Rubidium spectroscopy at high-pressure buffer gas conditions: detailed balance in the optical interaction of an absorber coupled to a reservoir

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.

Spectroscopy of high pressure rubidium-noble gas mixtures

“…For a noble buffer gas pressure in the 100 bar range, collisions occur on a 10 −11 s timescale, which is far faster than the upper electronic state lifetime of D-line transitions of alkali atoms. In this limit, a Boltzmannlike Kennard-Stepanov scaling between absorption and emission spectral profiles has been observed in rubidium-argon buffer gas mixtures [5,6]. Following a model developed by Sawicki and Knox [17] originally in the context of the description of dye spectra, the Kennard-…”

“…When using inert gases as a buffer gas, the quenching of optical transitions is weak, and atoms in their electronically excited state undergo a large number of collisions, see e.g. Refs [4][5][6][7] for earlier work on the optical spectroscopy of dense alkali-noble gas mixtures. Especially the effect of a high pressure environment of the lightest noble gas helium acting as a buffer gas on the spectra of other elements is of great interest in astronomy as it is the second-most abundant element and represents 24% of the total baryonic mass in the universe [8].…”

“…This has a number of well-known consequences for the spectral profiles: the Stokes shift and Kasha's rule [10][11][12][13], which to a certain extent are clearly present in our measured spectra. Additionally, the assumption of a thermalisation in the occupation of the excited state manifold can be tested by verifying the Kennard-Stepanov relation [14,15] which is a Boltzmann-type scaling of the ratio of absorption and emission spectral profiles with frequency, and is known to be fulfilled in a wide range of systems like semiconductor quantum wells, doped glasses, photoactive biomolecules, dyes and other mixtures of alkali-metals and noble gases [5,6,[16][17][18][19][20][21].…”

Sapphire optical viewport for high pressure and temperature applications

“…In our main experiment (see e.g. [7]) the cell equipped with the assembled viewports has been successfully operated with rubidium vapor at 300 °C operation temperature (≈ 0.4 mbar vapor pressure) subject to typically 200 bar of noble buffer gas pressure, and allowed to observe pressure broadened spectra, without observable degradation of the all metal/sapphire construction from the chemically aggressive rubidium vapor after several weeks.…”

“…Optical spectroscopy is a powerful technique in fields ranging from atomic physics over chemistry and geology to industrial usage [1][2][3][4][5] . In several applications, the sample under investigation is a medium at high temperature or high pressure [6][7][8][9][10] , leading to technical challenges in sealing the optical access to the medium's container.…”