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arXiv:1303.5233 · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Towards redistribution laser cooling of molecular gases: production of candidtate molecules SrH by laser ablation

P. Simon, Peter Moroshkin, L. Weller, A. Saß, Martin Weitz

Published Mar 11, 2013• 3 citations
arXivHTMLPDF

Abstract

Laser cooling by collisional redistribution of radiation has been successfully applied in the past for cooling dense atomic gases. Here we report on progress of work aiming at the demonstration of redistribution laser cooling in a molecular gas. The candidate molecule strontium monohydride is produced by laser ablation of strontium dihydride in a pressurized noble gas atmosphere. The composition of the ablation plasma plume is analyzed by measuring its emission spectrum. The dynamics of SrH molecular density following the ablation laser pulse is studied as a function of the buffer gas pressure and the laser intensity.

DOI
10.1117/12.2002379

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Laser cooling by collisional redistribution of radiation has been successfully applied in the past for cooling dense atomic gases. Here we report on progress of work aiming at the demonstration of redistribution laser cooling in a molecular gas. The candidate molecule strontium monohydride is produced by laser ablation of strontium dihydride in a pressurized noble gas atmosphere. The composition of the ablation plasma plume is analyzed by measuring its emission spectrum. The dynamics of SrH molecular density following the ablation laser pulse is studied as a function of the buffer gas pressure and the laser intensity.
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.
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.
We describe measurements demonstrating laser cooling of an atomic gas by means of collisional redistribution of radiation. The experiment uses rubidium atoms in the presence of several hundred bar of argon buffer gas pressure. Frequent collisions in the dense gas transiently shift a far-red detuned optical field into resonance, while spontaneous emission occurs close to the unperturbed atomic transition frequency. Evidence for the cooling is obtained via both thermographic imaging and thermographic deflection spectroscopy. The cooled gas has a density above 1021 atoms/cm3, yielding evidence for the laser cooling of a macroscopic ensemble of gas atoms.
We describe a simple experimental apparatus for laser ablation of barium monohydride (BaH) molecules and the study of their rovibrational spectra relevant to direct laser cooling. BaH is a promising candidate for laser cooling and ultracold fragmentation, both of which are precursors to novel experiments in many-body physics and precision measurement. We present a detailed analysis of the properties of ablation plumes that can improve the understanding of surface ablation and deposition technologies. A range of absorption spectroscopy and collisional thermalization regimes has been studied. We directly measured the Franck-Condon factor of the $B{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}({v}^{\ensuremath{'}}=0)\ensuremath{\leftarrow}X{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}({v}^{\ensuremath{'}\ensuremath{'}}=1)$ cycling transition. Prospects for production of a high luminosity cryogenic BaH beam are outlined.
We demonstrate the production of cold atomic strontium (Sr) and strontium-containing molecules (SrOH) in a cryogenic buffer-gas beam source via direct heating of strontium oxide (SrO) with 30 mJ laser pulses several milliseconds long. Here, $3.7(2)\ifmmode\times\else\texttimes\fi{}{10}^{14}$ Sr atoms are released, which represents a factor of 7 increase in atomic production per pulse compared to nanosecond-scale ablation laser pulses. A peak atomic density of $1.93(6)\ifmmode\times\else\texttimes\fi{}{10}^{12}$ atoms/cm3{\mathrm{cm}}^{3}cm3 is achieved, which corresponds to a factor of 2 increase relative to ablation. We further propose extensions of this method to other atomic and molecular species.
Polyatomic molecules, while ubiquitous in nature, were generally perceived as too complicated and "unruly" for the majority of quantum physics experiments. However, recent theoretical analyses have pinpointed the multiple advantages of using complex molecules for diverse applications such as the creation of a universal quantum computer, quantum simulation of non-trivial many-body states, and tests of fundamental physical laws. Throughout my dissertation research I focused on extending experimental techniques from atomic physics to complex molecules. The goal of my work was to not only demonstrate new ways to control molecules, but also develop a novel experimental platform for quantum science - complex polyatomic molecules - and provide tools for physicists to conquer new frontiers in physics and chemistry.\nThe main theme of this thesis is the use of laser radiation to control internal and external degrees of freedom for the triatomic radical strontium monohydroxide (SrOH). The research achievements presented here can be primarily divided into the following areas:\ni) out-of-equilibrium studies of vibrationally inelastic collisions at cryogenic temperatures,\nii) demonstration of the radiation pressure force and the direct laser cooling of polyatomic molecules, and\niii) the development of concepts for laser cooling of molecules with six and more atoms.\nFor the collisional studies, off-diagonal optical pumping was used to populate excited Sr-O stretching state and the ratio of elastic to vibrational quenching collisions was determined to be ~700. In the laser cooling work, using only laser light, the transverse temperature of the cryogenic buffer-gas beam of SrOH was reduced from 50 mK to below 1 mK, leading to an order of magnitude increase in phase space density of the molecular beam. Additionally, the prospects of using a coherent bichromatic force for optical deceleration of polyatomics were analyzed and experimentally characterized with the transverse deflection of the SrOH beam. Our experimental results with SrOH and theoretical analysis of more complex strontium alkoxides not only open a wide range of future directions for laser manipulation and trapping of polyatomic molecules but also help overturn long held assumptions about the difficulties of performing "traditional" atomic physics experiments with complex molecules.