HSOH - Role Model for an Internal Rotor

The content on this page was translated automatically.

The "HSOH" project focuses on understanding the internal dynamics of small molecules. Of particular interest are molecules that may also occur in the interstellar medium (ISM).
One such molecule is HSOH. It is believed to occur both in the ISM and in the atmospheres of planets containing sulfur.

The molecule is also very interesting for another reason: it is a relatively simple molecule consisting of four atoms linked together in a chain-like structure. A good understanding of this molecule can help us understand the internal dynamics of larger, more complex molecules.

HSOH is the intermediate molecule between HSSH and HOOH, with two distinct "arms": the SH group on one side and the OH group on the other. The molecule exhibits interesting internal dynamics; for example, rotation and vibration are coupled. In addition, the two arms (O-H and S-H) can rotate around the S-O axis (“torsion”).

The potential associated with the torsional motion exhibits two maxima or two minima. This so-called double-well potential leads to the splitting of the molecule’s rotational-vibrational levels, which is reflected in the spectrum by a splitting of the spectral lines. The principles governing this so-called torsional splitting of energy levels and spectral lines are not yet fully understood and are the subject of current research.

Fig. 2: The double-well potential of HSOH; the energy is plotted against the wave number on the vertical axis, and the angle between the two O-H and O-S arms is shown on the horizontal axis. The two minima and the two barriers, as well as their values, are clearly visible. The green lines indicate various energy levels of HSOH that arise as a result of torsional splitting into doublets.

To this end, we will conduct measurements at the SOLEIL synchrotron facility in France and obtain the first broadband spectrum of HSOH in the far-infrared frequency range. Using the significantly narrower-band but higher-resolution radiation sources in Kassel and Cologne, we will take a closer look at particularly interesting regions.
For these measurements, HSOH must be continuously regenerated because it is short-lived (with a lifetime of a few seconds). To do this, various precursor molecules are introduced into a strong radiofrequency discharge, and HSOH forms in the resulting plasma.

Radiofrequency discharge in the THz laboratory. The HSOH molecule is formed in the glowing plasma.

The goal of my work is to perform precise measurements in order to draw conclusions about the internal dynamics of HSOH. Based on these measurements, the molecular constants can be refined. These are variables used to describe the quantum mechanical states of the system.
Furthermore, the measurements can be used to test quantum mechanical models that describe the behavior of tunneling splitting, such as its magnitude in the ground state or in vibrationally excited states of the molecule. In close collaboration with theorists, the goal is to gain a better understanding of the internal dynamics of molecules.