We have simultaneously confined rubidium and metastable argon in a dual-species magneto-optical trap (MOT). Here a binary mixture of atomic species from different groups of the periodic table have been optically confined at ultracold temperatures. We describe the apparatus and characterize the individual, single species MOTs and the dual-species MOT. Both fluorescence and ion production are monitored. With both species trapped, we observe ~5106 85Rb atoms and ~2106 40Ar* atoms. Realization of the dual-species trap opens the way for detailed studies of Penning and associative ionization, photoassociative spectroscopy, and eventually for the production of bound, ultracold RbAr molecules

Busch, H. C.

Sukenik, C. I.

English

Old Dominion University

Old Dominion UniversityODU Digital CommonsPhysics Faculty Publications Physics11-2002Simultaneous Trapping of Rubidium andMetastable Argon in a Magneto-Optical TrapC. I. SukenikOld Dominion University, csukenik@odu.eduH. C. BuschOld Dominion UniversityFollow this and additional works at: https://digitalcommons.odu.edu/physics_fac_pubsPart of the Atomic, Molecular and Optical Physics CommonsThis Article is brought to you for free and open access by the Physics at ODU Digital Commons. It has been accepted for inclusion in Physics FacultyPublications by an authorized administrator of ODU Digital Commons. For more information, please contact digitalcommons@odu.edu.Repository CitationSukenik, C. I. and Busch, H. C., "Simultaneous Trapping of Rubidium and Metastable Argon in a Magneto-Optical Trap" (2002).Physics Faculty Publications. 12.https://digitalcommons.odu.edu/physics_fac_pubs/12Original Publication CitationSukenik, C.I., & Busch, H.C. (2002). Simultaneous trapping of rubidium and metastable argon in a magneto-optical trap. PhysicalReview A, 66(5), 51402. doi: 10.1103/PhysRevA.66.051402Simultaneous trapping of rubidium and metastable argon in a magneto-optical trapC. I. Sukenik and H. C. BuschDepartment of Physics, Old Dominion University, Norfolk, Virginia 23529~Received 15 August 2002; published 15 November 2002!We have simultaneously confined rubidium and metastable argon in a dual-species magneto-optical trap~MOT!. Here a binary mixture of atomic species from different groups of the periodic table have been opticallyconfined at ultracold temperatures. We describe the apparatus and characterize the individual, single speciesMOTs and the dual-species MOT. Both fluorescence and ion production are monitored. With both speciestrapped, we observe;53106 85Rb atoms and;23106 40Ar* atoms. Realization of the dual-species trapopens the way for detailed studies of Penning and associative ionization, photoassociative spectroscopy, andeventually for the production of bound, ultracold RbAr molecules.DOI: 10.1103/PhysRevA.66.051402 PACS number~s!: 32.80.Pj, 33.80.PsRecent advances in ultracold molecule production areopening new frontiers of research in low-temperature physicsand chemistry. Unlike atoms, molecules do not possess anoptical cycling transition and quickly become decoupledfrom a light field if the standard cooling and trapping tech-niques used for atoms are applied. As a result, other methodsare needed to produce ultracold molecules. To date, a numberof techniques to produce cold molecules have been demon-strated including electrodynamic deceleration of polar mol-ecules@1#, sympathetic cooling with a helium buffer gas@2#,and photoassociation of ultracold atoms in a magneto-opticaltrap ~MOT! @3# or Bose-Einstein condensate@4#. Photoasso-ciation uses well-established methods to cool the transla-tional energy of atoms which are then brought together toform a molecule. Initial experiments demonstrated the feasi-bility of photoassociating atoms into molecules producinghomonuclear dimers, the simplest pair to produce. Recently,attention has turned to the production of heteronucleardimers for low-temperature physics studies@5# as possiblecandidates for quantum computation schemes@6#, and forparity violation measurements@7#. Because atoms are photo-associated during collisions, the details of the molecule for-mation process depend strongly on the ultracold collisiondynamics which are rather different for the two classes ofdimers. This is because for low partial-wave scattering, thefirst excited state of homonuclear dimers with internuclearseparationR is characterized by a long-range 1/R3 resonantdipole interaction, whereas in the heteronuclear case it is themuch shorter range 1/R6 van der Waals interaction whichtypically dominates.In this paper we report on the realization of a dual-speciesMOT which simultaneously confines alkali-metal (85Rb) andmetastable noble-gas (40Ar* ) atoms. Little is known aboutthe interactions of such a mixture at low temperature. In fact,to our knowledge this is the first time that two species fromdifferent groups of the periodic table have been confinedsimultaneously in a MOT. We have constructed the dualMOT with the eventual goal of producing ultracold groundstate RbAr, a weakly bound van der Waals molecule. Weobserve;53106 85Rb atoms and;23106 40Ar* atoms ina cloud with an approximate diameter of 500mm. We havealso observed a modest interaction between the Rb and Ar*atoms confined in the dual MOT. The coexistence of the twospecies under nominal MOT conditions was not a foregoneconclusion given that no information is available on cross-species ionization rates at ultracold temperatures. Hence theobservation of a robust dual-species MOT containing Rb andAr* has important positive implications for further studies ofphotoassociation and molecule formation.In addition to the promise of producing a weakly boundheteronuclear molecule, the Rb1Ar* system contains somefeatures that differentiate it from single-species traps. Asnoted above, the homonuclear and heteronuclear collisiondynamics are considerably different at ultracold tempera-tures, where the details of the long-range part of the potentialare very important. In addition, with the considerable inter-nal energy of the Ar* , interspecies ionizing collisions can bestudied in detail and excited-state reaction products can beproduced. Even-numbered isotopes of the noble gases do nothave hyperfine structure, thereby allowing for the detailedstudy of the role of hyperfine structure of just one of thecollision partners. And finally, for suitables-wave scatteringlengths, it may be possible to employ sympathetic cooling ofthe noble gas by alkali metal atoms as an alternative route toachieving Bose-Einstein condensation of noble gases or toproduce binary Bose-Einstein condensates.The dual MOT is produced in a stainless steel, differen-tially pumped, vacuum chamber with three regions. Ru-bidium atoms are loaded into the MOT from a backgroundvapor which is produced by running a current (;3.4 A)through an alkali-metal dispenser~SAES Getters!. The meta-stable argon is loaded from an atomic beam. The MOT re-gion is pumped by a 50-l/s turbo pump and an 11-l/s ionpump. The pressure in the MOT chamber is monitored by theion pump current. A 170-l/s turbo pump is used in the sourceregion, and a 50-l/s turbo pump is used in conjunction with aliquid-nitrogen cold trap in an intermediate region, betweenthe MOT and the source. Anti-Helmholtz coils, placed exter-nal to the chamber, provide an axial magnetic-field gradientof 10 G/cm at the center of the trapping region.Metastable atoms are produced in a rf discharge, whichoperates at a frequnecy of 155 MHz and closely follows thedesign of Chenet al. @8#. We estimate the metastable flux tobe near the 431014 s21 sr21 reported in Ref.@8#. The dis-charge frequency is very close to the 2-m amateur radioband, allowing us to use an inexpensive rf amplifier~MirageRAPID COMMUNICATIONSPHYSICAL REVIEW A 66, 051402~R! ~2002!1050-2947/2002/66~5!/051402~4!/$20.00 ©2002 The American Physical Society66 051402-1B-34-G! and antenna tuner~MFJ-921! to operate the dis-charge. Ions are removed from the atomic beam by a pair ofelectric-field plates. The atomic beam~containing Ar andAr* ) then enters a Zeeman slower, where the Ar* atoms aretranslationally cooled in one dimension before entering theMOT capture zone. The Zeeman slower is constructed usinga single layer of 14 gauge magnet wire that is wound, withvariable pitch, around a standard 1.5 inch outer diameterstainless steel tube. A water cooled brass jacket is placedaround the Zeeman coil.Separate laser systems are used for each species. Anenergy-level diagram for both atoms is shown in Fig. 1. ASanyo DL7140-201 laser diode is injection locked to anexternal-cavity diode laser master~also a Sanyo DL7140-201! to produce the trapping and repumping light for Rb at780 nm. The repumping frequency is obtained by directmodulation of the slave current through a bias-T junction toproduce sidebands at the required frequency@9#. Typically,the sideband intensity is 10% of the fundamental. A Coherent899-01 Ti:sapphire laser with a Coherent 895 etalon assem-bly ~to narrow the linewidth! is used to produce the 811-nmtrapping light for 40Ar* . Note that with no hyperfine struc-ture in 40Ar, a repumper laser is not required.We have added a piezoelectric driven mirror~tweeter! tothe Ti:sapphire laser to allow for frequency stabilization.Without a reference cavity or some of the more sophisticatedlocking techniques often used in narrow-band lasers, we findthat the Ti:sapphire laser is susceptible to acoustic noisewhich translates to pronounced oscillations in the output fre-quency. The frequency jitter is on the order of the naturallinewidth of the trapped atoms, and hence does impact trapperformance. We are presently modifying our Ti:sapphire la-ser to better shield from acoustic perturbations and henceimprove the Ti:sapphire locking and Ar* MOT stability.Both laser are locked using phase-sensitive detection toseparate saturated absorption spectrometers. In the case ofRb, a small portion of light from the master laser passesthrough an acousto-optic modulator~AOM!, driven at 80MHz, and is downshifted in frequency before entering thesaturated absorption spectrometer. We then lock the laser tothe peak of theF53→F85224 crossover, resulting in atrapping frequency red detuned two natural linewidths fromtheF53→F854 cooling transition in85Rb. Since there areno crossover peaks in the40Ar* spectrum, the lockingscheme must be slightly different than for Rb. The output ofthe Ti:sapphire laser is split into three parts. One part passesthrough an AOM driven at 80 MHz, and is sent to the40Ar*saturated absorption spectrometer. Unlike the Rb setup, thelight is shifted to theblueof the 4s@3/2#2→4p@5/2#3 coolingtransition in40Ar* . This results in an output frequency of theTi:sapphire laser which is detuned 80 MHz to the red of thecooling transition. A second AOM then shifts the portion oflight used for trapping up by 68 MHz before being sent tothe MOT chamber, again resulting in a trapping laser fre-quency which is two natural linewidths from resonance. Athird AOM shifts the final portion of light down by another80 MHz, yielding a total red detuning of 160 MHz. This lightis then sent to the Zeeman slower.All light passes through single-mode optical fibers beforearriving at the MOT. Because of the proximity of the twowavelengths, identical optics, including wave plates, can beused for the Rb and Ar* light. Generally, this is quite ben-eficial, but it does complicate combining the two beams be-fore sending them into the MOT chamber because dichroicoptics for such a small wavelength separation are difficult tofabricate. After exiting their respective fibers, the light foreach species is identically collimated and combined on apolarizing beam-splitter cube. This results in a complete spa-tial overlap with no loss of light, but yields orthogonal po-larizations for the two colors. The polarization of both wave-lengths is then rotated by 45° with a half wave plate beforeimpinging on a second polarizing beam-splitter cube whichyields two beams each with an equal amount of identicallypolarized 780-nm and 811-nm light. One beam is sent to thevertical arm of the MOT, and the other beam is further spliton a 50-50 nonpolarizing beam-splitter cube and sent to thetwo horizontal arms of the MOT. Before entering the cham-ber, the linearly polarized light on each axis is made circularwith a 1/4 wave plate. Note that a single wave plate is usedfor both colors. After exiting the chamber, all three beamsare retroreflected after passing through a 1/4 wave plate. Al-though intensities on the three axes are not the same, the trapoperates well. The total laser power available after the outputof the fiber is 8 mW at 780 nm and 27 mW at 811 nm. Bothbeams are expanded to a 1/e2 diameter of 1 cm.The light for the Zeeman slower is also brought to thetrapping chamber via a single-mode fiber. After passingthrough a linear polarizer and 1/4 wave plate, 25 mW of lightropagates along the Ar* atomic beam axis. The 1/e2 diam-eter is;1 cm entering the chamber and the beam is slightlyfocusing as it propagates along the Zeeman slower.The fluorescence from the trapped Rb and Ar* atoms ismonitored by separate photomultipliers~Hamamatsu R928!,each equipped with a narrow bandpass interference filter~Melles Griot 03 FIL 056 for Rb, and Esco S908100 forAr* ). This allows us to independently observe the behaviorof each species. Two charge coupled device~CCD! cameras,placed at right angles to each other, ensure good spatial over-lap of the two traps. In addition, a channel electron multipler~Amptek MD-502! is placed 10 cm from the center of theFIG. 1. Partial energy-level diagrams for85Rb and 40Ar. Thedotted line indicates an electric dipole forbidden transition thatgives rise to the metastability of the 4s@3/2#2 state.RAPID COMMUNICATIONSC. I. SUKENIK AND H. C. BUSCH PHYSICAL REVIEW A66, 051402~R! ~2002!051402-2trap to monitor ion production. With the Rb laser blocked wefirst load the Ar* MOT. A typical loading curve for the Ar*MOT, obtained by measuring the Ar* fluorescence as a func-tion of time, is shown in the upper trace of Fig. 2. Concur-rently, we also record the ion counts as shown in the lowertrace of Fig. 2. As the MOT loads, both the atom number andthe density increase, yielding an increase in Penning andassociative ionization resulting from Ar* -Ar* collisions@10#. The oscillations visible in the loading curve arise fromlaser instabilities as discussed above. After the Ar* MOT hasloaded, we unblock the Rb laser, allowing the Rb trap toform in the presence of the Ar* MOT, as shown in Fig. 3.An interaction between the ultracold Rb and Ar* has beenobserved. As with single-species traps, light-assisted colli-sions have been observed to induce trap loss in mixed alkali-metal MOTs. In addition to the dominant trap loss mecha-nisms observed in alkali systems~ uch as radiative escape!,collisional ionization~Penning and associative! from Rb-Ar*collisions is expected to be a major, if not dominant, compo-nent of the interspecies trap loss rate. With 11.5 ev of internalenergy, a 4s@3/2#2 Ar* atom has sufficient energy to ionize aground-state Rb atom, whose ionization energy is 4.2 eV.With the experimental conditions reported here, the ob-served interaction between Rb and Ar* is relatively small.We have measured an 8% decrease in the number of trappedRb atoms after the Ar* MOT is allowed to load. Somewhatlarger interactions have been observed in dual-alkali metalMOTs containing Rb and K@11,12#, Rb and Na@13#, Rb andCs @14#, and Cs and Li@15#. However, neither our Rb trapnor our Ar* trap has yet been optimized for density; weexpect to observe an increase in the Rb-Ar* interaction atdifferent MOT settings. We did not observe a clear decreasein the Ar* MOT signal due to the presence of the Rb MOT,although noise on the Ar* MOT fluorescence~discussedabove! prevented us from drawing any conclusions about thereciprocity of trap loss in the dual MOT at this time.We are presently improving the stability of our Ti:sap-phire laser system in order to allow for a detailed study of theinteraction of the Rb and Ar* MOTs. We wish to fully char-acterize the collision dynamics of the system and preciselymeasure the interspecies collisional loss rates. We will alsobegin, shortly, to search for bound-state structure in the ex-cited states directly above the Rb 5S1/2 1Ar* 4s@3/2#2 level.In summary, we have realized the first dual-speciesmagneto-optical trap that confines alkali-metal~Rb! andmetastable noble-gas (Ar* ) atoms. The two species fromdifferent groups of the periodic table have been simulta-neously trapped at ultracold temperatures. We observe;53106 85Rb atoms and;23106 40Ar* atoms in a cloud withan approximate diameter of 500mm. Realization of the Rb-Ar* dual-species trap opens the way for detailed studies ofPenning and associative ionization, photoassociative spec-troscopy, and eventually for the production of ultracold RbArmolecules.We thank M.D. Havey for useful discussions. This workwas supported by Old Dominion University and the ThomasF. Jeffress and Kate Miller Jeffress Memorial Trust.@1# H.L. Bethlem, G. Berden, A.J.A. van Roij, F.M.H. Crompvo-ets, and G. Meijer, Phys. Rev. Lett.84, 5744~2000!.@2# J.D. Weinstein, R. deCarvalho, T. Guillet, B. Friedrich, andJ.M. Doyle, Nature~London! 395, 148 ~1998!.@3# N. Vanhaecke, W. de Souza Melo, B.L. Tolra, D. Comparat,and P. Pillet, Phys. Rev. Lett.89, 063001~2002!; C. Gabba-nini, A. Fioretti, A. Lucchesini, S. Gozzini, and M. 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Simultaneous Trapping of Rubidium and Metastable Argon in a Magneto-Optical Trap

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Simultaneous Trapping of Rubidium and Metastable Argon in a Magneto-Optical Trap

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