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CH3SH  –  methanethiol


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Methanol, CH3OH, is the most important complex molecule in star-forming regions. Much less is known about its sulfur homolog methanethiol, CH3SH, also known as methyl mercaptan, in particular at submillimeter wavelengths (or terahertz frequencies), which are important in particular for observations with the Atacama Large Millimeter Array, (ALMA), with the Stratospheric Observatory for Far-Infrared Astronomy (SOFIA), or with the Herschel satellite. ...more


Absorption Rotational Spectroscopy
Rotational spectroscopy is a key method to investigate molecules, radicals and ions. These species are capable of motions, in particular molecular rotation. If a permanent or induced dipole moment is existent, the species is called transient and the underlying energy states are quantized and accessible for electromagnetic waves. The energy distances between the rotational levels are such, that mostly the transition lines are in the cm- (up to 30 GHz), mm- (up to 300 GHz) and in part shorter wavelength ranges of the electromagnetic spectrum. Since the energies necessary to excite the rotational states are low, the typical temperatures in the ISM are sufficiently high for exiting these states (T = 5 K to much more than 100 K). The line frequencies of the transitions can be measured with great accuracy; this, in turn, gives precise molecular parameters, which allows calculating reliable predictions of new molecular lines which helps to identify new molecular species in space. Such line lists and the parameters of many species are available in our Cologne Database for Molecular Spectroscopy (
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With its exceptionally long absorption path, MIDAS-COINS is enhancing our sensitivity in the millimeter range since 2010.
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The Cologne Terahertz Spectrometer
In Cologne, high resolution, broadband scanning spectroscopy with microwave accuracy has been extended into the terahertz region (λ < 0.3 mm) by stabilization of continuously tunable backward wave oscillators (BWOs) from Russian fabrication. Precise measurements have been performed up to 1.3 THz, a frequency which has never been reached before directly using microwave techniques, i.e. without generation of harmonics.
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{Multiplier Chains}
{Atacama Large Millimeter Array}

Recent Results

∼18000 rotational and rovibrational transitions with J ≤ 40, K ≤ 15, and vt ≤ 2 were recorded between 49 and 550 cm–1 at the Canadian Light Source and used in the final fit. In addition, extensive spectral recordings between 1.1 and 1.5 THz were performed at the Universität zu Köln. Together with previous data between 7 and 358 GHz, more than 1700 transition frequencies with J ≤ 30, K ≤ 15, and vt ≤ 2 were determined with microwave accuracies. Employing a program, which is suitable to take large amplitude vibration into account, a satisfactory fit was obtained with 78 parameters used in the fit, see Xu et al. (2012).
Section of the rotational spectrum of methanethiol near 1375 GHz showing two labeled Q-branch transitions along with many other lines.
Employing the resulting CDMS catalog file, Müller et al. (2016) could carry out a thorough analysis of methanethiol in the star-forming region Sagittarius B2(N2). Lines within the ground and first excited torsional states were detected and yielded a rotational temperature of 180 K, slightly higher than 160 K derived for methanol. The column density od methanol is about a factor of 120 higher than that of methanethiol.
Details of the ALMA spectrum toward Sgr B2(N2) in black. The model of lines of CH3SH is shown in red and the model of all identified species is shown in green. The upper part shows emission features pertaining to the ground torsional state while the lower part displays those within the first excited torsional state.


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  • Funding by the Deutsche Forschungsgemeinschaft (DFG) via the collaborative research grant SFB 956 "Conditions and Impact of Star Formation" project B3, is gratefully acknowledged.
  • The following ALMA were used: ADS/JAO.ALMA#2011.0.00017.S and ADS/JAO.ALMA#2012.1.00012.S.
  • Holger Müller has been supported by BMBF for maintance of the Cologne Database for Molecular Spectroscopy, CDMS, initially in the framework of the Herschel/HIFI ICC, project FKZ 50OF0901, more recently in the framework of the ALMA Regional Centre (ARC) Node, project 5A11PK3.