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Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

Experimental Methods and corresponding Instruments

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Category: Spectroscopy

Double resonance rotational action spectroscopy

The double resonance rotational action spectroscopy scheme is a method that can be used to bring well established rovibrational schemes to the pure rotational domain. It works on the principle of redistributing the rotational population of the studied molecule ensemble (1. photon, usually THz radiation), effectively influencing the second action spectroscopy method, responsible for the signal detection. Wide range of schemes can be applied as a second process e.g. laser induced reactions (LIR), the messenger tagging technique, or the infrared multiphoton dissociation (IRMPD).
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COLTRAP

COLTRAP and FELion are two new generation 22-pole ion trap instruments developed and built in our laboratory. Both instruments offer unique possibilities to study the kinetics of ion-molecule reactions at low temperatures, and to use highly sensitive methods for spectroscopic studies of molecular ions. Whereas the COLTRAP instrument is located in the Cologne laboratories, FELion has been installed in October 2014 at the FELIX Laboratory (Radboud University Nijmegen, Netherlands).
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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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Jet Spectroscopy

Carbonaceous clusters are produced through laser ablation. High-energy UV laser pulses (355 nm) are focused onto a rotating rod composed of appropriate precursor material (graphite, SiC, etc.). Products are carried through a 1cm reaction channel by pulses of Helium gas kept at a backing pressure of 10-20 bar and expand adiabatically into the vacuum chamber. The background pressure in the vacuum chamber is kept below 0.1 mbar. With every single laser pulse a total amount of roughly 1013-1014 clusters of different sizes is produced. Molecules are investigated at high spectral resolution using infrared radiation provided by quantum cascade lasers or optical parametric oscillators.
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Carbon Cluster Experiment

In the carbon cluster experiment, part of the infrared probe radiation is guided through a reference gas cell and a reference interferometer for calibration purposes. The major fraction is guided into a vacuum chamber and through a Herriott-type multireflection cell where it is intersecting the free jet harboring the clusters perpendicularly close to the exit of a slit nozzle. After having passed the chamber, the infrared beam is detected using sensitive InSb or HgCdTe detectors and its signal is digitized using an USB oscilloscope.
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Jet Spectroscopy of Weakly Bound Molecular Complexes

The weakly bound complexes and small helium and hydrogen clusters can be efficiently produced in a supersonic jet expansion of a gas mixture into vacuum. The temperature of the sample in the jet expansion is on the order of a few Kelvin, and the complexes are stabilized in this nearly collision-free environment.
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OROTRON

The OROTRON spectrometer has become the most sensitive and probably the most powerful tool for investigating inherently extremely weak spectral features of molecular complexes and small clusters in the millimeter-wave (MMW) range [L.A. Surin, et.al.Rev. Sci. Instrum., 72, 2535 (2001)]. The key element of the spectrometer is a tunable OROTRON oscillator, which generates the radiation (2-3 mm) through the interaction of an electron beam with the electromagnetic field of an open Fabry-Perot resonant cavity. The supersonic molecular jet enters into the resonator perpendicularly to its axis. A high quality factor (Q ≈ 104) of the cavity results in 100 effective passes of the radiation through the jet. Absorption in the cavity causes changes of the electron current in the collector circuit and is detected by measuring these current changes.
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Light Induced Inhibition of Complex Growth (LIICG) and Rotational State-Dependent Attachment of He Atoms

LIICG is a novel action-spectroscopy scheme (see also LIR - Laser Induced Reactions technique) for measuring high-resolution ro-vibrational spectra of gas-phase molecular ions. This method makes use of an inhibition of Helium-attachment to vibrationally excited molecular ions. Furthermore, we also observed a change in the rate of Helium-attachment depending on the rotational state of the cold, stored molecular ions. This effect can be exploited to perform purely rotational action spectroscopy on a wide class of molecular ions. Both methods can, due to the low temperatures needed, only be employed in our two new 4 K 22-pole ion traps COLTRAP and FELion.
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COLTRAP

COLTRAP and FELion are two new generation 22-pole ion trap instruments developed and built in our laboratory. Both instruments offer unique possibilities to study the kinetics of ion-molecule reactions at low temperatures, and to use highly sensitive methods for spectroscopic studies of molecular ions. Whereas the COLTRAP instrument is located in the Cologne laboratories, FELion has been installed in October 2014 at the FELIX Laboratory (Radboud University Nijmegen, Netherlands).
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FELion

COLTRAP and FELion are two new generation 22-pole ion trap instruments developed and built in our laboratory. Both instruments offer unique possibilities to study the kinetics of ion-molecule reactions at low temperatures, and to use highly sensitive methods for spectroscopic studies of molecular ions. Whereas the COLTRAP instrument is located in the Cologne laboratories, FELion has been installed in October 2014 at the FELIX Laboratory (Radboud University Nijmegen, Netherlands).
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Laser induced reactions (LIR)

Laser induced reactions (LIR) belong to the family of "action spectroscopy" methods. In the special case of LIR, changes of the rate coefficient of an endothermic ion-molecule reaction serve to detect the excitation of the parent ionic species. This offers not only the possibility of performing very high sensitivity spectroscopy on transient ions (a number of only 1000 ions per trapping period is enough), but LIR can yield information on state-selected reaction rate coefficients and lifetimes of excited states.
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LIRTrap

The 22-pole ion trap apparatus LIRTrap is used both for kinetic and spectroscopic characterization of ions.
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Reconstruction of Molecular Energy Levels Using Combination Differences: From Lines to states without a Model

For molecules which lack an appropriate model (as CH5+) common data evaluation methods which are based on the assignments of the measured lines to those of the model cannot be applied. Instead pattern recognition methods have to be applied to the measured data to reconstruct at least the energy levels of the molecule (without quantum numbers). Such a pattern recognition method is the Ritz combination principle which is more than 100 years old. It is based on calculating all possible CDs from the measured lines and searching for cumulationg values. This method has been enhanced a lot to be reliably applicable even to very dense spectra as that of CH5+.
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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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