Rotational Spectra of the Carbon Chain Negative Ions C ₄ H ^- and C ₈ H ^-

Abstract

The rotational spectra of the butadiyne anion C 4 H Ϫ and the octatetrayne anion C 8 H Ϫ have been detected in the laboratory. Precise spectroscopic constants for these closed-shell molecules have been obtained, which enable their rotational spectra to be calculated to high accuracy throughout the radio band. Deep astronomical searches can now be undertaken in essentially any molecular source, including TMC-1 and IRC ϩ10216, where the negative ion C 6 H Ϫ has recently been detected. The large dipole moments and high binding energies of both anions make them good candidates for astronomical detection.We recently detected the radio spectrum of the negative ion C 6 H Ϫ in the laboratory and showed that this suprisingly large carbon chain anion is the carrier of the unidentified harmonic sequence of molecular lines B1377 observed by Kawaguchi et al. (1995) in the circumstellar shell of the carbon star IRC ϩ10216 (McCarthy et al. 2006). The radio spectra of other molecular anions should now be detectable in the laboratory and in space, especially linear closed-shell carbon chains similar in structure to C 6 H Ϫ . Here we report laboratory detection of the next shorter and the next longer members of the series, the butadiyne anion C 4 H Ϫ and the octatetrayne anion C 8 H Ϫ . Precise transition frequencies over most of the radio band are now available for astronomical searches for these anions.Like C 6 H Ϫ , the C 4 H Ϫ anion has been observed in both the millimeter-wave band with a free space (FS) spectrometer (Gottlieb et al. 2003) and the centimeter-wave band with a Fourier transform microwave (FTM) spectrometer (McCarthy et al. 2000). The longer anion C 8 H Ϫ has only been detected with the highly sensitive FTM instrument, since its rotational lines are fairly weak. Seventeen rotational transitions between 9 and 363 GHz of C 4 H Ϫ and nine successive rotational transitions between 9 and 19 GHz of C 8 H Ϫ have been measured to an accuracy approaching 1 part in 10 7 (see Tables 1 and 2).In the FS spectrometer, C 4 H Ϫ was produced by a DC glow discharge through a flowing mixture of argon (15%) and acetylene (85%) at a total pressure of ≤10 mtorr with the discharge cell walls cooled to 100 K. As with C 6 H Ϫ , the optimum discharge current (∼150 mA) was substantially lower than that which produces the most intense lines of the neutral radical (∼400 mA). At this lower current, lines of C 4 H Ϫ were only about 8 times weaker than those of either fine-structure component of C 4 H; hyperfine structure of C 4 H collapses at high J and is not resolved at millimeter wavelengths, but spin-doubling does not collapse and is resolved. Taking the different partition functions and dipole moments into account, the abundance of the anion is found to be ∼0.14% that of the neutral. The absorption J p 37 R 36 line of C 4 H Ϫ is shown in Figure 1; a signal-to-noise ratio of ∼75 was obtained in only 25 minutes of integration, allowing line frequencies to be measured to 10-20 kHz.In the FTM spectrometer, the anions were produced under disc...

Theoretical study of radiative electron attachment to CN, C2H, and C4H radicals

“…(1) and (2), as well as using Eq. (6). Figure 4 shows the DREA cross sections obtained for CN − , C 2 H − , and C 4 H − .…”

“…Six anions have been detected so far in the ISM: C 6 H − [1][2][3][4][5], C 4 H − [6], C 8 H − [7], C 3 N − [8], C 5 N − [2,5], and CN − [9]. The possibility for atomic anions, such as H − , to be formed in the ISM by REA was first suggested by McDowell [10] in 1961.…”

Infrared diode laser spectroscopy

“…These studies have led to mass use of the Doppler shift method differenti− ating anions from cations on the basis of measuring a spec− trum of a single line, subsequently identifying laboratory spectra of a series of anions [78][79][80] …”

Absolute angle-differential vibrational excitation cross sections for electron collisions with diacetylene

“…Diacetylene is of interest for astronomy-it is a potential precursor for the H−C≡C−C≡C − anion, recently detected in interstellar space [1,2] and it was detected in the upper layers of planetary atmospheres (Titan, Uranus) [3,4], where free electrons also occur. Electron collisions with diacetylene are of interest also in technological plasmas [5,6].…”