TWIST

The nocturnal view of the stars has always sparked scientific discoveries. Today, astronomical observations are carried out using observatories such as the Atacama Large Millimeter Array or the James Webb Space Telescope. Observations rely on spectroscopy across wide regions of the electromagnetic spectrum, extending far beyond the range visible to the human eye. In the submillimeter and terahertz regions (Submm/THz), rotational spectra of molecules can be measured, which has led to numerous detections of molecules in space. With the discovery of molecules such as ammonia and water in the 1960s at the latest, astronomical spectroscopy gave rise to astrochemistry. It investigates the presence and distribution of molecules in cold interstellar regions as well as in warm atmospheres. A crucial factor for detecting molecules in space is the careful preparation of reference data for observations. This arises from the interplay of computer-assisted simulations of molecular rotation and vibration, and their experimental observation in specialized laboratories. The FWF Schrödinger-funded TWIST project is embedded within this complex interaction between experiment and theory, focusing on a topic that has received little attention so far. Molecules tend to interact with one another and thus form weakly bound molecular clusters. These interactions are essential for particle formation and for enabling chemical reactions. The theoretical description of molecular clusters that can be observed spectroscopically under laboratory conditions is extremely challenging. This is due both to the internal flexibility of the molecules themselves and to the relative motions between weakly bound molecules. In this work, such molecular clusters are investigated experimentally and theoretically. At TU Wien, analysis is carried out using matrix-isolation Fourier transform infrared spectroscopy. Complementary measurements of the clusters are performed at the University of Bologna using chirped-pulse Fourier transform microwave spectroscopy and Submm/THz spectroscopy. This combination will yield globally unique reference datasets. To enable robust interpretation of these data, the corresponding spectral regions are covered by numerically solving the molecular Schrödinger equation. This is done using the Watson Hamiltonian within the framework of the rotational-vibrational configuration interaction (RVCI) method in collaboration with the University of Stuttgart, and using the generalized approach for arbitrary Hamiltonians (GENIUSH) in cooperation with Eötvös Lornd University in Budapest. Based on the results of the TWIST project, strategies can be developed for the comprehensive investigation of weakly bound molecular clusters using spectroscopy.