1. 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, Köln, Germany.

2. Thioformaldehyde, H2CS a Planar asymmetric molecule
Asymmetry parameter κ = –0.99 a
Dipole moment along
a-axis: μa = Debye
(Fabricant et al. 1977)
a-type transitions
(ΔKa = 0, ±2…, ΔKc = ±1, ±3…)

3. Previous Spectroscopic Studies
Rotational spectroscopy
* H2CS, microwave(1)
* H2CS, D2CS, H213CS, H2C34S, microwave in 1-70 GHz(2)
* H2CS, millimeter-wave in GHz(3)
* H2C33S, microwave in 1-37 GHz(4)
* H2CS, H2C34S, FIR in cm-1(5)
* HDCS, millimeter-wave in 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

4. S-bearing Molecules in the Interstellar
(H2S, SO, SO2, OCS, H2CS…)
O,B star
S-compounds S
Ultra-compact HII region
Heated up!
H2S? OCS?
grain mantle
T = K
n = cm-3
H2CS abundance can be chemical clock
→ Age of heated gas
Cold background

5. Odin Observation toward Orion KL
Old Prediction
extrapolated from
low freq. data
Transition
Odin J'
Ka'
Kc'
J''
Ka''
Kc''
Obs. Freq.
(MHz) 14 1 13 12
487663 16 15
547315 2
548927 4
549201 11 3
549407
549452
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…)

6. Experimental Conditions
FASSST Spectrometer
@ OSU
GHz with 3 BWOs
100 scans accumulation
up- & down-ward scans averaged
SO2 for frequency calibration
better than 100 kHz uncertainty
Terahertz Spectrometer
@ U. Cologne
GHz with a BWO
Phase-lock-loop (PLL) technique
→ frequency stabilization
~5 kHz experimental uncertainty
higher frequencies intended….
Production of H2CS
Pyrolysis of trimethylene-sulfide [(CH2)3S] (Sigma-Aldrich Co.)
at 680 ℃ (at OSU), 1300 ℃ (at Cologne)

7. Spectrum Assignment Predictions with SPFIT/SPCAT(1) program suites
based on previous microwave & millimeter-wave (& FIR) transitions of H2CS, H2C34S, H2C33S, H213CS.
CAAARS(2) (Computer Aided Assignment of Asymmetric Rotor Spectra) program for the FASSST spectrum
Pickett, J. Mol. Spectrosc., 148, 371 (1991),
Medvedev et al. J. Mol. Struct. 742, 229 (2005)

8. Intensities agree with natural abundance ratio
FASSST Spectrum of H2CS
85,4←75,3
85,3←75,2
H2CS
80,8←70,7
H2C34S
×10
Intensities agree with natural abundance ratio
32S : 33S : 34S = 95 : 0.75 : 4.2
12C : 13C = 99 : 1
H2C33S
H213CS
×100
×100

9. Terahertz Spectrum of H2CS
1913,6←1813,5
1913,7←1813,6
R-branch (ΔJ = +1)
ΔK = 0
P-branch (ΔJ = –1)
ΔK = 2

10. Summary of Transitions Newly Assigned
Previous data
+ Microwave
+ FIR transitions
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.
Ka up to 15!
Ka up to 11!

11. Analysis with SPFIT
Effective rotational Hamiltonian for asymmetric-top molecules in Watson’s S-reduced form
HR = AJa2 + BJb2 + CJc2
– DJJ4 – DJKJ2Ja2 – DKJa4
+ 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

12. New transition frequency predictions
Molecular Constants
New transition frequency predictions
for 4 isotopic species!

13. Frequency Comparison [MHz] between Odin & New Prediction
Transition
Odin
New
Prediction
Residuals J'
Ka'
Kc'
J''
Ka''
Kc''
Obs. Freq.
Calc. Freq. 14 1 13 12
487663
–0.3 16 15
547315
7.0 2
548927
6.7 4
549201
12.7 11
12.3 3
549407
5.0
549452
4.8

14. Line Surveys of Orion-KL by Schilke et al.
1) GHz (1997)
Frequency Comparison [MHz]
Extrapolated from
low frequency data

15. Line Surveys of Orion-KL by Schilke et al.
2) GHz (2001)
Extrapolated from
low frequency data

16. The Equilibrium Structure of H2CS

17. 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)