Available Theses

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Ethyl Cyanide

Ethyl cyanide (C2H5CN) is a complex organic molecule which was detected in space. Its many low-lying vibrationally excited states, the internal rotation of the methyl group, and hyperfine structure splitting due to the presence of the 14N nucleus result in a dense and complicated spectrum.

The lowest singly excited vibrational states are already described. However, the doubly excited states are also of high interest and are not understood yet.

To decipher these doubly excited states different advanced techniques will be used. DM-DR spectroscopy allows to understand the relationship between transitions (further information on DM-DR in our paper). On the software side, Loomis-Wood plots will facilitate the identification of series. Additionally, predictions extrapolated from the singly excited states will support the analysis.

The appeal of this thesis is diverse. First, the topic allows different approaches. The DM-DR technique can be used to identify series without any a priori knowledge. In contrast, very good predictions for the states of interest can solve the problem on their own. So the student will be able to focus on the methods that suit them most. Secondly, ethyl cyanide is of high interest for astronomers and therefore the results of this thesis would directly help other groups in the institute and world wide.

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Pyrolysis of Phosphaalkynes

Not all molecules of astronomical interest are readily available. Many of these molecules are not stable in the labarotory and therefore have to be produced in the experiment. Our pyrolysis experiment allows to do exactly this, by using readily available precursors. The precursors are exposed to heat in the pyrolysis oven and the resulting new molecules are directly examined in the absorption cell.

The focuses of this thesis are the experimental work on the experiment itself and the subsequent analysis of the measurements.

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Automating Molecule Fitting

For each examined molecule a quantum mechanical model is created. Typically, a generic Hamiltonian is fitted to the molecule of interest.

This procedure can be quite time consuming and is - especially for simple molecules - quite straightforward. Therefore, this thesis aims to automate the process of creating quantum mechanical models for simple molecules.

First, the general idea of how to fit a quantum mechanical model to a molecule is understood. Then, from these rather loose rules, an algorithm is developed that reproducibly performs this fitting. In the last step, this algorithm is implemented with currently used software and applied to real molecules.

This thesis suits students that want to combine the aspects of coding and analysing molecules. Advanced coding knowledge is helpful for this topic.

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Determining Uncertainties

For measured molecules we create a quantum mechanical model by fitting a Hamiltonian to the energy levels. The energy levels, or rather the energy differences between them, are in turn given by the frequencies of their transitions in the spectrum.

The positions of these transitions are obtained by fitting a suitable lineshape to the data. However, the uncertainty of the frequency positions is a little less straightforward. Most fitting routines also specify an uncertainty for the parameters. However, in our case other factors have additional influence. E.g. other close-by transitions or standing waves might influence the lineshape. Thus, we want to create a more robust way to determine these uncertainties.

Different approaches to this problem can be chosen. Some of these include, a statistical approach, the use of machine learning, or a mathematical description.

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Analysing the Datasets of the CDMS

The Cologne Database for Molecular Spectroscopy (CDMS) is our own database for rotational and vibrational spectra of molecules. Currently, the database holds a little more than 1000 entries. The idea of this thesis is to analyze this dataset of datasets. Some of the guiding questions could be, how comparable are parameters over different molecules, how are the correlations of different parameters or molecule values.

This topic is rather explorative and there is not a single clear goal from the beginning. This makes it an ideal topic to use ones creativity and intuition to examine the datasets of the CDMS.

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Understanding the Autler-Townes Effect and Population Changes

Double-modulation double-resonance (DM-DR) and double-resonance spectroscopy are based on two effects: Population changes and the Autler-Townes effect. This thesis examines the interplay of these two effects. The Autler-Townes effect causes a splitting and an intensity change in the molecule, whereas population changes only causes an intensity change.

Both, DM-DR and DR spectroscopy, use a second frequency source. The idea is to measure lines with different power settings of this second source. The effect of the Autler-Townes effect and thereby the splitting of the line depends on the power of this second source. Therefore, the intensity change caused by the Autler-Townes effect can be calculated from the splitting and substracted from the total intensity change. This results in the intensity change connected with population changes.

This thesis has a clear goal and a clear path. It is therefore very suitable for a Bachelor thesis.

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Comparing DM-DR Schemes

DM-DR spectroscopy is a powerful tool to examine the relation of transitions. The basic idea is, that a second frequency source is used together with the Autler-Townes effect which allows to split transitions into two less intense and frequency shifted transitions. By measuring the difference of the spectrum with and without this second source, we are able to find transitions that share an energy level.

However, there are different ways to realize the difference of the two spectra. The goal of this thesis is to characterize the different available methods and find their advantages and disadvantages.

This thesis focuses on experimental techniques and the theory connected with the Autler-Townes effect. It has a clear goal and a clear path.