Siegel der Universität

Universität zu Köln
Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut

Experimental Methods and corresponding Instruments

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Category: Spectroscopy / Supersonic Jets

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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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, 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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