Investigation of the condensation and freezing behavior of freely suspended supercooled molecular gases and particles in the nano to millimeter size range

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In the project supercooled molecules as well as nano, micro and millimeter droplets shall be investigated with respect to their condensation, aggregation and freezing behavior in a gaseous vicinity. For this purpose, optical methods as high resolution infrared spectroscopy, Raman spectroscopy, terahertz spectroscopy (by use of synchrotron sources), and high-speed filming will be applied. Theoretical support will especially be delivered by collaborators through computer simulations. From our group finite element methods for the investigation of heat production and distribution during freezing as well as the discrete dipole approximation method for optical particle characterization will be used. One main focus will be a new, improved collisional cooling system which shall be developed for cooling, supercooling and spectroscopy of molecular gases, nano and micro aerosols in the temperature region of 4 to 400 K. For this system our experiences of the last 15 years with the old well-tried system will be considered as well as other technical improvements. As a key technology a more powerful sample-gas inlet system for supercooling of gases shall be constructed. Performing high resolution spectroscopy of supercooled gases we aim at an improved understanding of the intra-molecular dynamics via global potential hyper-surfaces. Highly qualitative measuring data shall be delivered for data bases as HITRAN or GEISHA under consideration of applications, e.g., in environmental sciences. Concerning aerosols, water and other substances stand in the focus as they are important for the physical chemistry of the atmospheres of the Earth and other planets and their satellites and also for interstellar gases. Especially the freezing behavior of microdroplets within an aerosol cloud shall be unraveled (concerning dynamics, size-, shape-, phase-, and structure-evolution, heat production and transport, optical constants); last but not least for the interpretation of Raman lidar data. Concerning freely suspended droplets from water and aqueous solutions in the millimeter size range, the first rapid freezing stage which passes in milliseconds shall be investigated and characterized as well as the evolving dendritic network ice. During the first project stage we observed an electric effect showing freezing potentials of up to 6 V. This effect shall be unraveled now. The influence of the asymmetric network structure of the dendritic ice on the ion distribution within the final ice shall be explained, especially the impact on the particle chemistry and electric potential generation in thunderclouds. Furthermore, we will study the analogy of the formation of regular dendritic ice to the formation of snowflakes in air, as well as the transition towards smaller particles in the micrometer size range.