Two-photon excitation and absorption spectroscopy of gaseous and supercritical xenon

Constructing lasers in the vacuum-ultraviolet spectral range (VUV; 100 – 200 nm) is difficult, as excited state lifetimes scale as 1/ω³, necessitating high pump powers to achieve population inversion. Alternatively, a coherent source in this wavelength regime could be based on a Bose-Einstein condensate of photons, as first realized in our group in 2010 for visible-spectral-range photons. In the used experimental scheme, a photon gas is confined to a wavelength-size optical microcavity, which is filled with a liquid dye solution. The dye molecules exhibit a thermalized internal level structure and in repeated absorption and emission cycles the two-dimensional photon gas is thermally equilibrated. We here propose the adaption of this scheme for the condensation of VUV photons, using dense xenon gas as a thermalization mediator for the short-wavelength photons. Aiming at the identification of possible excitation channels for such a vacuum-ultraviolet photon condensate, two-photon transitions from the 5p⁶ to the 5p⁵6p and 5p⁵6p’ states are probed, using two photons of identical wavelengths in the UV. Further, absorption measurements of the 5p⁶ → 5p⁵6p transitions are presented, involving one light field in the VUV and an auxiliary one near 500 nm wavelength. This nondegenerate scheme aims at increasing the reabsorption probability for photons emitted on the second excimer continuum near 172 nm wavelength.