Laser Excitation and Fourier Transform Emission Spectroscopy of ScS R. S. Ram Department of Chemistry, University of Arizona, Tucson, AZ J. Gengler, T. C. Steimle Department of Chemistry, Arizona State University, Tempe, AZ P. F. Bernath Department of Chemistry, University of Waterloo, Waterloo, Ont. Canada N2L 3G1

Introduction Chemical importance Diatomic molecules such as ScS represent simple metal systems where d electrons take part in the bonding. Such molecules provide ideal models for the understanding the bonding and reactivity in transition metal systems. Astrophysical importance Like diatomic oxides and hydrides, sulfide molecules may also be found in the atmospheres of cool stars. TiS and ZrS have been identified in S-type Mira variable stars and there is strong possibility that ScS may also be found. If found, their spectra will provide data on the abundance of sulfur in cool stars.

Previous work on ScS The ScS bands were first observed by Stringat and Fenot in 1976 and were classified into A 2 Π-X 2 Σ + and B 2 Σ + -X 2 Σ + systems. The bands were double-headed with a splitting of ~0.23 cm -1 close to the hyperfine doubling of cm -1 observed for ScO. [Stringat and Fenot, CJP 54, 2293 (1976)] A rotational analysis of the 0-0 band of the B 2 Σ + -X 2 Σ + system was obtained and spectroscopic constants were determined. [Fenot, Femenias and Stringat, JMS 78, 40 (1979)] Ab initio predictions of spectroscopic properties have been made at the SDCI and CPF level of theory [Bauschlicher and Langhoff, JCP 85, 5936 (1986)] The 1-0 band (not 0-0 as stated in the paper!) of the B 2 Σ + -X 2 Σ + system was measured using the a laser ablation/ molecular beam source and dipole moments as well as spectroscopic and hyperfine constants were obtained for ScS. [Steimle, Marr and Goodridge, JCP 107, (1997)] Present work The emission spectrum of the A 2 Π-X 2 Σ + and B 2 Σ + -X 2 Σ + systems of ScS has been observed in the cm -1 region using a Fourier transform spectrometer. The 0-0 and 1-0 bands have also been measured by laser excitation spectroscopy. Rotational analysis of the 0-1, 0-0 and 1-0 bands of the A 2 Π 3/2 - X 2 Σ + and 0-1, 0-0, 1-0, 2-0 and 3-1 bands of the B 2 Σ + -X 2 Σ + systems has been obtained and improved spectroscopic constants have been determined for the observed electronic states of ScS.

Fig. 1. DC discharge – furnace setup Experimental Setup for FTS Observations Beamsplitter UV Number of scans: 4 Detectors: Midrange Si diode Signal-to-noise ratio: 8 Filter: RG645Furnace Temp.: 2125ºC Resolution: 0.05 cm -1 He pressure:261 Torr

Fig. 2. An overview of the emission spectrum of ScS observed using the FT spectrometer at Kitt Peak.

Observations for ScS FTS Observations: B 2 Σ + -X 2 Σ cm -1 J= A 2 Π 3/2 -X 2 Σ Laser Excitation Spectroscopy: B 2 Σ + -X 2 Σ + 0-0, 1-0 bands A 2 Π -X 2 Σ band

Fig. 3. A portion of the emission spectrum marking band heads of the B 2 Σ + -X 2 Σ + transition of ScS.

Fig. 4. Labeling of branches in the of the A 2 Π- X 2 Σ + and B 2 Σ + - X 2 Σ + systems of ScS

Fig. 5. Four R heads of the 0-0, B 2 Σ + - X 2 Σ + band of ScS arising due to spin splitting in the B 2 Σ + state and hyperfine splitting of 0.23 cm -1 in the X 2 Σ + state

Table 1. Spectroscopic constants (in cm -1 ) for the B 2 Σ + -X 2 Σ + system of ScS Note: The ground state spin splitting constant was fixed at cm -1, a value reported by Steimle et al. [ JCP 107, (1997)].

a Unpublished results by Steimle et al. Spectroscopic Constants (in cm -1 ) for the A 2 Π-X 2 Σ + System of ScS

Conclusion Financial support for this work was provided by the NASA laboratory astrophysics program. We thank the National Solar Observatory for access to the FTS and Mike Dulick for experimental assistance. Acknowledgements The A 2 Π 3/2 -X 2 Σ + and B 2 Σ + -X 2 Σ + bands of ScS have been observed in a high temperature furnace as well as laser excitation spectroscopy. A rotational analysis of a number of bands have provided the following equilibrium constants for the low-lying electronic states of ScS. X 2 Σ + : B e = (77) cm -1, α e = (92) cm -1, r e = (42) Å A 2 Π 3/2 : B e = (29) cm -1, α e = (29) cm -1, r e = (17) Å B 2 Σ + :  e = (27) cm -1,  e x e =2.0700(75) cm -1, B e = (67) cm -1, α e = (29) cm -1, r e = (40) Å