Laser induced fluorescence spectroscopy of the (A)over-tilde 3Πi←(X)over-tilde 3Σ- transition of the CCS radical

The (A) over tilde (3)Pi (i)<--(X) over tilde (3)Sigma (-) electronic transition of a carbon-chain molecule CCS has been observed for the first time by laser induced fluorescence spectroscopy. The species was generated with a pulsed discharge of 0.5% C2H2 and 0.5% CS2 diluted in Ar via a pulsed supersonic jet. A number of bands in the 690-675 nm and 600-615 nm regions (14 500-14 800, 16 200-16 600 cm(-1)) were observed and assigned to those of CCS using ground state combination differences. Three of the bands in each region were found to belong to the three (A) over tilde (3)Pi (i) spin-orbit components of the (A) over tilde (3)Pi (i)<--X (3)Sigma (-) transition (T-v=14 662.777(4) and 16 423 cm(-) (1)), and an effective set of rotational constants for the upper states were obtained. The higher energy band was heavily perturbed, preventing the determination of effective constants for this band. These bands have been tentatively assigned as two successive bands in the (A) over tilde (3)Pi (i)(n,0,0) progression with a resultant effective value of 1760 cm(-1) for the (A) over tilde (3)Pi (i) nu (1) vibrational fundamental. Dispersed fluorescence spectra from the (A) over tilde (3)Pi (i)<--X (3)Sigma (-) band of CCS have also been observed, yielding definitive experimental information on the vibrational structure of the X (3)Sigma (-) ground state for the first time. The three harmonic frequencies were determined to be 1708.2(40) cm(-1), 862.5(19) cm(-1), and 269.3(13) cm(-1) for omega (1) (CC stretch), omega (2) (CS stretch), and omega (3) (CCS bend), respectively. (C) 2001 American Institute of Physics.