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Short-Range Photoassociation and Trilobite-Like States of Ultracold Rb2

Michael Bellos

Published Jan 1, 2013• 1 citation
OpenAlex

Abstract

I demonstrate the formation of rubidium dimers via photoassociation and spontaneous emission at short internuclear distances. This short-range photoassociation allows the formation of deeply bound molecules, including, for instance, the lowest rovibrational level of the lowest triplet state: a(0,0). This particular state is interesting because it is metastable against radiative decay, and possibly long-lived in optical and/or magnetic traps. In a second experiment, I demonstrate the population of trilobite-like excited states of Rb2. Trilobite-like states are bound by an unusual chemical bond, and can have large physical dimensions and electric dipole moments. I demonstrate the population of these states through the ultraviolet excitation of weakly bound molecules. By using the same ultraviolet excitation scheme, I place a new upper-bound on the minimum energy required to ionize 85Rb2 molecules.

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Formation of ultracold RbCs molecules by photoassociation

Nadia Bouloufa-Maafa, M Aymar, Olivier Dulieu +1 more

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.

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Formation of ultracold metastable RbCs molecules by short-range photoassociation

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Ultracold metastable RbCs molecules are observed in a double species magneto-optical trap through photoassociation near the Rb(5S(1/2)) + Cs(6P(3/2)) dissociation limit followed by radiative stabilization. The molecules are formed in their lowest triplet electronic state and are detected by resonance enhanced two-photon ionization through the previously unobserved (3)(3)Π ← a (3)Σ(+) band. The large rotational structure of the observed photoassociation lines is assigned to the lowest vibrational levels of the 0(+) or 0(-) excited states correlated to the Rb(5P(1/2)) + Cs(6S(1/2)) dissociation limit. This demonstrates the possibility of inducing direct photoassociation in heteronuclear alkali-metal molecules at a short internuclear distance, as pointed out earlier [J. Deiglmayr et al., Phys. Rev. Lett., 2008, 101, 13304].

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Experimental study of the formation of ultracold RbCs molecules by short-range photoassociation

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We have experimentally studied the formation of ultracold RbCs molecules by photoassociation at short internuclear range followed by spontaneous emission. The observation of new excited levels with respect to our previous work [Gabbanini and Dulieu, Phys. Chem. Chem. Phys. 13, 18905 (2011)] on both sides of the $\text{Rb}(5{S}_{1/2})+\text{Cs}(6{P}_{3/2})$ dissociation limit gives information on the state responsible for molecule formation. The molecules are mainly formed in deeply bound levels of the electronic triplet ground state. The observed photoassociation transitions present differences in terms of saturation.

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Utlracold Rb2 Molecules: Formation, Detection and Trapping in an Optical Dipole Trap

Hyewon Pechkis

This thesis comprises two topics in the field of the ultracold atomic and molecular physics, both involving formation of ultracold 85Rb 2 by photoassociation (PA). Typically, these ground-state molecules are formed mainly in the very highest vibrational levels, rather than the low-v levels that will be required for most future investigations of ultracold molecular physics and chemistry. The first topic of my research is a method for circumventing this problem: resonant enhancement (RE) of photoassociative ground-state molecule production. The key idea of RE is to photoassociate pairs of colliding atoms into an excited state that has a bimodal vibrational probability distribution, with one peak at long range to enhance the photoassociation rate and another at shorter range to enhance radiative decay into deeply bound levels of the ground state. This can be arranged thanks to the strong spin-orbit perturbative interaction between the two 0+u states converging to two different asymptotes, (5s + 5p1/2) and (5s + 5 p3/2). This method enhances ground-state molecule formation in the region well below the dissociation limit. The experimental work described here was done at the University of Connecticut in collaboration with C. P. Koch at the Institut für Theoretische Physik, Freie Universität Berlin, Germany, who performed the theoretical analysis [1].^ The second part of my thesis is the investigation of ultracold molecules in an optical dipole trap. We have constructed a quasi-electrostratic optical trap (QUEST) for 85Rb using a focused CO2 laser [2]. This provides a very high density of ultracold atoms, up to 10 12 cm-3, providing much higher photoassociation rates. It also keeps the molecules trapped once they have formed. The QUEST provides an ample trapping lifetime for both trapped atoms and molecules, yielding about 4-5×106 atoms and about 9×105 high-v molecules. The long lifetime of the trapped molecules and atoms, combined with their high density, allows us to study interactions such as atom-molecule or molecule-molecule collisions in the ultracold regime. A preliminary experiment on Rb2 molecular decay is described. ^

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Short-pulse photoassociation in rubidium below the D1 line

Christiane P. Koch, Ronnie Kosloff, Françoise Masnou-Seeuws

Photoassociation of two ultracold rubidium atoms and the subsequent formation of stable molecules in the singlet ground and lowest triplet states is investigated theoretically. The method employs laser pulses inducing transitions via excited states correlated to the $5S+5{P}_{1∕2}$ asymptote. Weakly bound molecules in the singlet ground or lowest triplet state can be created by a single pulse while the formation of more deeply bound molecules requires a two-color pump-dump scenario. More deeply bound molecules in the singlet ground or lowest triplet state can be produced only if efficient mechanisms for both pump and dump steps exist. While long-range $1∕{R}^{3}$ potentials allow for efficient photoassociation, stabilization is facilitated by the resonant spin-orbit coupling of the ${0}_{u}^{+}$ states. Molecules in the singlet ground state bound by a few wave numbers can thus be formed. This provides a promising first step toward ground-state molecules which are ultracold in both translational and vibrational degrees of freedom.

arXiv:physics/0511235•Physical Review A•Apr 13, 2006•73 citations

Experimental evidence for an isotopic effect in the formation of ultracold ground-state rubidium dimers

A. Fioretti, Olivier Dulieu, C. Gabbanini

We perform a comparative study of the formation of ultracold rubidium dimers in their ground state with both 85Rb and 87Rb isotopes. Ultracold rubidium molecules are created through photoassociation of ultracold atoms into rovibrational levels of the 0+u(5S1/2 + 5P1/2) excited state, which interact through resonant coupling (Dion et al 2001 Phys. Rev. Lett. 86 2253) with rovibrational levels of the 0+u(5S1/2 + 5P3/2) excited state. The photoassociated levels decay by spontaneous emission down to rovibrational levels of the X1Σ+g ground state. The resonant coupling is expected to enhance the formation of ultracold molecules in low-lying ground-state levels. By modelling the ionization spectrum of these ultracold molecules, we show that this coupling is larger for 87Rb2 molecules than for 85Rb2 molecules.

Journal of Physics B Atomic Molecular and Optical Physics•Aug 6, 2007•17 citations
I demonstrate the formation of rubidium dimers via photoassociation and spontaneous emission at short internuclear distances. This short-range photoassociation allows the formation of deeply bound molecules, including, for instance, the lowest rovibrational level of the lowest triplet state: a(0,0). This particular state is interesting because it is metastable against radiative decay, and possibly long-lived in optical and/or magnetic traps. In a second experiment, I demonstrate the population of trilobite-like excited states of Rb2. Trilobite-like states are bound by an unusual chemical bond, and can have large physical dimensions and electric dipole moments. I demonstrate the population of these states through the ultraviolet excitation of weakly bound molecules. By using the same ultraviolet excitation scheme, I place a new upper-bound on the minimum energy required to ionize 85Rb2 molecules.
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.
Ultracold metastable RbCs molecules are observed in a double species magneto-optical trap through photoassociation near the Rb(5S(1/2)) + Cs(6P(3/2)) dissociation limit followed by radiative stabilization. The molecules are formed in their lowest triplet electronic state and are detected by resonance enhanced two-photon ionization through the previously unobserved (3)(3)Π ← a (3)Σ(+) band. The large rotational structure of the observed photoassociation lines is assigned to the lowest vibrational levels of the 0(+) or 0(-) excited states correlated to the Rb(5P(1/2)) + Cs(6S(1/2)) dissociation limit. This demonstrates the possibility of inducing direct photoassociation in heteronuclear alkali-metal molecules at a short internuclear distance, as pointed out earlier [J. Deiglmayr et al., Phys. Rev. Lett., 2008, 101, 13304].
We have experimentally studied the formation of ultracold RbCs molecules by photoassociation at short internuclear range followed by spontaneous emission. The observation of new excited levels with respect to our previous work [Gabbanini and Dulieu, Phys. Chem. Chem. Phys. 13, 18905 (2011)] on both sides of the Rb(5S1/2)+Cs(6P3/2)\text{Rb}(5{S}_{1/2})+\text{Cs}(6{P}_{3/2})Rb(5S1/2​)+Cs(6P3/2​) dissociation limit gives information on the state responsible for molecule formation. The molecules are mainly formed in deeply bound levels of the electronic triplet ground state. The observed photoassociation transitions present differences in terms of saturation.
This thesis comprises two topics in the field of the ultracold atomic and molecular physics, both involving formation of ultracold 85Rb 2 by photoassociation (PA). Typically, these ground-state molecules are formed mainly in the very highest vibrational levels, rather than the low-v levels that will be required for most future investigations of ultracold molecular physics and chemistry. The first topic of my research is a method for circumventing this problem: resonant enhancement (RE) of photoassociative ground-state molecule production. The key idea of RE is to photoassociate pairs of colliding atoms into an excited state that has a bimodal vibrational probability distribution, with one peak at long range to enhance the photoassociation rate and another at shorter range to enhance radiative decay into deeply bound levels of the ground state. This can be arranged thanks to the strong spin-orbit perturbative interaction between the two 0+u states converging to two different asymptotes, (5s + 5p1/2) and (5s + 5 p3/2). This method enhances ground-state molecule formation in the region well below the dissociation limit. The experimental work described here was done at the University of Connecticut in collaboration with C. P. Koch at the Institut für Theoretische Physik, Freie Universität Berlin, Germany, who performed the theoretical analysis [1].^ The second part of my thesis is the investigation of ultracold molecules in an optical dipole trap. We have constructed a quasi-electrostratic optical trap (QUEST) for 85Rb using a focused CO2 laser [2]. This provides a very high density of ultracold atoms, up to 10 12 cm-3, providing much higher photoassociation rates. It also keeps the molecules trapped once they have formed. The QUEST provides an ample trapping lifetime for both trapped atoms and molecules, yielding about 4-5×106 atoms and about 9×105 high-v molecules. The long lifetime of the trapped molecules and atoms, combined with their high density, allows us to study interactions such as atom-molecule or molecule-molecule collisions in the ultracold regime. A preliminary experiment on Rb2 molecular decay is described. ^
Photoassociation of two ultracold rubidium atoms and the subsequent formation of stable molecules in the singlet ground and lowest triplet states is investigated theoretically. The method employs laser pulses inducing transitions via excited states correlated to the 5S+5P1∕25S+5{P}_{1∕2}5S+5P1∕2​ asymptote. Weakly bound molecules in the singlet ground or lowest triplet state can be created by a single pulse while the formation of more deeply bound molecules requires a two-color pump-dump scenario. More deeply bound molecules in the singlet ground or lowest triplet state can be produced only if efficient mechanisms for both pump and dump steps exist. While long-range 1∕R31∕{R}^{3}1∕R3 potentials allow for efficient photoassociation, stabilization is facilitated by the resonant spin-orbit coupling of the 0u+{0}_{u}^{+}0u+​ states. Molecules in the singlet ground state bound by a few wave numbers can thus be formed. This provides a promising first step toward ground-state molecules which are ultracold in both translational and vibrational degrees of freedom.
We perform a comparative study of the formation of ultracold rubidium dimers in their ground state with both 85Rb and 87Rb isotopes. Ultracold rubidium molecules are created through photoassociation of ultracold atoms into rovibrational levels of the 0+u(5S1/2 + 5P1/2) excited state, which interact through resonant coupling (Dion et al 2001 Phys. Rev. Lett. 86 2253) with rovibrational levels of the 0+u(5S1/2 + 5P3/2) excited state. The photoassociated levels decay by spontaneous emission down to rovibrational levels of the X1Σ+g ground state. The resonant coupling is expected to enhance the formation of ultracold molecules in low-lying ground-state levels. By modelling the ionization spectrum of these ultracold molecules, we show that this coupling is larger for 87Rb2 molecules than for 85Rb2 molecules.