Ultraviolet spectroscopy and thermodynamics of ultradense gas mixtures

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Between the absorption and emission lines of dense atomic and molecular media, such as dye solutions and alkali buffer gas mixtures at high pressure, in many cases there exists a universal thermodynamic scaling, the Kennard-Stepanov relation, a manifestation of detailed balance. In preliminary work, this enabled our group to demonstrate the first Bose-Einstein condensation of photons, as well as the first demonstration of collisionally-induced laser cooling of gases. The immediate goal of this research project is the demonstration of a new spectroscopic method for the determination of the absolute temperature. In the framework of the planned investigations, spectroscopic measurements of absorption and emission performed in preliminary work on dense ensembles, where the collisional broadening reaches the thermal energy in frequency units, will be extended to samples which are already gaseous at room temperature. We intend to investigate gas mixtures of the noble gas xenon, as well as small molecules, with, for example, argon as a buffer gas at 100-200 bar pressure, which will be made possible by measurements in the ultraviolet and vacuum ultraviolet spectral regions. By recording absorption and emission spectra at high gas densities, the thermodynamic Kennard-Stepanov relation for atomic and molecular gas mixtures can be verified, which should allow an optical frequency measurement of the temperature. A future perspective of the work is the realization of Bose-Einstein condensates in the vacuum-ultraviolet spectral regime, as coherent sources of light in this wavelength range, which is difficult to access for usual lasers, for example for lithography or medical applications. The work will also form the basis for experiments on collisionally-induced laser cooling of "macroscopic" ensembles of dense gases starting from room temperature.