A Missing Link: The Rotational Spectrum, Geometrical Structure, and Astronomical Discovery of Disilicon Carbide, SiCSi M.-A. Martin-Drumel, C. A. Gottlieb, N. J. Reilly, M. C. McCarthy Laboratory measurements J. Baraban, P. B. Changala, S. Thorwirth, J. F. Stanton Theoretical calculations J. Cernicharo, M. Agúndez, L. Velilla Prieto, M. Guélin, C. Kahane, N. A. Patel, G. Quintana-Lacaci, K. H. Young Radio astronomy Talk RD12 ISMS, 70 th Meeting, June 2015 Champaign-Urbana, Illinois

 Fundamental importance; only Si m C n n+m=3 cluster whose ground state structure and bonding have so far eluded laboratory characterization  Believed to be highly stable; one of the most abundant molecular fragments in vaporization of solid SiC  Chemical equilibrium calculations predict it to be highly abundant in IRC+10216; may play a key role in formation of SiC dust grains Motivation

Fundamental interest Quantum Monodromy Chemical bonding and structure Reilly et al. JCP (Comm.) (2015 )

Vaporization of solid SiC dominated by Si, Si 2 C and SiC 2

Long predicted to be abundant in IRC Inner winds of asymptotic giant branch (AGB) are one of two sources for interstellar dust grains Existence of SiC grains established years ago from emission band at 11.3  m; SiCSi thought to be a key species in nucleation processes

Long predicted to be abundant in IRC+10216

Previous theoretical & experimental work Ground state PES subject of many calculations owing to low barrier to linearity Grev & Schaefer, JCP (1985); Diercksen et al., CPL (1985); Rittby, JCP (1991); Spielfiedel, et al., JPC (1996), etc. Consensus is now bent structure of 1 A 1 symmetry, a shallow bending angle ( °), and modest dipole moment (0.9 D) Several infrared bands measured in cold matrices Kafafi, et al. JPC (1983); Presilla-Marquez & Graham, JCP (1991) Recent low-resolution optical spectra and dispersed fluorescence Steglich & Maier, ApJ (2015); Reilly et al. JCP (Comm.) (2015)

Key findings Rotational spectrum measured by FT and DR up to 210 GHz; mmw absorption lines detected as well Singly-substituted isotopic spectroscopy performed. Precise molecular structure derived in combination with new variational calculations More than 110 astronomical lines detected by both a single dish (IRAM 30m) and an interferometer (SMA) SiCSi found to be highly abundant both in laboratory and in space Rotational transitions now detected from all three vibrationally excited states

Challenging target for rotational detection Extremely sparse low-J spectrum, owing to C 2v symmetry and shallow bending angle Frequency (GHz) Relative Int. faint lines Predicted lines between 5 and 50 GHz, assuming T rot = 2 K

Challenging target for rotational detection Extremely sparse low-J spectrum, owing to C 2v symmetry, and shallow bending angle Frequency of b-type lines very sensitive to bending angle; a 1° change in the apex angle shifts lines by ~2 GHz Zero-point correction is significant: shifts A e (~62 GHz) down by 2-3 GHz Strongest line ( ) predicted near 42 GHz, above our frequency ceiling

Challenging target for rotational detection Extremely sparse low-J spectrum, owing to C 2v symmetry and shallow bending angle Frequency (GHz) Relative Int. T rot = 2 K faint lines Search range of spectrometer wide search not possible!

Challenging target for rotational detection Extremely sparse low-J spectrum, owing to C 2v symmetry, and shallow bending angle Solution: search for Si 13 CSi first; substitution reduces A value by 10% or about 4 GHz Frequency of b-type lines very sensitive to bending angle; a 1° change in the apex angle shifts lines by ~2 GHz Zero-point correction is significant: shifts A e (~62 GHz) down by 2-3 GHz Strongest line ( ) predicted near 42 GHz, above our frequency ceiling

Experimental approach Use double resonance to extend frequency range ( GHz+) Cavity FTWM (5-43 GHz) + pin-hole nozzle + electrical discharge Combination: rapid formation and stabilization of rotationally cold molecules in multiple minima on PES; T vib may be much higher

K a =0K a =1 J= E/k (K) E/h (GHz) 10 Offset (kHz) 29 Si 13 CSi Si 13 CSi 0 = MHz 0 = MHz Doppler hfs Initial discovery FT DR 10 sec 15 min

K=0K=1K= E/k (K) E/h (GHz) 10 8 Total of 22 lines by FT and DR; up to J=20 New lines yield  K; T rot well above 3 K Follow-up work FT DR 20 1,19 —19 2,18 E LO =85 K ! 12 min Offset (kHz) Doppler

Structural determination: VPT2 versus variational corrections ConstantSiCSiSi 13 CSi 29 SiCSi 30 SiCSi A0A B0B C0C 0 VPT2 corrections: AA BB CC  SE Variational FC-CCSD(T)/PVQZ : AA BB CC  0 SE

Variational rovibrational treatment Si Potential energy curve Quartic force field 1D variational wavefunction Bending mode is highly anharmonic— perturbative methods (VPT2) not appropriate Exact nuclear motion calculation required to quantitatively predict vibrational/zero-point corrections to rotational constants C

r e SE Structure ParameterCCSD(T)/PVTZ (fc)CCSDT(Q)/PVTZ (fc)delta (Q) Best estimate R C-Si /Å Apex angle (°)

Vibrational satellites VPT2 a Variational b ExperimentDifference (%) v 2 =1 (bend) A 0 -A v /-1.4 B 0 -B v /-3.2 C 0 -C v /-3.3 v 3 =1 (sym str.) A 0 -A v /-2.3 B 0 -B v /-3.4 C 0 -C v /-3.6 Units: MHz a FC-PVQZ VPT2 b FC-CCSD(T)/PVQZ

mmw absorption spectra Frequency (MHz) 31 1,31 —30 0,30 9 min rms of 20 kHz with 42 laboratory lines (22 FT and DR, and 20 mmw) and 15 constants; about 10 times smaller than combined lab+astro fit (~660 kHz) SiCSi is about 15 times lower than SiC 2

Chirped detection shots (3 hours)

Chirped detection shots (3 hours)

Chirped detection shots (3 hours)

Sampling of astronomical data SMA: maps from average of 4 lines Close coordination with radio astronomy IRAM 30m: selected lines, 112 in total

Rotational T and radial distribution Maps and change in line shape with suggest SiCSi concentrated near central star (6”), but with extended distribution Appears abundant in dust formation zone Evidence for several zones Derived abundance is comparable to that of SiC 2 ; lines are ~100 weaker owing to smaller dipole moment and larger partition function

Future Work Explore further bending potential MODR to understand complex excited states ALMA interferometry Larger Si m C n clusters Acknowledgments Acknowledgments Damian Kokkin Sam Palmer Paul Antonucci