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Universität zu Köln
Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut


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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 ( ...more


Figure. 1: The MIDAS COINS spectrometer, this instrument is equipped with two 7 m and one 8 m long pyrex tubes (100 mm inner diameter). The three tubes can be switched optically in series, by applying double path scheme a total absorption length of 44 m can be achieved. This spectrometer is well suited for measuring complex organic molecules in the laboratory at low vapor pressures to guarantee small line widths.
Figure 2: The 3 mm wavelength spectrometer, this instrument is equipped with two 5 m long pyrex tubes (100 mm inner diameter) one tube is used for RF discharge experiments and partly covered by an aluminium foil to shield the RF power.
Figure 3: The RF discharge in operation.


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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With its exceptionally long absorption path, MIDAS-COINS is enhancing our sensitivity in the millimeter range since 2010.
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