Between the absorption and the emission spectral lineshapes 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 (
5p6) xenon atoms subject to up to 80bar of helium or krypton buffer gas pressure, respectively, in the 220-260nm 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
5p6↔5p56s single-photon transition near 147nm wavelength of xenon atoms subject to 80bar 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.
Spectroscopy of gases under high-pressure conditions is of interest in various fields such as plasma physics and astrophysics. Recently, it has also been proposed to utilize a high-pressure noble gas environment as a thermalization medium to extend the wavelength range of photon Bose-Einstein condensates to the vacuum-ultraviolet regime, from the presently accessible visible and near-infrared spectral regimes. In this work, we report on experimental results of two-photon spectroscopy of gaseous and supercritical xenon for pressures as high as 95 bar, probing the transitions from the
5p6 electronic ground-state to the
5p56p and $5{p}^{5}6{p}^{\ensuremath{'}}$ excited-state configurations. Aiming at the exploration of possible pumping schemes for future vacuum-ultraviolet photon condensates, we have recorded degenerate two-photon excitation spectra of such dense xenon samples. In further measurements, we have investigated whether irradiation of an auxiliary light field can enhance the reabsorption of the emission on the second excimer continuum of xenon, which is subject to a large Stokes shift. To this end, absorption measurements have been conducted, driving the $5{p}^{6}\ensuremath{\rightarrow}5{p}^{5}6p$ two-photon transitions nondegenerately.
Bose-Einstein condensation, an effect long known for material particles as cold atomic gases, has in recent years also been observed for photons in microscopic optical cavities. Here, we report absorption and emission spectroscopic measurements on the lowest electronic transition ($5{p}^{6}\ensuremath{\rightarrow}5{p}^{5}6s$) of xenon, motivated by the search for a thermalization medium for photon Bose-Einstein condensation in the vacuum-ultraviolet spectral regime. We have recorded pressure-broadened xenon spectra in the 135- to 190-nm wavelength regime at conditions near the critical point. The explored pressure and temperature ranges include high-pressure gaseous xenon below the critical pressure and supercritical xenon at room temperature as well as liquid xenon close to the boiling point near the critical pressure.