submillimeter-wave spectroscopy of thioformaldehyde, h 2 cs, in its ground state atusko maeda, ivan...
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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)