium constants using vibration–rotation coupling constants calculated ab initio was compared with a large‐scale coupled cluster RCCSD(T) calculation. The calculated vibration–rotation coupling constants and vibrational frequencies should aid future investigations of vibrationally excited CCCN. Less extensive RCCSD(T) calculations are reported here for CCCCH. The equilibrium geometries, excitation energies (Te), and dipole moments of the A2Π excited electronic state in CCCN and CCCCH were also calculated. We estimate that Te=2400±50 cm−1 ...