Rotationally resolved A (2)Pi <- X (2)Pi electronic spectra of cyanodiacetylene and dicyanoacetylene cations

The 0(0)(0) bands of the A (2)Pi <-- X (2)Pi and A (2)Pi(g)<-- X (2)Pi(u) t ransitions of cyanodiacetylene and dicyanoacetylene cations have been rotat ionally analyzed at Doppler-limited resolution using frequency modulation a bsorption spectroscopy. The ions were generated in a liquid-nitrogen cooled hollow cathode modulated discharge incorporated in a White cell. Analysis of the rotational structure yields molecular parameters of the cations in t he two electronic states, confirming linear geometries. The inferred spin-o rbit and rotational constants are discussed in terms of the electronic stru cture of the isoelectronic cations. The spectrum of dicyanoacetylene cation reveals Lambda-type doubling consistent with the presence of a nearby (2)S igma(g)(+) state. The second order spin-orbit splitting of the A (2)Pi(g) s tate is analyzed to predict the existence of the (2)Sigma(g)(+) state below the A (2)Pi(g) state. (C) 1999 American Institute of Physics. [S0021-9606( 99)01441-5].