High-resolution electronic spectroscopy of a nonlinear carbon chain radical C6H4+

Abstract

A high-resolution gas-phase spectrum of a molecular absorption band around 604 nm is assigned as due to an electronic transition of a nonlinear C 6 H 4 ϩ planar species starting from its 2 AЉ electronic ground state. The spectrum is observed in direct absorption by cavity ringdown spectroscopy through a supersonic planar discharge through a mixture of acetylene in helium. The spectrum has a clear rotational and K-type structure. This allows an accurate determination of the B and C rotational constants and an estimate for the A rotational constant in ground and electronically excited states. The resolved spectrum of the fully deuterated species C 6 D 4 ϩ has been obtained as well. The results are compared both to the outcome of ab initio geometry optimizations and low-resolution absorption spectra in 6 K neon matrices obtained after mass-selective deposition.

Mass spectrometric and laser spectroscopic characterization of a supersonic planar plasma expansion

“…Besides the poly-acetylene progression HC n H + , with n as high as 8, a number of other carbon chain cations-for example, species of the series C 2n H 4 + are found. Such radicals are currently under research in spectroscopic studies; C 6 H 4 + , for example, was just recently shown to have a non-linear non-cyclic ground state structure [12].…”

“…Rotationally resolved spectra, as previously obtained for carbon chain cations and carbon chain radicals using cavity ring down spectroscopy (see e.g., Refs. [12,13,22]) are within range and of particular astrophysical interest in view of the role carbon chain anions are expected to play in interstellar chemistry [23].…”

“…Such an expansion provides a Doppler free environment and combines high molecular densities and relatively long absorption path lengths with effective adiabatic cooling. During the last decade a number of planar plasma sources has been designed, using discharge [6,7], ablation [8], photolysis [9] and electron ionization [10] tech-niques, with spectroscopic applications ranging from cluster ions [11] to long and highly unsaturated carbon chain radicals [12]. The major disadvantage of such a plasma for spectroscopic use is that a large variety of species is formed simultaneously in the expansion.…”

Rotationally resolved electronic spectroscopy of a nonlinear carbon chain radical C6H4+

“…Recently, a nonlinear planar carbon chain radical C 6 H þ 4 , HACBCACBCACH@CH þ 2 (hereafter ''C 6 H þ 4 '' indicates this structural isomer, Fig. 1), was detected in a supersonic planar discharge by cavity ring down (CRD) spectroscopy in the 604 nm region [6]. The related C 4 H þ 4 and C 8 H þ 4 nonlinear carbon chains were also produced and detected in this plasma [7].…”

“…1) was recorded using high-resolution cw-CRD spectroscopy as part of this work. Additionally, an unassigned 585.3 nm absorption band, produced by a molecule having the same mass as C 6 H þ 4 , was observed in a 6 K neon-matrix [6]. To identify the carrier, this frequency region was scanned in the gas phase using cw-CRD spectroscopy, and a rotationally resolved band was observed at 581 nm.…”