1. A LABORATORY AND THEORETICAL INVESTIGATION OF THE SILICON SULFUR MOLECULES H 2 SiS AND Si 2 S. MICHAEL C. MCCARTHY 1, PATRICK THADDEUS 1, HARSHAL GUPTA 2, SVEN THORWIRTH 3, JÜRGEN GAUSS 4 and FRANÇOIS SHINDO 1 1 Harvard-Smithsonian Center for Astrophysics, Cambridge MA, USA. 2 Institute for Theoretical Chemistry, University of Texas, Austin, USA 3 Max-Planck-Institut für Radioastronomie, Bonn, Germany 4 Institut für Physikalische Chemie, Johannes-Gutenberg-University, Mainz, Germany

2. Motivations Astronomical interest: Significant fraction of molecules with Si or S in circumstellar shells ex: 1/3 of the 37 molecules in IRC+10216 Closely related in composition with SiS: Importance of SiS in the photochemistry of IRC+10216 High abundance: seven isotopes including 29 Si 34 S and 30 Si 34 S Rotational transitions high J (20-19) vibrational excited levels (v=3) maser action But little information on other molecules containing both Si and S

3. Motivations Laboratory work on similar systems: Extensive spectroscopy on the three isovalent molecules of silanethione (H 2 SiS): Formaldehyde (H 2 CO), H 2 CS, silanone (H 2 SiO) rotational spectrum of radical HSiS detected in glow discharge cell Brown et al., J. Mol. Struct. 413-414, 537(1993) Theoretical studies on both H 2 SiS and Si 2 S fairly polar (H 2 SiS μ~3D) Most stable isomeric form Lai et al., Inter. J. Quant. Chem. 82, 14 (2001) Davy and Schaefer, Chem. Phys. Lett. 255, 171 (1996)

4. This work Joint experimental and theoretical study of H 2 SiS and Si 2 S Lab investigation: Detection of the rotational spectra by FTM spectroscopy (5-43GHz) Rotational spectra of many isotopes (~10 species) Rotational spectra from several vibrational levels H 2 SiS in mmw band (200-376GHz) in glow discharge Derive precise rotational and centrifugal distortion constants Theoretical investigation with coupled-cluster (CC) methods: Vibrational energies of GS, vibration-interaction constants Empirical and theoretical structures

5. The Search Both planar, prolate asymmetric-top, closed-shell molecules with C 2ν symmetry Si 2 S Two Si atoms: Bose-Einstein statistics b-type transitions (K a K c =ee or oo) Fundamental transition 1 1,1 →0 0,0 at 11.4 GHz H 2 SiS Two equivalent H: ortho-para statistics Triplet structure split by ~750 MHz a-type transitions Fundamental transition 1 0,1 →0 0,0 at 15.7 GHz

6. Centimeter band investigation Discharge: SiH 4 +H 2 S diluted in Ne, 1000 V 90 individual scans of 0.4 MHz, 2h FTM spectrometer 5-43 GHz, supersonic beam with T of 1-3 K Intense lines observed For Si 2 S : 13 transitions (J=5, K a =3) 3 Isotopes (Si 34 SSi, 29 SiSSi, 30 SiSSi) 8 spectroscopic constants, rms~few kHz For H 2 SiS : 4 transitions because B+C~ 15GHz Measurement of H 2 Si 33 S hfs ( 33 S, 0.76%) Additional study in mm (200-376GHz) 58 transitions in mm up to J=23 and K a =5 10 spectroscopic constants, rms~10 kHz

7. Vibrational excitation Intense vibrational satellites for several rotational lines of H 2 SiS (and normal species) in Ne Quenched by H 2 Determination of the vibration-rotation constants α i B and α i C B v -B 0 =-v α i B => ν 6 mode (b 2, SiH 2 in plane rock) calculated at 618 cm -1 (MHz) α6Aα6A α6Bα6B α6Cα6C Experimental -12.825.67 Theory* 84605.474-14.0506.919 *fc-CCSD(T)/cc-pV(Q+d)Z

8. r Si-S (Å)r H-Si (Å)HSiH˚ Best estimate 1.93711.4743110.23 r e emp 1.93571.4735110.33 r Si-S (Å)r Si-Si (Å)SiSSi˚ Best estimate 2.13072.375667.76 r e emp 2.13012.374467.74 Best estimate: fc-CCSD(T)/cc-pV ∞ Z + Δcore/cc-pCVQZ +ΔT/cc-pVTZ + ΔQ/cc-pVDZ r e emp : r 0 after zero-point vibrational correction in fc-CCSD(T)/cc-pV(Q+d)Z level theory Structures derived from this work Si=SSi-Si dimer r e (Å)1.9292.246 Comparison with known bond lengths

9. Conclusion Extensive experimental and theoretical study of H 2 SiS and Si 2 S H 2 SiS μ =2.67 D Si 2 S μ=0.66 D Derived spectroscopic constants more than adequate for astronomical search and detection Intense lines of both species observed in laboratory: Si 2 O, cis and trans-HSiSH

10. H 2 SiS in the millimeter band Freq. modulated MW spectrometer, discharge cell range of measurements 230-377GHz Complete observations in cm band 58 transitions up to J=23 and K a =5 Best fit: 10 spectroscopic constants, rms~10 kHz Intensity ratio SiS/H 2 SiS~1000 Discharge: SiH 4 +H 2 S+Ar, 150 mA Ptot=55 mTorr, T=-120˚C Integration time 23s/MHz (v=0) 14-13 16 0,16 -15 0,15

11. Si 2 S/isotopes spectroscopic constants

13. H 2 SiS/isotopes spectroscopic constants

15. Vibrational energies

16. Vibration-rotation interaction