A LABORATORY STUDY OF C ₃ H ⁺ AND THE C ₃ H RADICAL IN THREE NEW VIBRATIONALLY EXCITED ² Σ STATES USING A PIN-HOLE NOZZLE DISCHARGE SOURCE

Abstract

Rotational lines of the positive molecular ion C 3 H + and of the neutral C 3 H radical in three new vibrationally excited states with 2 Σ symmetry have been detected in a supersonic molecular beam in the centimeter-wave band. The fundamental rotational line of the ion is quite weak, but is observed with similar intensity in a dc discharge through several different hydrocarbon gases when helium is the buffer gas. Under these conditions, the fractional abundance of C 3 H + relative to C 3 H is estimated to be of order 10 −4 , i.e., toward the lower end of the ratio (10 −3 -10 −4 ) found for protonated ions using the same discharge nozzle. For each new 2 Σ state of the C 3 H radical, spectroscopic constants, including those describing hydrogen hyperfine structure, have been determined to high precision. Lines of one 2 Σ state (B = 11271 MHz) are particularly intense in our molecular beam; for this state and a second one (B = 11306 MHz), millimeter-wave transitions have also been observed between 180 and 340 GHz using a long path dc glow absorption spectrometer. On the basis of intensity measurements with this spectrometer, the inferred rotation-vibration constant α, and theoretical calculations, the state with B = 11271 MHz is tentatively assigned to the ν 5 bending mode, predicted to lie ∼300 cm −1 above ground.

The Cologne Database for Molecular Spectroscopy, CDMS, in the Virtual Atomic and Molecular Data Centre, VAMDC

“…In some cases, initial assignments were based on astronomical data only, such as in the cases of C 5 N − [50] and C 3 H + [51]. Laboratory rotational data have been obtained for the latter cation in the meantime [52,53], in great part because of the dispute concerning the carrier of the lines. Data from astronomical observations can even be important if they only supplement laboratory data and if they have lower accuracies, such as in the case of extensive Herschel-HIFI data on the cyclic SiC 2 molecule [54].…”

Gas-Phase Vibrational Spectroscopy of the Hydrocarbon Cations l-C3H+, HC3H+, and c-C3H2+: Structures, Isomers, and the Influence of Ne-Tagging

“…The recent detection of the linear l-C 3 H + isomer via its rotational transitions in several regions of the interstellar medium 11,21,22 has triggered several laboratory and theoretical spectroscopic studies on this closed-shell hydrocarbon ion. In order to identify the astronomical lines, high-resolution rotational spectra were recorded using rotational action spectroscopy in a cryogenic ion trap 23 and Fourier transform microwave spectroscopy, 24 providing accurate spectroscopic parameters for its vibrational ground state. The observed large centrifugal distortion parameter prompted Huang et al 25 to question the original identification of the astronomical lines as belonging to l-C 3 H + based on quantum chemical calculations, resulting in a much smaller value.…”

Aromatics and Cyclic Molecules in Molecular Clouds: A New Dimension of Interstellar Organic Chemistry

“…In some instances it has been also used to reproduce a rotational line that exactly agrees in frequency with an unassigned astronomical feature or sequence of lines, e.g., B 1377, the carrier of which was shown to C 6 H – . Owing to high detection sensitivity, this variant is also extremely well-suited for isotopic studies, and detection of highly reactive intermediates, such as molecular ions, that are produced in low steady state abundance . Although wide spectral surveys are possible, these become inefficient with increasing IFBW.…”