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arXiv:2411.01209 · Physical Review Research

Efficient associative ionization of ultracold lithium and the long-lived Li 2 +

N. Joshi, Vaibhav Mahendrakar, M. Niranjan, Raghuveer Singh Yadav, E. Krishnakumar, Amrendra Pandey, Romain Véxiau, Olivier Dulieu, S. A. Rangwala

Published Jun 10, 2026• 0 citations
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Abstract

We report on the efficient production of Li 2 + ions in a magneto-optical trap (MOT) of Li atoms prepared in an ensemble of 2 S , 2 P , and 3 S states. A linear Paul trap surrounding the MOT is used to trap and investigate the time evolution of the ion populations. Associative ionization of Li( 2 P 3 / 2 ) and Li( 3 S 1 / 2 ) atoms, with a giant cross section of ∼ 2.2 × 10 6 a 0 2 , is the dominant Li 2 + production channel. Li 2 + decays to Li + via photodissociation and also in the absence of light, which are investigated experimentally and theoretically. A fraction of the Li 2 + ions are long-lived in the presence of the MOT light, implying that they are deeply bound and hence immune to dissociation, which opens up the possibility for studying Li 2 + molecular ion interactions with alkali atoms.

DOI
10.1103/z7xy-hmlb

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We report on the efficient production of Li 2 + ions in a magneto-optical trap (MOT) of Li atoms prepared in an ensemble of 2 S , 2 P , and 3 S states. A linear Paul trap surrounding the MOT is used to trap and investigate the time evolution of the ion populations. Associative ionization of Li( 2 P 3 / 2 ) and Li( 3 S 1 / 2 ) atoms, with a giant cross section of ∼ 2.2 × 10 6 a 0 2 , is the dominant Li 2 + production channel. Li 2 + decays to Li + via photodissociation and also in the absence of light, which are investigated experimentally and theoretically. A fraction of the Li 2 + ions are long-lived in the presence of the MOT light, implying that they are deeply bound and hence immune to dissociation, which opens up the possibility for studying Li 2 + molecular ion interactions with alkali atoms.
A two element magneto-optical trap (MOT) in standard configuration for Na and 7Li or 6Li is used to cool and trap Na and one Li isotope simultaneously or separately. The metal background vapor out of which the atoms are trapped comes from dispensers. The two atom clouds can be slightly shifted what makes it easy to overlap them. The number and density of the clouds were determined and the temperature measured roughly. The mutual influence of the clouds was investigated and turned out to be smaller than the noise. Preliminary experiments working towards the production of heteronuclear cold molecules by photoassociation were performed, showing the well known production of Na2 long- range molecules. Additionally, the photoionization cross- section was determined by ionizing cold atoms in the excited state by UV-laser light. The results are in reasonable agreement with data obtained elsewhere using synchrotron radiation.
In this work, a reaction microscope with a magneto-optical trap for 6Li atoms, was extended by an optical dipole trap in order to be able to investigate in detail laser-induced atomic and molecular ionization dynamics in a cold quantum gas. \nThe optical dipole trap was operated at a full trap depth of 2.3 mK for 6Li atoms and about \n1% of the atoms in the MOT could be transferred into the dipole trap, with 1/e storage times \nexceeding 5 s. The optically trapped ensemble of 6Li atoms was used as a target for ionisation with intensive and broadband femtosecond laser pulses (λ = 750−820 nm, P = 10^11−10^14 W/cm2, Δt = 30 fs) and allowed to perform kinematically complete experiments, in which 6Li+ ions \nas well as photoelectrons were measured coincidentically. \nAs a first application, in this work, a series of association- and ionization mechanisms, which \nled to production of molecular 6Li2+ ions, were investigated with trapped lithium atoms. \nIn photoassociative ionization, two atoms collide, which were previously lifted into the asymptotic 2p − 3s potential energy curve by ladder excitation. During the collision the atoms autoionize into the 12Σg+(6Li2+) groundstate of the molecular ion, since these two potential energy curves exhibit an avoided crossing. This process was observed when magneto-optically trapped atoms were illuminated with the femtosecond laser. \nIn the dipole trap, using single-color photoassociation, excited state molecules were produced \nin high-lying vibrational states 11Σg+(ν = 65) and 13Σg+(v = 57) and spectroscopically investigated. A fraction of the excited state molecules decay via fluorescence into the molecular ground state. The 11Σg+(ν = 38)(6Li2) ground state molecules created via the singlet res- \nonance 11Σu+(ν = 65)(Li∗2) were detected via direct 3 photon ionisation. The momentum spectra show very low kinetic energies for the photo electrons of below 100 meV. Therefore \nin the molecular ion only vibrational states of Li2+ are getting populated, which are directly below the 3-photon transition energy. \nFinally, a stepwise ionization mechanism was identified, which leads into the continuum via \nan intermediate molecular state of 6Li2∗ after photoassociation. The starting point is a pho- \ntoassociated excited state molecule 11Σu+(ν = 65)(6Li∗2), which absorbs two photons of the dipole trap laser (λ = 1070 nm ± 2 nm). This happens via an intermediate molecular state \n31Σg+(2s + 3s), after which it leads into the 12Σg+(6Li2+) potential.
A two element magneto-optical trap (MOT) in standard configuration for Na and 7Li or 6Li is used to cool and trap Na and one Li isotope simultaneously or separately. The metal background vapor out of which the atoms are trapped comes from dispensers. The two atom clouds can be slightly shifted what makes it easy to overlap them. The number and density of the clouds were determined and the temperature measured roughly. The mutual influence of the clouds was investigated and turned out to be smaller than the noise. Preliminary experiments working towards the production of heteronuclear cold molecules by photoassociation were performed, showing the well known production of Na2 long- range molecules. Additionally, the photoionization cross- section was determined by ionizing cold atoms in the excited state by UV-laser light. The results are in reasonable agreement with data obtained elsewhere using synchrotron radiation.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
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.
Our goal is the preparation and study of ultra- cold plasma and Rydberg matter. We assembled an ex- perimental setup for laser cooling of lithium atoms in a magneto-optical trap (MOT). Trapping of 7 Li atoms in a MOT is demonstrated for a wide range of cooling laser detunings. The number density of 7 Li atoms at different ground-state sublevels is measured by using a probe tun- able laser. The temperature of the trapped atoms is esti- mated. Obtained information is important for the prepara- tion of ultra-cold plasma and highly-excited atoms. duction of recombination in such systems. Our theoreti- cal works show the appearance of short and long-range order (7) and a large reduction of the recombination rate (a few orders of magnitude) in the ultra-cold mixture of Rydberg atoms and plasma in the presence of a strong magnetic field (8-11). The ability to control the decay and crystallization processes in an ensemble of Rydberg atoms could find an application in quantum storage and quan- tum computers. The suggested experiments can model the behavior of the plasma recombination in experiments on creating anti-hydrogen (12-14), and help to increase the number of ground state anti-hydrogen atoms in a strong magnetic field. It is noteworthy that lithium is an element which is very close to hydrogen.
We present spectra of ultracold 7Li85Rb^{7}\mathrm{Li}^{85}\mathrm{Rb}7Li85Rb molecules in their electronic ground state formed by spontaneous decay of weakly bound photoassociated molecules. Beginning with atoms in a dual-species magneto-optical trap, weakly bound molecules are formed in the 4(1) electronic state, which corresponds to the ${B}^{1}\ensuremath{\Pi}$ state at short range. These molecules spontaneously decay to the electronic ground state and we use resonantly enhanced multiphoton ionization to determine the vibrational population distribution in the electronic ground states after spontaneous emission. Many of the observed lines from the spectra are consistent with transitions from the ${X}^{1}{\ensuremath{\Sigma}}^{+}$ ground electronic state to either the ${B}^{1}\ensuremath{\Pi}$ or the ${D}^{1}\ensuremath{\Pi}$ electronic state that has been previously observed, with levels possibly as low as ${X}^{1}{\ensuremath{\Sigma}}^{+}({v}^{\ensuremath{'}\ensuremath{'}}=2)$ being populated. We do not observe decay to weakly bound vibrational levels of the ${X}^{1}{\ensuremath{\Sigma}}^{+}$ or ${a}^{3}{\ensuremath{\Sigma}}^{+}$ electronic state in the spectra. We also deduce a lower bound of $3900 \mathrm{cm}{}^{\ensuremath{-}1}$ for the dissociation energy of the LiRb+\mathrm{LiRb}{}^{+}LiRb+ molecular ion.