Rotationally resolved gas-phase spectrum of the (A)over-tilde 2Σ+-(X)over-tilde 2Π3/2 electronic transition for the cyanogen halide radical cation ICN+

A rotationally resolved gas-phase absorption spectrum of the (A) over tilde (2)Sigma(+)-(X) over tilde (2)Pi(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) over tilde (2)Sigma(+)-(X) over tilde (2)Pi(3/2) electronic states, respectively, and the band origin to be 18262.083(3) cm(-1). The rotational constant ratio beta {= (B'-B '')/B ''}was determined to be -2.8%. The beta 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.