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C2H5OH  –  ethanol


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Ethanol (CH3CH2OH) is a heavier homologue of the ubiquitous methanol (CH3OH) and an isomer of dimethyl ether. Ethanol is abundant enough in warmer and denser parts of star-forming regions that minor isotopologues may be observable, especially those involving one 13C or one D. Relative abundances of isotopologues, of isomers, and of homologues may help to constrain their chemical formation pathways. ...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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{Atacama Large Millimeter Array}

Recent Results

Ethanol species with one 13C

We characterized the two 13C isotopic species of ethanol employing isotopically enriched samples. Spectra were measured between 80 and 800 GHz. The analyses were limited to quantum numbers for which the anti conformer can be treated as separate species. The accurate data have been published by Bouchez et al. (2012).

Both 13C isotopomers were identified unambiguously later in an investigation of alkanols and alkanethiols in Sagittarius B2(N2), see below.

Example spectra for 13C1 ethanol of (a) a single line 121,12 – 110,11 and (b) a line showing asymmetric torsional splitting 126,7 – 125,8.
Ethanol-2-13C spectral recording near 89.1 GHz. Total integration time was 40 ms per point, the step size 25 kHz. The scanning time for this spectrum was about 15 minutes, no baseline correction has been carried out yet.

Ethanol species with one D

Subsequently, we investigated the three isotopologues of singly deuterated ethanol (C2H5OD, CH3CHDOH and CH2DCH2OH). Initial calculations were based on the results of J. P. Culot, who published measurements of deuterated ethanol between 26 and 58 GHz in 1969. From these data, we predicted transitions for higher frequencies with H. M. Pickett's programs SPFIT and SPCAT. Absorption lines have been measured between 38 and 129 GHz using MIDAS-COINS, a similar spectrometer was used to cover 360 to 500 GHz. The region from 210 to 365 GHz was covered with a backward wave oscillator spectrometer. Our new data were published by Walters et al. (2015).

Detail of the rotational spectrum of CH2DCH2OH showing the origin of the Ka = 8 ← 7 Q-branch of the asymmetric conformer. The lines are labeled with their J quantum numbers.

Ethanol observations with ALMA toward Sagittarius B2(N2)

Belloche et al. (2016) used the Atacama Large Millimeter/submillimeter Array (ALMA) in its Cycles 0 and 1 to carry out a molecular line survey of the giant molecular cloud Sagittarius (Sgr) B2(N) near the Galactic Center at 3 mm (84.1 to 114.4 GHz). Müller et al. (2016) detected both 13C isotopomers toward the secondary hot core Sgr B2(N2) with a 12C/13C ratio of 25, typical of the Galactic Center region.
Details of the ALMA spectrum toward Sgr B2(N2) in black. The model of lines of 13CH3CH2OH is shown in red and the model including all assigned species is shown in green. The horizontal dashed line displays the 3σ detection limit.
The interpretation of the spectrum of the main isotopolgue of ethanol was initially hampered by intensity issues. Some lines predicted to be strong appeared to be absent or much weaker whereas others were stronger than predicted. Intensities in spectra with extensive torsion-rotation interaction (or vibration-rotation interaction) can depend strongly on the signs of the dipole moment components relative to those of the interactzion parameters. Modifying just one out of five signs of dipole moment components solved the intensity issues as can be seen below. We created a modified CDMS catalog entry for ethanol.
Details of the ALMA spectrum toward Sgr B2(N2) in black. The model of lines of CH3CH2OH is shown in red. The upper part shows the initial model based on the JPL catalog entry whereas the lower part displays the modified model which is now available in the CDMS catalog.


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  • The investigations have been funded by the Deutsche Forschungsgemeinschaft (DFG) within the Collaborative Research Centre 956 "Conditions and Impact of Star Formation", project B3.
  • The following ALMA were used: ADS/JAO.ALMA#2011.0.00017.S and ADS/JAO.ALMA#2012.1.00012.S.
  • Travel for A.B. and A.W. was partially supported by the French CNRS program “Physique et Chimie du Milieu Interstellaire” (PCMI).