Rotationally resolved gas-phase spectrum of the A∼ ²Σ⁺–X∼ ²Π_3/2 electronic transition for the cyanogen halide radical cation ICN⁺

A rotationally resolved gas-phase absorption spectrum of the A∼ ²Σ⁺–X∼ ²Π_3/2 electronic transition of the cyanogen iodide radical cation ICN⁺ was observed by cavity ring-down spectroscopy for the first time. This cation was produced in a supersonic planar discharge jet through a mixture of ICN in helium. By the aid of a program for rotational, vibrational and electronic spectra PGOPHER, the rotational constants were determined to be 0.10700(12) and 0.11002(12) cm⁻¹ for the A∼ ²Σ⁺ and X∼ ²Π_3/2 electronic states, respectively, and the band origin to be 18262.083(3) cm⁻¹. The rotational constant ratio β {= (B′−B″)/B″} was determined to be −2.8%. The β values for FCN⁺, ClCN⁺, BrCN⁺, and ICN⁺ were also evaluated theoretically by CAM-B3LYP/CEP-121G using Gaussian 09W. The rotational profile of the absorption band and its temperature dependence for ClCN⁺, one of the important candidates for Diffuse Interstellar Bands, were simulated, aiding us in the identification of this cation as in interstellar space.