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Universität zu Köln
Mathematisch-Naturwissenschaftliche Fakultät
Fachgruppe Physik

I. Physikalisches Institut


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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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Boundary Element Method (BEM)

To find the electric potential of an rf multipole trap we have to find a solution for the ''Laplace'' equation: ΔΦ=0 The movement of a particle in a rapidly oscillating field can described by an effective trapping potential. It is calculated by taking the time-average over one period of the fast oscillation rf field . This effective potential can be expressed as: Ueffective = qΦdc+ q²⁄(4mΩ²)·(∇Φrf)² with Φdc the static part of the potential, Φrf the rf part, Ω the rf frequenz and m the mass of the particle.
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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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Collision Dynamics

Experiments are performed in a variable temperature 22-pole ion trap. In the trap, mass selected ions are typically stored over seconds. Upon storage they are accommodated to the ambient trap temperature via He buffer gas cooling. The reactant gas is admitted to the trap at a fixed but variable number density. The trap content is analysed as a function of the trapping time. Rate coefficients are derived from the simulation of the underlying kinetics solving a set of rate equations. This method has been applied to several astrophysically interesting systems.
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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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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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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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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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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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Molecular Symmetry

The usual rotation group, SO(3), is conveniently used to describe the symmetry of the rotational wave functions of molecules. Internal rotation, on the other hand, is described by SO(2) symmetry since the rotation axis is fixed with respect to the rigid framework. Combining these two motions leads to the formulation in a five-dimensional rotational symmetry group SO(5). Applied to the example of CH5+, this group is capable of describing the permutation symmetry elements of the molecular symmetry group in terms of five-dimensional rotations. We showed that an "equivalent rotation" treatment is actually impossible in three dimensions implying a conventional description of the molecular wave functions in terms of rotational wave functions to fail.
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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 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 (
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Semi Classical Aproach

Useful descriptions of the intermediate regime between the well-understood quantum mechanical "world'' and the classical limit include the use of semi-classical calculations, which we use to determine the rotational energy spectra of different molecules at high J-quantum numbers. Basis of the semiclassical approach is the rotational energy surface (RES, cf. Fig. 1) which is found by writing the quantum mechanical Hamiltonian in terms of a "classical vector" (Jx, Jy,Jz)T using two angles and the fixed length |J|. To get to quantization conditions, one can use the so-called WKB- or Sommerfeld quantization rules, which were first applied to standard problems like the quantum harmonic oscillator. Here we can use them analogously to find conditions for the quantization of the energy levels. An alternative approach makes use of results of quantum chaos theory. This apporach is completely coordinate free and involves only geometrical and topological features of the classical dynamics, which makes it more useful than the heuristic picture of the earlier works. In that approach one uses the Gutzwiller trace formula for the description of the density of states, where the poles signals the energy levels. Going one step further, we can use the symmetry projected Green function in the derivation of the density of states and hence find different quantization conditions for the various representations of the molecular symmetry group.
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