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arXiv:2503.10553 · Physical Review A

One- and two-photon spectroscopy with a test of the Kennard-Stepanov relation in high-pressure two-species xenon–noble-gas mixtures

Eric Boltersdorf, Thilo vom Hövel, Jeremy Andrew Morín Nenoff, Frank Vewinger, Martin Weitz

Published Jun 23, 2025• 0 citations
arXivHTMLPDF

Abstract

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 ($5p^6$) 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 $5p^6 \leftrightarrow 5p^56s$ 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.

DOI
10.1103/m77f-7r1j

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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 5p65{p}^{6}5p6 electronic ground-state to the 5p56p5{p}^{5}6p5p56p 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.
Other than for the three-dimensonal case of blackbody radiation, low-dimensional photon gases can exhibit Bose- Einstein condensation, as been experimentally demonstrated in the visible spectral regime first in 2010 using a dye-filled optical microcavity [1]. Here a short, wavelength sized spacing of cavity mirrors provides a low- frequency cutoff imprinting a spectrum of photon energies restricted to well above the thermal energy, and photons thermalize by repeated absorption and (re-)emission processes on the dye molecules to room temperature. As a prominent feature, a Bose-Einstein condensate of photons is as a coherent light source. This is of special interest in the vacuum-ultraviolet spectral regime (100 nm-200 nm, VUV), as it is difficult to realize lasers in this wavelength range. This is mainly due to the1/ω31/\omega^{3}1/ω3-scaling of excited state lifetimes, which requires increasingly high pump powers at short wavelengths to create population inversion in a laser medium.
Spectroscopy of alkali-metal--buffer-gas mixtures at high pressures from single-digit to several hundred bars in the regime of substantial collisional broadening is relevant in a wide range of fields, ranging from collisional redistribution cooling to laboratory astrophysics. Here we report on spectroscopic measurements of dense rubidium--noble-gas mixtures recorded in a pressure cell equipped with soldered sapphire optical viewports, which allows for the controlled realization of extreme conditions of high temperature and high pressure in a table top laboratory experiment. In the gas cell, we have recorded absorption and emission spectra of rubidium subject to 188 bars of helium buffer-gas pressure at 548-K temperature. The spectra to good accuracy follow a Boltzmann-type Kennard-Stepanov frequency scaling of the ratio of absorption and emission spectral profiles. Further, the long optical path length in the cell allowed us to both record spectra of rubidium-argon mixtures at moderate temperatures and high pressures and observe redistributional laser cooling in this system.
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.
Abstract Optical spectroscopy of atoms and molecules is a field where one usually operates very far from thermal equilibrium conditions. A prominent example is spectroscopy of thin vapors, where the pump irradiation leads to a non-equilibrium distribution within the electronic structure that is well shielded from the environment. Here we describe experimental work investigating absorption and emission lines of rubidium vapor subject to a noble buffer gas environment with pressure 100–200 bar, a regime interpolating between usual gas phase and liquid/solid state conditions. Frequent elastic collisions in the dense buffer gas sample cause a large coupling to the environment. We give a detailed account of recent observations of the Kennard–Stepanov scaling, a Boltzmann-like thermodynamic frequency scaling between absorption and emission profiles, for both atomic and molecular rubidium species in the gaseous environment. Our observations are interpreted as due to the thermalization of alkali-noble gas submanifolds in both ground and electronically excited states respectively. Both pressure broadening and shift of the high pressure buffer gas D -lines system are determined. We also discuss some prospects, including possible advances in collisional laser cooling and optical thermometry.