Xenon-helium gas mixtures as thermalization mediator for vacuum-ultraviolet photon Bose-Einstein condensation

Bose-Einstein condensation, previously mostly associated with cold atomic ensembles, has in our and other groups in recent years been realized with visible spectral range photons in material-filled optical microcavities. A quantum gas of light, confined to the microcavity inducing a non-trivial low-energy ground state, is subject to repeated absorption and emission cycles in a liquid dye solution, driving the photon gas to thermal equilibrium at ambient temperature, as understood from the collision-induced thermalization of the rovibronic substructure of the involved molecular states. We propose an experimental approach for the realization of a Bose-Einstein condensate of photons in the vacuum-ultraviolet (100 – 200 nm), a spectral regime in which the construction of lasers is inherently difficult due to short excited state lifetimes. Intriguingly, a photon condensate requires no population inversion, while nevertheless constituting a source of coherent and monochromatic light. Our current proposal envisages transitions between the 5p⁶ and 5p⁵6s states of lightly bound xenon-helium quasi-molecules at around 100 bar total pressure to be employed in the thermalization scheme. We here report on spectroscopic absorption and emission data, exploring suitable pressure and admixture conditions. Further, the competing influence of homonuclear xenon-xenon excimers is assessed.