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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 this year (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 (see TELMI).
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Terahertz laboratory emission spectrometer (TELMI)

The modern developments of highly sensitive superconductor-insulator-superconductor (SIS) and hot electron bolometer (HEB) based heterodyne receivers 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 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. 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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