Ã2Σ+g → X̃2Πu emission band system of dicyanoacetylene radical cation in the gaseous phase

The emission spectrum of the dicyanoacetylene radical cation has been observed in the 580–720 nm wavelength region as a result of low energy electron impact excitation in a crossed-beam arrangement. The band system is attributed to the òΣ⁺_g → X?²Π_u electronic transition by comparison with the photoelectron spectroscopic and calculated data on the states of dicyanoacetylene cation. The frequencies of the three Σ⁺_g vibrational fundamentals in the ground cation state have been deduced from the emission spectrum. The lifetime of the lowest vibrational level of the òΣ⁺_g state of dicyanoacetylene cation was determined to be 13 ± 2 ns. Emission could not be detected from the corresponding states, òΣ⁺, of fluorocyanoacetylene and cyanoacetylene cations, and these results are discussed.