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PHOTODISSOCIATION OF ULTRACOLD MOLECULES

Robin Côté

Published Jan 1, 1998• 0 citations
OpenAlex

Abstract

Translationally cold molecules can be produced during photoassociation experiments on ultracold atoms. We explore the photodissociation of these ultracold molecules in given rovibrational levels of their lowest electronic state into the excited electronic states. More precisely, we study the dissociation of ${^{7}}Li_{2}$ molecules in specific rovibrational level of the triplet electronic state $\\alpha^{3}\\Sigma^{+}_{u}$ to the $2s+2p$ limit. We observe structures at extremely low energies. We also explore the dissociation of the vibrational levels of the excited electronic states formed during photoassociation experiments on ultracold atoms. They are produced in specific rovibrational levels, and due to the presence of shape resonances, it is found that for certain energy regimes, the cross sections are strongly enhanced. Precise calculations will be presented for lithium.

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We have obtained high-lying vibrational spectra for the $\\rm 1\\sp3\\Sigma\\sbsp{g}{+}$ and $\\rm 1\\sp1\\Sigma\\sbsp{u}{+}$ excited states of both $\\rm \\sp7Li\\sb2$ and $\\rm \\sp6Li\\sb2$ via photoassociation of colliding ultracold lithium atoms confined in a magneto-optical trap. Photoassociative spectroscopy of ultracold atoms is a powerful tool for probing molecular states near the dissociation energy that are inaccessible by other methods. For an ultracold lithium vapor, the distribution of unbound ground state energies is so small that precise free-bound spectroscopy is possible. Molecular hyperfine structure is resolved. We calculate accurate dissociation energies for both the $\\rm 1\\sp3\\Sigma\\sbsp{g}{+}$ and $\\rm 1\\sp1\\Sigma\\sbsp{u}{+}$ excited states of Li$\\sb2$. A model potential is constructed from a combination of experimental and theoretical information, and used to assign vibrational quantum numbers. We observe rotational structure and assign rotational quantum numbers.

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We demonstrate the existence of a new mechanism for the formation of ultracold molecules via photoassociation of cold cesium atoms. The experimental results, interpreted with numerical calculations, suggest that a resonant coupling between vibrational levels of the 0+u (6s+6p1/2) and (6s+6p3/2) states enables formation of ultracold molecules in vibrational levels of the ground state well below the 6s+6s dissociation limit. Such a scheme should be observable with many other electronic states and atomic species.

Physical Review Letters•Mar 12, 2001•120 citations
Translationally cold molecules can be produced during photoassociation experiments on ultracold atoms. We explore the photodissociation of these ultracold molecules in given rovibrational levels of their lowest electronic state into the excited electronic states. More precisely, we study the dissociation of 7Li2{^{7}}Li_{2}7Li2​ molecules in specific rovibrational level of the triplet electronic state alpha3Sigmau+\\alpha^{3}\\Sigma^{+}_{u}alpha3Sigmau+​ to the 2s+2p2s+2p2s+2p limit. We observe structures at extremely low energies. We also explore the dissociation of the vibrational levels of the excited electronic states formed during photoassociation experiments on ultracold atoms. They are produced in specific rovibrational levels, and due to the presence of shape resonances, it is found that for certain energy regimes, the cross sections are strongly enhanced. Precise calculations will be presented for lithium.
The formation of ultracold metastable RbCs molecules is observed in a double species magneto-optical trap through photoassociation below the 85 Rb(5S 1/2 ) + 133 Cs(6P 3/2 ) dissociation limit followed by spontaneous emission. The molecules are detected by resonance enhanced two-photon ionization. Using accurate quantum chemistry calculations of the potential energy curves and transition dipole moment, we interpret the observed photoassociation process as occurring at short internuclear distance, in contrast with most previous cold atom photoassociation studies. The vibrational levels excited by photoassociation belong to the 5th 0 + or the 4th 0 − electronic states correlated to the Rb(5P 1/2, 3/2 ) + Cs(6S 1/2 ) dissociation limit. The computed vibrational distribution of the produced molecules shows that they are stabilized in deeply bound vibrational states of the lowest triplet state. We also predict that a noticeable fraction of molecules is produced in the lowest level of the electronic ground state.
We recently reported the formation of ultracold LiCs molecules in the rovibrational ground state X1sigma+, v" = 0,J" = 0 (J. Deiglmayr et al., Phys. Rev. Lett., 2008, 101, 133004). Here we discuss details of the experimental setup and present a thorough analysis of the photoassociation step including the photoassociation line shape. We predict the distribution of produced ground state molecules using accurate potential energy curves combined with an ab initio dipole transition moment and compare this prediction with experimental ionization spectra. Additionally we improve the value of the dissociation energy for the X1sigma+ state by high resolution spectroscopy of the vibrational ground state.
Ultracold collisions between Cs atoms and Cs2{\mathrm{Cs}}_{2}Cs2​ dimers in the electronic ground state are observed in an optically trapped gas of atoms and molecules. The Cs2{\mathrm{Cs}}_{2}Cs2​ molecules are formed in the triplet ground state by cw photoassociation through the outer well of the ${0}_{g}^{\ensuremath{-}}({P}_{3/2})$ excited electronic state. Inelastic atom-molecule collisions converting internal excitation into kinetic energy lead to a loss of Cs2{\mathrm{Cs}}_{2}Cs2​ molecules from the dipole trap. Rate coefficients are determined for collisions involving Cs atoms in either the F=3F=3F=3 or F=4F=4F=4 hyperfine ground state, and Cs2{\mathrm{Cs}}_{2}Cs2​ molecules in either highly vibrationally excited states (${v}^{\ensuremath{'}}=32--47$) or in low vibrational states (${v}^{\ensuremath{'}}=4--6$) of the $a\text{ }^{3}\ensuremath{\Sigma}_{u}^{+}$ triplet ground state. The rate coefficients $\ensuremath{\beta}\ensuremath{\sim}{10}^{\ensuremath{-}10}\text{ }\text{ }{\mathrm{cm}}^{3}/\mathrm{s}$ are found to be largely independent of the vibrational and rotational excitation indicating unitary limited cross sections.
We have obtained high-lying vibrational spectra for the rm1sp3Sigmasbspg+\\rm 1\\sp3\\Sigma\\sbsp{g}{+}rm1sp3Sigmasbspg+ and rm1sp1Sigmasbspu+\\rm 1\\sp1\\Sigma\\sbsp{u}{+}rm1sp1Sigmasbspu+ excited states of both rmsp7Lisb2\\rm \\sp7Li\\sb2rmsp7Lisb2 and rmsp6Lisb2\\rm \\sp6Li\\sb2rmsp6Lisb2 via photoassociation of colliding ultracold lithium atoms confined in a magneto-optical trap. Photoassociative spectroscopy of ultracold atoms is a powerful tool for probing molecular states near the dissociation energy that are inaccessible by other methods. For an ultracold lithium vapor, the distribution of unbound ground state energies is so small that precise free-bound spectroscopy is possible. Molecular hyperfine structure is resolved. We calculate accurate dissociation energies for both the rm1sp3Sigmasbspg+\\rm 1\\sp3\\Sigma\\sbsp{g}{+}rm1sp3Sigmasbspg+ and rm1sp1Sigmasbspu+\\rm 1\\sp1\\Sigma\\sbsp{u}{+}rm1sp1Sigmasbspu+ excited states of Lisb2\\sb2sb2. A model potential is constructed from a combination of experimental and theoretical information, and used to assign vibrational quantum numbers. We observe rotational structure and assign rotational quantum numbers.
The first part of this project involved continued development of theoretical models of diatomic molecular electronic level structure for application to the production of ultracold diatomic molecules via photoassociation. Recently, there has been considerable interest in producing molecules at the temperatures achieved for laser-cooled atoms, for applications to "coherent chemistry," studies of molecule-atom and molecule-molecule interactions. Bose condensates of molecules and for quantum computing. It is advantageous to produce cold molecules from laser-cooled atoms. but this requires an accurate model of the energy level structure near the dissociation limit for excited atoms, and for other intermediate states. With this in mind. we have modeled experimental photoassociation data from Rb2. spectroscopic data on the lowest excited states of Na2, and hyperfine structure of the lowest triplet state of Cs2. The second part involved continued development of experimental techniques for intense atomic beam collimation using a bichromatic laser field. We solved the equations for evolution of atoms in such a laser field, expressed the results in the form of trajectories on the Bloch sphere for a two-level atoms, and then used these results to design and perform efficient atomic beam reflection experiments. At the same time, we are developing methods to use these highly collimated beams of metastable helium atoms for precision lithography, using sensitized surfaces.
We demonstrate the existence of a new mechanism for the formation of ultracold molecules via photoassociation of cold cesium atoms. The experimental results, interpreted with numerical calculations, suggest that a resonant coupling between vibrational levels of the 0+u (6s+6p1/2) and (6s+6p3/2) states enables formation of ultracold molecules in vibrational levels of the ground state well below the 6s+6s dissociation limit. Such a scheme should be observable with many other electronic states and atomic species.