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Universität zu Köln
Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

Research Projects

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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 cm- (up to 30 GHz) and mm- (up to 300 GHz) wavelength range 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 > 100 K). The line frequencies of the transitions can be measured with great accuracy; this in turn gives precise molecular constants, which allows calculating reliable predictions of new molecular lines which helps to indentify new molecular species in space. Such line lists and constants for many species are available in our Cologne Database for Molecular Spectroscopy (www.
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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}

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.
Section of the rotational spectrum of methanethiol near 1475 GHz showing two labeled Q-branch transitions along with many other lines.


External Links


  • Funding by the Deutsche Forschungsgemeinschaft (DFG) via the collaborative research grant SFB 956, project B3, is gratefully acknowledged.
  • 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.