Submillimeter-wave Spectroscopy of Thioformaldehyde, H2CS,

in its Ground State

Atusko Maeda, Ivan Medvedev, Eric Herbst,Mandfred Winnewisser, Frank C. De Lucia ,

Department of Physics, The Ohio State University, Columbus, OH 43210, USA;

Holger S. P. Müller, Christian P. Endres & Stephan Schlemmer

I. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany.

Thioformaldehyde, H2CS

Dipole moment alonga-axis: μa = 1.6483 Debye

(Fabricant et al. 1977)

a

Planar asymmetric moleculeAsymmetry parameter κ = –0.99

a-type transitions(ΔKa = 0, ±2…, ΔKc = ±1, ±3…)

Previous Spectroscopic Studies

Rotational spectroscopy* H2CS, microwave(1)

* H2CS, D2CS, H213CS, H2C34S, microwave in 1-70 GHz(2)

* H2CS, millimeter-wave in 104-244 GHz(3)

* H2C33S, microwave in 1-37 GHz(4)

* H2CS, H2C34S, FIR in 16-60 cm-1(5)

* HDCS, millimeter-wave in 90-400 GHz(6)

J = 27, Ka = 0-2

J = 53, Ka = 0-9

(1) Johnson & Powell 1970, (2) Johnson, Powell & Kirchhoff 1971,(3) Beers et al. 1972, (4) Brown et al. 1987, (5) Mc Naughton & Bruget 1993 (6) Minowa et al. 1997

S-bearing Moleculesin the Interstellar

Ultra-compact HII region

O,B star

T = 100-300 K

n = 106-108 cm-3

Cold background

(H2S, SO, SO2, OCS, H2CS…)

grain mantle

H2S? OCS?

S-compounds

H2CS abundance can be chemical clock→ Age of heated gas

S

Heated up!

Odin Observation toward Orion KL

Transition Odin

J' Ka' Kc' J'' Ka'' Kc''Obs. Freq.

(MHz)

14 1 13 13 1 12 487663

16 0 16 15 0 15 547315

16 2 15 15 2 14 548927

16 4 13 15 4 12 549201

16 4 12 15 4 11 549201

16 3 14 15 3 13 549407

16 3 13 15 3 12 549452

Old Predictionextrapolated from

low freq. data

6.3

15.3

16.6

16.8

16.4

12.2

11.9

Obs.(Odin)-Pred.(old)

Laboratory measurement at higher frequency!!

(at least better prediction…)

Experimental Conditions

FASSST Spectrometer @ OSU

• 120-370 GHz with 3 BWOs• 100 scans accumulation • up- & down-ward scans averaged• SO2 for frequency calibration• better than 100 kHz uncertainty

• Production of H2CSPyrolysis of trimethylene-sulfide [(CH2)3S] (Sigma-Aldrich Co.)

at 680 ℃ (at OSU), 1300 ℃ (at Cologne)

Terahertz Spectrometer @ U. Cologne

• 570-670 GHz with a BWO• Phase-lock-loop (PLL) technique

→ frequency stabilization• ~5 kHz experimental uncertainty• higher frequencies intended….

Spectrum Assignment

• Predictions with SPFIT/SPCAT(1) program suitesbased on previous microwave & millimeter-wave (& FIR) transitions of H2CS, H2C34S, H2C33S, H2

13CS.

• CAAARS(2) (Computer Aided Assignment of Asymmetric Rotor

Spectra) program for the FASSST spectrum

(1) Pickett, J. Mol. Spectrosc., 148, 371 (1991), http://spec.jpl/nasa.gov(2) Medvedev et al. J. Mol. Struct. 742, 229 (2005)

FASSST Spectrum of H2CS

Intensities agree with natural abundance ratio32S : 33S : 34S = 95 : 0.75 : 4.2

12C : 13C = 99 : 1

H2CS H2C34S

H2C33S H213CS

×10

×100 ×100

80,8←70,7

85,4←75,3

85,3←75,2

Terahertz Spectrum of H2CS

1913,6←1813,5

1913,7←1813,6

R-branch (ΔJ = +1)ΔK = 0

P-branch (ΔJ = –1)ΔK = 2

Summary of Transitions Newly Assigned

• H2CS (200 transitions)

a-type R: J=3-18, Ka=0-15, Q: J=15-44, Ka=1-2

P: J=13,14, Ka=0.

• H2C34S (140 transitions)

a-type R: J=3-19, Ka=0-11, Q: J=15-26, Ka=1.

• H2C33S (360 transitions) with I=3/2

a-type R: J=3-19, Ka=0-11, Q: J=16-19, Ka=1.

• H213CS (150 transitions)

a-type R: J=3-19, Ka=0-11, Q: J=16-23, Ka=1.

Previous data

+ Microwave + FIR transitions

+ Microwave + FIR transitions

+ Microwave

+ Microwave

Ka up to 15!

Ka up to 11!

Analysis with SPFIT

• Effective rotational Hamiltonian for asymmetric-top molecules in Watson’s S-reduced form

HR = AJa2 + BJb

2 + CJc2

– DJJ4 – DJKJ2Ja2 – DKJa

4

+ d1J2[(J+)2 + (J–)2] + d2[(J+)4 + (J–)4]

+ (higher order centrifugal distortion terms)

* For 33S species with I = 3/2, Hyperfine interaction Hamiltonian is added

Hhyp = HeQq + HRI

Molecular Constants

New transition frequency predictionsfor 4 isotopic species!

Frequency Comparison [MHz] between Odin & New Prediction

Transition OdinNew

Prediction Residuals

J' Ka' Kc' J'' Ka'' Kc'' Obs. Freq. Calc. Freq.

14 1 13 13 1 12 487663 487663.321 –0.3

16 0 16 15 0 15 547315 547307.994 7.0

16 2 15 15 2 14 548927 548920.315 6.7

16 4 13 15 4 12 549201 549188.338 12.7

16 4 12 15 4 11 549201 549188.727 12.3

16 3 14 15 3 13 549407 549402.035 5.0

16 3 13 15 3 12 549452 549447.188 4.8

Line Surveys of Orion-KL by Schilke et al.

1) 325-365 GHz (1997) Frequency Comparison [MHz]

Extrapolated fromlow frequency data

338.0 338.1 338.2 338.3

Old prediction338.0808 GHz

Observed338.0832

Line Surveys of Orion-KL

by Schilke et al.

626.4 626.5 626.6 626.7

Old prediction626.482 GHz

Lab. Obs.626.496 GHz

641.3 641.4 641.5 641.6

Lab. Obs.641.437 GHz

Old prediction641.421 GHz

2) 607-725 GHz (2001)

Extrapolated fromlow frequency data

The Equilibrium Structure of H2CS

r(CH) / Å r(CS) / Å (HCH)

exp. (αs from MP2) 1.0866 (9) 1.6093 (3) 116.50 (15)

exp. (from r0)a 1.0856 (21) 1.6110 (8) 117.21 (35)

MP2/cc-pV(Q+d)Z 1.0853 1.6069 116.36

CCSD(T)/cc-pV(Q+d)Z 1.0866 1.6140 116.30

a) Turner et al., JMSp 88 (1981) 402

Conclusion• Rotational spectra of thioformaldehyde in the ground state were

observed for 4 isotopic species; H2CS, H2C34S, H2C33S, H213CS.

• Discrepancies between new lab. measurements and old predictions at high Ka(≥4) were found at most 3 MHz in the FASSST spectrum.

• Intensity ratios of lines of isotopic species agree with natural abundance ratio, supporting the isotopic assignments.

• R-branch transitions with J ≤ 19 & Ka ≤ 15, Q-branch transitions with J ≤ 44 & Ka ≤ 2 have been assigned.

• New predictions/measurements allow unambiguous identification of interstellar lines.

• Precise equilibrium molecular structure of H2CS is determined.

Satellite band spectra were found and assigned to ν2, ν3, ν4, and ν6

Global analysis →→→→→ WG04 (Microwave session)