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Universität zu Köln
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Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

Experimental Methods and corresponding Instruments

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Category: Spectroscopy / Gas Cells

Chirped-Pulse Spectroscopy

Jet and FTMW spectroscopy have replaced classical rotational spectroscopy in the microwave and millimeter wave regime as more complex molecules need higher sensitivity and lower temperatures to be detected in the laboratory. But having their own limitations, like short repetition cycles with jet valves, taking spectra and finding lines still used to be a time consuming task. With the advancement of semiconductor technology new methods of signal creation and detection were developed. With the availability of arbitrary waveform generators the FTMW spectroscopy was extended using a chirped pulse instead of a single frequency excitation signal.
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Waveguide Chirped-Pulse Experiment


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Cologne Spectrometer for cold Molecules in Interstellar Clouds (COSMIC)


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Emission Spectroscopy

Traditionally the vast majority of lab spectroscopy measurements have been absorption measurements, where a strong tunable source can provide a high signal-to-noise ratio. With today's high sensitivity of state-of-the-art astronomical heterodyne receivers and their ever growing instantaneous bandwidth, emission spectroscopy becomes a more and more interesting alternative, which will ultimately outperform absorption spectroscopy in terms of scanning speed at a not comparable, but desired signal-to-noise ratio. In 2014 the emission spectroscopy method has been employed in our laboratory for the first time for measurements of rotational spectra of complex molecules of astrophysical demand at 800 GHz. In 2016 we employed a 100 GHz room-temperature emission spectrometer for the first time (accepted IAU proceedings 2017). In 2017 we obtained first results using an emission spectrometer coupled to a highly sensitive SIS-mixer operational between 300 and 400 GHz, coincident with ALMA Band 7.
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100 GHz room-temperature emission spectrometer

The modern developments of highly sensitive room-temperature low-noise amplifiers in combination with room-temperature mixers as well as advances in digital Fast Fourier transform spectrometers (FFTSs) for the space and ground observatories make also laboratory emission spectroscopy very attractive, in the light of the possibility of fast measurements of high resolution broadband spectra with high sensitivity and precise line intensities. Thus, emission spectroscopy is of interest for the spectroscopy itself, allowing fast broadband spectral measurements, and for the physical chemistry, providing measurements of absolute line intensities and shapes.
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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 (www.CDMS.de).
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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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MIDAS-COINS

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