Stationary molecules in well-defined internal states are of broad interest for physics and chemistry. In physics, this includes metrology 1-3 , quantum computing 4,5 and manybody quantum mechanics 6,7 , whereas in chemistry, stateprepared molecular targets are of interest for uni-molecular reactions with coherent light fields 8,9 , for quantum-stateselected bi-molecular reactions 10-12 and for astrochemistry 12. Here, we demonstrate rotational ground-state cooling of vibrationally and translationally cold MgH + ions, using a lasercooling scheme based on excitation of a single rovibrational transition 13,14. A nearly 15-fold increase in the rotational ground-state population of the X 1 + electronic groundstate potential has been obtained. The resulting ground-state population of 36.7 ± 1.2% is equivalent to that of a thermal distribution at about 20 K. The obtained cooling results imply that cold molecular-ion experiments can now be carried out at cryogenic temperatures in room-temperature setups. At present there is a strong interest within the scientific community to produce and experiment with cold molecules. As standard laser-cooling schemes developed for atomic species cannot be applied to molecules, in the recent past tremendous effort has been put into developing schemes to cool molecules by other means. Several schemes for producing translationally cold and strongly bound neutral molecules in a single quantum state have been demonstrated. These include buffer-gas cooling of magnetic dipolar molecules in magnetic traps 15 , Stark deceleration and trapping of molecules with permanent 16,17 or induced 18 electric dipole moments, formation of molecules through Feshbach resonances followed by transfer to the vibrational ground state of electronic molecular potentials by stimulated Raman adiabatic passage schemes 19 and photoassociative molecule formation directly in the rovibrational ground state 20 or by transfer to the vibrational ground state through the application of shaped femtosecond laser pulses 21. With respect to molecular ions, the well-established technique of buffer-gas cooling is the only method that has so far led to the production of translationally as well as internally cold molecular ions 22. Although buffer-gas cooling is simple and generally applicable, it limits both the effective internal and translational temperature to the kelvin range, and owing to frequent collisions with the buffer-gas atoms it prevents strong spatial localization as well as utilization of internal coherences. Sympathetic cooling of trapped molecular ions, through the Coulomb interaction with laser-cooled atomic ions, has, on the other hand, proven highly effective for reaching translational temperatures in the millikelvin range, where the ions become spatially localized in the form of so-called Coulomb crystals 23. The sympathetic cooling scheme does, however, not significantly influence the molecular ion's internal degrees of freedom, owing to the very distant Coulomb interactions within the crystalline structures. ...
We have produced and cooled the molecular ions MgH ϩ and MgD ϩ in a linear Paul trap. These ions were generated by the photochemical reactions Mg ϩ (3p 2 P 3/2 )ϩH 2 (D 2 )→MgH ϩ (MgD ϩ )ϩH (D), and identified by the radial separation in the trap of ions with different charge-to-mass ratios. The molecular translational motion was cooled sympathetically by Coulomb interaction with laser-cooled Mg ϩ ions to a temperature estimated to be below 100 mK. Ordered structures ͑ion crystals͒ containing more than 1000 ions, with more than 95% being molecular ions, were obtained. Such translationally cold and well-localized samples of molecular ions could become very useful for molecular physics and chemistry.
Plasmas of Mg 1 ions, containing more than 10 5 ions, have been observed to reach well-ordered (crystalline) states by applying laser cooling. The crystals are highly elongated with up to ten concentric cylindrical shells surrounding a central string. Such large structures have not previously been observed in a Paul trap. The amplitude of the micromotion of the ions can be larger than the shell spacings. As the diameter changes along the crystals, sharp transitions are observed when new shells form, in good agreement with molecular dynamics simulations. The predictions from simulations of how ordering develops with decreasing temperature are also confirmed. [S0031-9007(98)07188-9] PACS numbers: 32.80. Pj, 42.50.Vk, 52.25.Wz Clouds of laser-cooled, trapped ions have previously been observed to condense and to exhibit quasicrystalline spatial order in Penning [1,2] and Paul (radiofrequency or rf) traps [3][4][5]. A classical, infinite, one-component plasma undergoes a transition from liquidlike behavior to a body-centered-cubic (bcc) lattice when the ratio G of the Coulomb energy between adjacent particles to the random thermal kinetic energy exceeds 175 [6]. In contrast, for finite plasmas molecular dynamics (MD) simulations predict formation of concentric shells with near-hexagonal ordering within the shell [7,8]. (Though these structures have no long-range periodic order, they are often referred to as crystals, a usage we follow here.)In Paul traps the rf field drives micromotion, which is a modulation (at the rf frequency) of the secular harmonic motion in the effective trapping potential. The magnitude of the micromotion increases with an ion's distance from the trap central axis, and such motion can couple energy from the rf drive into the random motion of the ions through their mutual Coulomb interaction. This so-called rf heating (see, e.g., [9,10]) is widely assumed to limit attainable crystal sizes. The kinetic energy associated with the micromotion can be several orders of magnitude higher than the thermal energy at which spatial ordering occurs in static potentials, so the coupling of micromotion into thermal motion can be expected to be critical for crystal formation. The noninertial constraints and the continually changing shape of the cloud in the rf field can affect the ordering process and the resultant structure of the crystals in interesting ways that at present are poorly understood.Crystals consisting of at most five shells have been attained earlier [5] in a ring rf trap, and simulations with up to 512 ions in a standard Paul trap, with the emphasis on the averaged position of ions in the crystal [11] have been reported previously. By contrast, the largest ordered systems so far were seen in Penning traps, where more than 10 5 ions have been crystallized and show evidence of central bcc structures [2].In this Letter, we present evidence for Coulomb crystals of the largest transverse size observed in a linear Paul trap [12], as well as evidence on how the ordering develops gradually as the r...
We demonstrate a simple and nondestructive method for identification of a single molecular ion sympathetically cooled by a single laser cooled atomic ion in a linear Paul trap. The technique is based on a precise nondestructive determination of the molecular ion mass through a measurement of the eigenfrequency of a common motional mode of the two ions. The demonstrated mass resolution is sufficiently high that molecular ion mass doublets can potentially be distinguished from each other. The obtained results represent an important step towards single molecule gas phase chemical physics.
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