Between the absorption and the emission spectral line shapes of dense atomic and molecular media, such as dye solutions and alkali-noble buffer gas mixtures at high pressure, in many cases there exists a universal scaling, the Kennard-Stepanov relation, which is a manifestation of detailed balance. This relation plays a crucial role in recent Bose-Einstein condensation experiments of visible-spectral photons in, e.g., dye-solution-filled optical microcavities. It has recently been proposed to use high-pressure xenon–noble-gas mixtures as a thermalization medium for vacuum-ultraviolet regime photons, so as to extend the achievable wavelength range of such Bose-Einstein-condensed optical sources from the visible to the vacuum-ultraviolet regime. In this work, we report two-photon excitation spectroscopy measurements of ground-state ($5p^6$) xenon atoms subject to up to $80\mathrm{bar}$ of helium or krypton buffer gas pressure, respectively, in the 220–260 nm wavelength range. The study of such two-photon spectra is of interest, e.g., for the exploration of possible pumping schemes of a future vacuum-ultraviolet photon Bose-Einstein condensate. We have also recorded absorption and emission spectra of the $5p^6\leftrightarrow 5p^{5}6s$ single-photon transition near 147 nm wavelength of xenon atoms subject to $80\mathrm{bar}$ of krypton buffer gas pressure. We find that the ratio of absorption and emission shows a Kennard-Stepanov scaling, which suggests that such gas mixtures are promising candidates as a thermalization medium for a Bose-Einstein condensate of vacuum-ultraviolet photons.

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• Bose gases
• Bose-Einstein condensates
• Tunable lasers
• Fluorescence spectroscopy
• Optical absorption spectroscopy
• Spectroscopy