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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

A Kennard-Stepanov relation study on redistributional laser cooling in dense gaseous ensembles

Stavros Christopoulos, Anne Saß, Peter Moroshkin, Lars Weller, Roberto Cota, Benedikt Gerwers, Katharina Knicker, Martin Weitz

Published Mar 10, 2015• 2 citations
Publisher

Abstract

We report on experiments investigating laser cooling of atomic gases by collisional redistribution of radiation, a technique applicable to dense mixtures of alkali metals with noble gases. Thermal deflection spectroscopy is one of the methods used to measure the temperature change of the laser-cooled gas. In this work we describe experiments focusing on a different technique for precise determination of the local temperature achieved by the cooling within the gas cell. We investigate the Kennard-Stepanov relation, a thermodynamic, Boltzmann-type scaling between the absorption and emission spectral profiles of an absorber, which applies in many liquid state dye solutions as well as in semiconductor systems. To this end, absorption and emission spectra of rubidium atoms and dimers in dense argon buffer gas environment have been recorded. We demonstrate experimentally that the Kennard-Stepanov relation between absorption and emission spectra is well fulfilled for the collisionally broadened atomic and molecular transitions of the system, which allows for the extraction of the thermodynamic temperature.

DOI
10.1117/12.2075940

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We report on experiments investigating laser cooling of atomic gases by collisional redistribution of radiation, a technique applicable to dense mixtures of alkali metals with noble gases. Thermal deflection spectroscopy is one of the methods used to measure the temperature change of the laser-cooled gas. In this work we describe experiments focusing on a different technique for precise determination of the local temperature achieved by the cooling within the gas cell. We investigate the Kennard-Stepanov relation, a thermodynamic, Boltzmann-type scaling between the absorption and emission spectral profiles of an absorber, which applies in many liquid state dye solutions as well as in semiconductor systems. To this end, absorption and emission spectra of rubidium atoms and dimers in dense argon buffer gas environment have been recorded. We demonstrate experimentally that the Kennard-Stepanov relation between absorption and emission spectra is well fulfilled for the collisionally broadened atomic and molecular transitions of the system, which allows for the extraction of the thermodynamic temperature.
The Kennard-Stepanov relation describes a thermodynamic, Boltzmann-type scaling between the absorption and emission spectral profiles of an absorber, which applies in many liquid state dye solutions as well as in semiconductor systems. Here we examine absorption and emission spectra of rubidium atoms in a dense argon buffer gas environment. We demonstrate that the Kennard-Stepanov relation between absorption and emission spectra is well fulfilled in the collisionally broadened atomic gas system. Our experimental findings are supported by a simple theoretical model.
Laser cooling via collisional redistribution of fluorescence utilizes dense mixtures of alkali metals with noble buffer gases. Typical pressure values of the buffer gas are of the order of a few hundred bar, ensuring a frequent number of collisions between the two atomic species. The energy levels of the alkali atoms are thus perturbed so that excitation using far red-detuned laser light becomes feasible. Energy is then extracted via spontaneous emission occuring close to the unperturbed atomic resonances. Optimization of the cooling effect strongly depends on excitation conditions, namely the choice of detuning of the cooling beam, which in turn is dependent on the used pressure. We here report on spectroscopy measurements of atomic rubidium under high pressure buffer gas conditions. While thermal deflection spectroscopy has been previously used to measure the temperature change of the laser-cooled gas, an alternative approach for temperature measurements utilizing the Kennard-Stepanov relation is also investigated. We here exhibit that the collisionally thermalized atomic resonances well follow this thermodynamic scaling, allowing for temperature extraction of the dense gas mixture.
We study laser cooling of atomic gases by collisional redistribution of fluorescence. In a high pressure buffer gas regime, frequent collisions perturb the energy levels of the alkali atoms; which enables the absorption of a far red detuned irradiated laser beam. Subsequent spontaneous decay occurs close to the unperturbed resonance frequency, leading to a cooling of the dense gas mixture by redistribution of fluorescence. Thermal defection spectroscopy indicates large relative temperature changes down to and even below room temperature starting from an initial cell temperature near 700K. We are currently performing a detailed analysis of the temperature distribution in the cell. As we expect this cooling technique to work also for molecular-noble gas mixtures, we also present initial spectroscopic experiments on alkali-dimers in a dense buffer gas surrounding.
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.
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.
We describe experiments on the laser cooling of both helium-rubidium and argon-rubidium gas mixtures by collisional redistribution of radiation. Frequent alkali-noble gas collisions in the ultradense gas, with typically 200 bar of noble buffer gas pressure, shift a highly red detuned optical beam into resonance with a rubidium Dline transition, while spontaneous decay occurs close to the unshifted atomic resonance frequency. The technique allows for the laser cooling of macroscopic ensembles of gas atoms. The use of helium as a buffer gas leads to smaller temperature changes within the gas volume due to the high thermal conductivity of this buffer gas, as compared to the heavier argon noble gas, while the heat transfer within the cell is improved.