Formation of ultracold metastable RbCs molecules by short-range photoassociation
aa r X i v : . [ phy s i c s . a t o m - ph ] J u l Formation of ultracold metastable RbCs molecules by short-rangephotoassociation
C.Gabbanini ∗ a and O.Dulieu b Received Xth XXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XXFirst published on the web Xth XXXXXXXXXX 200X
DOI: 10.1039/b000000x
Ultracold metastable RbCs molecules are observed in a double species MOT through photoassociation near theRb(5S / )+Cs(6P / ) dissociation limit followed by radiative stabilization. The molecules are formed in their lowest tripletelectronic state and are detected by resonant enhanced two-photon ionization through the previously unobserved ( ) P ← a S + band. The large rotational structure of the observed photoassociation lines is assigned to the lowest vibrational levels of the0 + , − excited states correlated to the Rb(5P / )+Cs(6S / ) dissociation limit. This demonstrates the possibility to induce directphotoassociation in heteronuclear alkali-metal molecules at short internuclear distance, as pointed out in [J. Deiglmayr et al. ,Phys. Rev. Lett. , 13304 (2008)]. The field of cold molecules received a large attention inthe last decade due to important advances and potentiallynew applications in several domains like fundamental tests inphysics, molecular clocks, molecular spectroscopy, dynam-ics of cold reactions, controlled photochemistry studies andquantum computation . In particular cold polar molecules(i.e. exhibiting a permanent electric dipole moment) thanks tolong range dipolar interactions have been proposed for quan-tum information . The large anisotropic interaction betweencold polar molecules is expected to give rise to quantum mag-netism and to novel quantum phases . Polar molecules canbe manipulated by external electric fields, allowing for thecontrol of elementary chemical reactions at very low temper-atures .The techniques that have produced up to now molecules inthe ultracold temperature range are magnetoassociation andphotoassociation (PA). PA of laser-cooled atoms followed bystabilization via spontaneous emission has been very suc-cessful for many homonuclear and heteronuclear alkali-metalmolecules . The molecules are typically formed in high vi-brational levels of the ground singlet or triplet state. The pop-ulation can be transferred to lower vibrational levels, eventu-ally to the v = or coherent optical pro-cesses. In particular the stimulated Raman adiabatic passage(STIRAP) process has demonstrated to be a powerful tool forquantum degenerate gases starting from molecules producedby magnetoassociation , and recently also for ultracoldmolecules created by PA in a magneto-optical trap . Another possible approach is to perform vibrational cooling by a prop-erly shaped laser . For both methods a good spectroscopicknowledge of the molecule under study and a favourable rovi-brational population distribution are crucial, therefore it is im-portant to investigate different formation paths.For what concerns ultracold RbCs molecules, their forma-tion in the ground a S + state has been reported by PA belowthe lowest excited asymptote Rb(5 S / )+Cs(6 P / ) and sub-sequent spontaneous emission . Rb and Cs atoms wereexcited into the 0 − state resulting from the coupled ( ) S + and b P states with the decay process producing RbCs in theground triplet state . The molecules were detected by stateselective REMPI through the coupled ( ) S + and B P states.Some RbCs molecules were transferred in a following experi-ment to the v = X S + ground state by an incoher-ent optical pumping process . The process was based on the c S + , B P and b P coupled states, allowing to circumvent thetriplet-singlet forbidden transition, using two lasers for pump-ing and dumping the molecules from the starting level ( v ′ =37level of the a S + state).In this paper a different formation path of tripletRbCs molecules is reported, that involves PA near theRb(5S / )+Cs(6P / ) dissociation limit followed by radiativestabilization. The produced molecules are detected by reso-nant enhanced two-photon ionization through the previouslyunobserved ( ) P ← a S + band. a Istituto Nazionale di Ottica, INO-CNR, U.O.S. Pisa ”Adriano Gozzini”, viaMoruzzi 1, 56124 Pisa, Italy Fax: 39-0503152522; Tel: 39-0503152529; E-mail: [email protected] b Laboratoire Aim´e Cotton, CNRS, Bˆat. 505, Univ Paris-Sud,F-91405 Orsay Cedex, France. ?? ||
Ultracold metastable RbCs molecules are observed in a double species MOT through photoassociation near theRb(5S / )+Cs(6P / ) dissociation limit followed by radiative stabilization. The molecules are formed in their lowest tripletelectronic state and are detected by resonant enhanced two-photon ionization through the previously unobserved ( ) P ← a S + band. The large rotational structure of the observed photoassociation lines is assigned to the lowest vibrational levels of the0 + , − excited states correlated to the Rb(5P / )+Cs(6S / ) dissociation limit. This demonstrates the possibility to induce directphotoassociation in heteronuclear alkali-metal molecules at short internuclear distance, as pointed out in [J. Deiglmayr et al. ,Phys. Rev. Lett. , 13304 (2008)]. The field of cold molecules received a large attention inthe last decade due to important advances and potentiallynew applications in several domains like fundamental tests inphysics, molecular clocks, molecular spectroscopy, dynam-ics of cold reactions, controlled photochemistry studies andquantum computation . In particular cold polar molecules(i.e. exhibiting a permanent electric dipole moment) thanks tolong range dipolar interactions have been proposed for quan-tum information . The large anisotropic interaction betweencold polar molecules is expected to give rise to quantum mag-netism and to novel quantum phases . Polar molecules canbe manipulated by external electric fields, allowing for thecontrol of elementary chemical reactions at very low temper-atures .The techniques that have produced up to now molecules inthe ultracold temperature range are magnetoassociation andphotoassociation (PA). PA of laser-cooled atoms followed bystabilization via spontaneous emission has been very suc-cessful for many homonuclear and heteronuclear alkali-metalmolecules . The molecules are typically formed in high vi-brational levels of the ground singlet or triplet state. The pop-ulation can be transferred to lower vibrational levels, eventu-ally to the v = or coherent optical pro-cesses. In particular the stimulated Raman adiabatic passage(STIRAP) process has demonstrated to be a powerful tool forquantum degenerate gases starting from molecules producedby magnetoassociation , and recently also for ultracoldmolecules created by PA in a magneto-optical trap . Another possible approach is to perform vibrational cooling by a prop-erly shaped laser . For both methods a good spectroscopicknowledge of the molecule under study and a favourable rovi-brational population distribution are crucial, therefore it is im-portant to investigate different formation paths.For what concerns ultracold RbCs molecules, their forma-tion in the ground a S + state has been reported by PA belowthe lowest excited asymptote Rb(5 S / )+Cs(6 P / ) and sub-sequent spontaneous emission . Rb and Cs atoms wereexcited into the 0 − state resulting from the coupled ( ) S + and b P states with the decay process producing RbCs in theground triplet state . The molecules were detected by stateselective REMPI through the coupled ( ) S + and B P states.Some RbCs molecules were transferred in a following experi-ment to the v = X S + ground state by an incoher-ent optical pumping process . The process was based on the c S + , B P and b P coupled states, allowing to circumvent thetriplet-singlet forbidden transition, using two lasers for pump-ing and dumping the molecules from the starting level ( v ′ =37level of the a S + state).In this paper a different formation path of tripletRbCs molecules is reported, that involves PA near theRb(5S / )+Cs(6P / ) dissociation limit followed by radiativestabilization. The produced molecules are detected by reso-nant enhanced two-photon ionization through the previouslyunobserved ( ) P ← a S + band. a Istituto Nazionale di Ottica, INO-CNR, U.O.S. Pisa ”Adriano Gozzini”, viaMoruzzi 1, 56124 Pisa, Italy Fax: 39-0503152522; Tel: 39-0503152529; E-mail: [email protected] b Laboratoire Aim´e Cotton, CNRS, Bˆat. 505, Univ Paris-Sud,F-91405 Orsay Cedex, France. ?? || Experiment
The experiment is done in a double species magneto-opticaltrap (MOT) loaded from vapor and produced inside a UHVmetal chamber. The rubidium vapor pressure is establishedby running current through a metal dispenser while for Cs ametal reservoir is used, separated from the main chamber bya valve. The cooling laser for cesium is a DFB diode laser(150 mW power), with frequency tuned two linewidths belowthe 6 S / ( F = ) → P / ( F = ) transition. For Rb thecooling laser is a DFB diode laser (80 mW power), frequencytuned two linewidths below the 5 S / ( F = ) → P / ( F = ) transition. Two other diode lasers tuned on transitions fromthe other ground hyperfine levels of the two atoms act as re-pumpers. The repumper beams pass through acousto-opticmodulators and the first diffracted order beams are used forthe MOT. In this way the repumper beams can be almost ex-tinguished, with the remaining intensity due to the extintionratio of the AOM. All lasers are frequency-locked in separatedcells through saturated absorption spectroscopy.The double MOT has separated horizontal arms withretroreflected beams for the two species, while it has super-posed beams along the vertical direction, that is also the axis ofthe quadrupole magnetic field. This configuration allows forindependent control of the alignment and therefore for maxi-mizing the overlap of the two cooled samples. The overlap ismonitored by two CCD cameras along orthogonal axis. Thedouble species MOT captures a similar number of Cs and Rbatoms (nearly 10 ) with densities of a few 10 cm − .The PA step is performed by a tapered amplifier that is in-jected by a dedicated DFB diode laser. A light power of 0.7W is available to be focused on the trapped sample. Thanksto its large bandwidth and to robust injection, the tapered am-plifier remains injected for all the tuning range of the DFBdiode laser, that by changing its temperature exceeds thirtywavenumbers. The absolute PA laser frequency is measuredby a wavemeter, while a Fabry-P´erot interferometer monitorsthe frequency scan. The FP interferometer can be also used tofrequency lock the DFB laser.The molecules are resonantly ionized by a laser pulse, thatis given by a dye laser (Quantel TDL50) pumped by the sec-ond harmonics of a Nd:YAG laser (Continuum Surelite I-20,with 20 Hz repetition rate and 7 ns time width). The dye laseroperates with LDS 698 dye and covers the frequency rangefrom 13800 to 14800 cm − . The pulsed beam, with energy ofabout 1 mJ, is softly focused to the cold sample. The pulsedlaser is fired after that the repumping lasers of the double MOThave been almost extinguished for about 10 ms, making tem-poral dark SPOTs. This has the double advantage of an ini-tial increase of the atomic densities and, even more impor-tant, a decrease of the collisional losses (both single speciesand inter-species ) that are particularly strong with atoms in the excited states. The time sequence of the experiment isshown in Fig.1a. After the laser pulse the produced atomicand molecular ions are repelled by a grid, separated by time-of-flight and detected by a microchannel plate. The ion signalsare recorded by gated integrators, and a typical example is dis-played in Fig.1b. -3 I ( a . u . ) -6 t (s) t (ms) PA and cooling lasers repump.laserspulsed laserONON OFF a)b)
Fig. 1 (a) Time sequence of the experiment. (b) an example of atime-of-flight record; the three peaks from left to right correspond toRb + , Cs + and RbCs + ion signals, respectively. The RbCs molecule formation has been investigated byscanning the PA laser over all its tuning range below theRb(5S / )+Cs(6P / ) dissociation limit (about 15 cm − ) andby changing wavelength region of the pulsed laser for detec-tion. As it can be deduced from Fig.2 and as discussed by Ker-man et al , the PA process below the Rb(5S / )+Cs(6P / )dissociation limit has the drawback to induce predissocia-tion to states correlated to the lowest excited asymptote (i.e.Rb(5S / )+Cs(6P / )), however this does not preclude to findspecific paths that efficiently produce (meta)stable moleculesby radiative stabilization. In fact RbCs molecule formationhas been observed only when the PA laser is tuned -8.1 cm − below the asymptote and with a REMPI laser wavelengthof about 707 nm. The time-of-flight spectrum of Fig.1b isrecorded under such conditions.No other PA lines have been observed over all the scannedregion. In particular no line with small B v which could belongto the Rb(5S / )+Cs(6P / ) asymptote has been observed, in | ?? E n e r gy ( c m - ) (3) P (4) S + Rb(5s)+Cs(6s) X S + (0 + ) a S + (0 - ,1) P A (3) S + (2) P (2) P + - Rb(5s)+Cs(5d)Rb(5s)+Cs(6p )Rb(5s)+Cs(6p )Rb(5p )+Cs(6s)Rb(5p )+Cs(6s) ( )( )( ) Fig. 2
Relevant potential curves for the RbCs formation processand detection discussed here. Note that states with total electronicangular momentum projection W = c symmetry notation.Some curves correlated to Rb(5 P / , / )+Cs(6 S ) are also labelledwith their Hund’s case a symmetry. The upward arrow suggests theprobable path for the photoassociation (PA), and numbereddownward arrows possible paths for spontaneous emission towardstable RbCs molecules. accordance with the hypothesis of a strong predissociation.The PA line has a simple rotational structure that is plottedin Fig.3. The three peaks are due to the J = , , − . The J = + ionswith the PA laser locked at 852.946 nm ( J = I ( a . u . ) D E (cm -1 ) Fig. 3
Rotational structure of the PA line at -8.1 cm − . The threepeaks correspond to J=0,1,2 rotational levels of the excitedmolecular state. I ( a . u . ) D E (MHz)
Fig. 4
Stark effect on the J=1 line: the line is recorded for twodifferent values of static electric field E el with E redel > E blueel . in Fig.5. The spectrum extends in the 700-715 nm wave-length range and has a complex structure. The observationsare consistent with the explanation that triplet ground statemolecules are produced and subsequently ionized by REMPIthrough the previously unobserved ( ) P ← a S + band. The ( ) P state has a local minimum and it is correlated with theRb(5 S )+Cs(5 D ) dissociation limit .As it can be deduced from Fig.2, the bound states of the a S + state oscillate in the region r ≥ a , while in order tomatch the RbCs + potential curve, the ionization should occurat r ≤ a . The Franck-Condon factors for the ( ) P ← a S + band have been calculated to be particularly favorable . An-other weaker detection region has been observed in the 683-693 nm wavelength range (Fig.6). It is tentatively assigned tothe ( ) S + ← a S + band. As shown in Fig.2, the ( ) S + isalso a double well state correlated with the Rb(5 S )+Cs(5 D )dissociation limit, but its local minimum is above that limit,implying that the supported bound states have finite lifetimes.The interpretation of the molecular bands, that will be the sub-ject of a future study, will allow to determine the vibrational ?? ||
Stark effect on the J=1 line: the line is recorded for twodifferent values of static electric field E el with E redel > E blueel . in Fig.5. The spectrum extends in the 700-715 nm wave-length range and has a complex structure. The observationsare consistent with the explanation that triplet ground statemolecules are produced and subsequently ionized by REMPIthrough the previously unobserved ( ) P ← a S + band. The ( ) P state has a local minimum and it is correlated with theRb(5 S )+Cs(5 D ) dissociation limit .As it can be deduced from Fig.2, the bound states of the a S + state oscillate in the region r ≥ a , while in order tomatch the RbCs + potential curve, the ionization should occurat r ≤ a . The Franck-Condon factors for the ( ) P ← a S + band have been calculated to be particularly favorable . An-other weaker detection region has been observed in the 683-693 nm wavelength range (Fig.6). It is tentatively assigned tothe ( ) S + ← a S + band. As shown in Fig.2, the ( ) S + isalso a double well state correlated with the Rb(5 S )+Cs(5 D )dissociation limit, but its local minimum is above that limit,implying that the supported bound states have finite lifetimes.The interpretation of the molecular bands, that will be the sub-ject of a future study, will allow to determine the vibrational ?? || istribution of the produced RbCs molecules. Similar results,at different detuning of the PA laser, are obtained for RbCsmolecules, starting from a double species trap of Rb and Csatoms.A precise value for the molecular formation rate cannot begiven because the ionization and detection efficiencies are notknown. However the RbCs + detection rate is just a factortwo lower than the Cs + detection rate measured in the sameexperimental condition photoassociating cold cesium atomsthrough a giant line and detecting by REMPI through the ( ) P g ← a S + u band. If a similar ionization efficiency is as-sumed, the RbCs formation rate can be estimated to be above10 molecules/s. I ( a . u . ) h n L (cm -1 ) Fig. 5
Spectrum of RbCs + molecular ions recorded by scanning thepulsed dye laser in the 700-715 nm wavelength range. The PA laseris locked on the J = I ( a . u . ) h n L (cm -1 ) Fig. 6
Spectrum of RbCs + molecular ions recorded by scanning thepulsed dye laser in the 683-694 nm wavelength range. The PA laseris locked on the J = The most striking feature of the present observation is thelarge rotational constant of the detected line, and the absenceof neighboring lines. This strongly suggests that it corre-sponds to the direct population of a vibrational level lying at the bottom of a molecular potential well located in the energyrange of the Rb(5S / )+Cs(6P / ) dissociation limit. Fig.2 il-lustrates the probable process using the potential curves in-cluding molecular spin-orbit computed in Ref. . Indeed,several potential wells correlated to the Rb(5P / )+Cs(6S / )dissociation limit have their minimum located just belowRb(5S / )+Cs(6P / ): an isolated W = c notation with W being the projection of the total electronicangular momentum on the molecular axis) mostly induced bya ( ) P state, and a group of four states ( W = + , − , , ( ) P state. Inthe following, these states are numbered by increasing energy ( ) ( ) − , ( ) + , ( ) ( )
2, respectively.The observed structure starts with a J = ( ) − , ( ) + molecular states above.The measured rotational constant is in good agreement withthe ones computed in Ref. (0.0136 cm − and 0.0133 cm − ,respectively). The lower levels of these potentials can be pop-ulated by the PA laser in the vicinity of the inner turning pointof the lowest triplet a S + potential, as illustrated by the arrowmarked ”PA” in Fig.2. In contrast with what is generally re-ported (see for instance Ref. ), this result demonstrates thepossibility to achieve PA of a pair of different ultracold atomsat short distances. There is only one another observation ofthis kind reported up to now : the lowest levels of the ( ) P state correlated to the Li(2 S )+Cs(6 P ) dissociation limit ofthe LiCs molecule have been successfully populated by PAfrom the inner turning point of the a S + potential as well. Theapparent violation of the spin selection rule is explained by thesmall spin-orbit mixing of the ( ) P with triplet states , alsoexpected in RbCs.Once such excited levels are populated, they naturally de-cay by spontaneous emission down to a broad range of a S + vibrational levels as pictured by arrow (2) in Fig.2 startingfrom levels of the ( ) P manifold. As the spin-orbit is large inthe RbCs molecule, another possible path (arrow (1) in Fig.2)could be the decay from the ( ) P state (or the (4)1 state inHund’s case c ) as it is likely coupled to neighboring tripletstates just like in LiCs. The precise distribution of the popu-lated vibrational levels will be analyzed from the spectra re-produced in Figs.5 and 6. It is also likely that some of theexcited levels decay down to the lowest levels of the X S + ground state (see arrow (3) in Fig.2). This should be analyzedusing another detection scheme where excited singlet stateswould be reached by the first photon of the REMPI process. The formation of translationally and rotationally cold RbCsmolecules in a double species MOT is observed, and assigned | ?? o the short-range photoassociation of ultracold pair of atomsfollowed by radiative stabilization to the lowest metastablestate, a S + . They are detected by resonant enhanced two-photon ionization through two different paths including thepreviously unobserved ( ) P ← a S + band. Further experi-ments should explore the vibrational distribution of the pro-duced ultracold molecules, as well as the possibility to createground state molecules through other isolated PA lines whichare expected in the same energy range. In particular detun-ings below the -8 cm − region investigated here should beexplored, as several other levels are expected below the en-ergy of the Rb(5S / )+Cs(6P / ) limit. This would providethe precise location of the v = ( ) ( ) − , ( ) + , ( ) ( ) i.e. at positivedetunings of the PA laser above Rb(5S / )+Cs(6P / )) shouldbe possible as well, as recently observed by Bellos et al. inthe same issue of PCCP. Such simple formation path of RbCsmolecules can be useful as a first step for a further opticalmanipulation that modifies the internal degrees of freedom toproduce a sample of ultracold polar molecules. Acknowledgments
The author wishes to thank A.Fioretti for helpful discussions,D.Comparat for lending some experimental equipment andM.Voliani and F.Pardini for technical support.
References
Cold Molecules, Theory,Experiment, Applications , CRC Press, 2009.2 O. Dulieu and C. Gabbanini,
Rep.Progr.Phys. , 2009, , 086401.3 L. D. Carr, D. DeMille, R. V. Krems and J. Ye, New J.Phys. , 2009, ,055049.4 D. DeMille, Phys. Rev. Lett. , 2002, , 067901.5 P. Rabl and P. Zoller, Phys. Rev. A , 2007, , 042308.6 R. Barnett, D. Petrov, M. Lukin and E. Demler, Phys. Rev. Lett. , 2006, ,190401.7 M. A. Baranov, M. S. Mar’enko, V. S. Rychkov and G. V.Shlyapnikov, Phys. Rev. A , 2002, , 013606.8 R. V. Krems, Inter. Rev. in Phys. Chem. , 2005, , 24.9 J. M. Sage, S. Sainis, T. Bergeman and D. DeMille, Phys. Rev. Lett. , 2005, , 203001.10 F. Lang, K. Winkler, C. Strauss, R. Grimm and J. H. Denschlag, Phys.Rev. Lett. , 2008, , 133005.11 K.-K. Ni, S. Ospelkaus, M. H. G. de Miranda, A. Peer, B. Neyenhuis, J. J.Zirbel, S. Kotochigova, P. S. Julienne, D. S. Jin and J. Ye,
Science , 2008, , 231.12 J. G. Danzl, E. Haller, M. Gustavsson, M. J. Mark, R. Hart, N. Bouloufa,O. Dulieu, H. Ritsch and H.-C. N¨agerl,
Science , 2008, , 1062.13 K. Aikawa, D. Akamatsu, M. Hayashi, K. Oasa, J. Kobayashi, P. Naidon,T. Kishimoto, M. Ueda and S. Inouye,
Phys. Rev. Lett. , 2010, ,203001. 14 M. Viteau, A. Chotia, M. Allegrini, N. Bouloufa, O. Dulieu, D. Comparatand P. Pillet,
Science , 2008, , 232.15 A. J. Kerman, J. M. Sage, S. Sainis, T. Bergeman and D. DeMille,
Phys.Rev. Lett. , 2004, , 033004.16 A. J. Kerman, J. M. Sage, S. Sainis, T. Bergeman and D. DeMille, Phys.Rev. Lett. , 2004, , 153001.17 M. Harris, P. Tierney and S. Cornish, J. Phys. B: Atomic Molecular andOptical Physics , 2008, , 035303.18 H. Fahs, A. Allouche, M. Korek and M. Aubert-Frecon, J. Phys. B , 2002, , 1501.19 A.R.Allouche, M.Korek, K.Fakherddine, A.Chaalan, M.Dagher, F.Taherand M.Aubert-Fr´econ, J. Phys. B , 2000, , 2307.20 R. Beuc, M. Movre, B. Horvatic and G. Pichler, Chem. Phys. Lett. , 2007, , 236.21 A. Fioretti, D. Comparat, A. Crubellier, O. Dulieu, F.Masnou-Seeuws andP. Pillet,
Phys. Rev. Lett. , 1998, , 4402.22 N. Bouloufa, E. Favilla, M. Viteau, A. Chotia, A. Fioretti, C. Gabbanini,M. Allegrini, M. Aymar, D. Comparat, O. Dulieu and P. Pillet, Mol.Phys. ,2010, , 2355.23 K. M. Jones, E. Tiesinga, P. D. Lett and P. S. Julienne,
Rev. Mod. Phys. ,2006, , 483.24 J. Deiglmayr, A. Grochola, M. Repp, K. M¨ortlbauer, C. Gl¨uck, J. Lange,O. Dulieu, R. Wester and M. Weidem¨uller, Phys. Rev. Lett. , 2008, ,133004.25 J. Deiglmayr, P. Pellegrini, A. Grochola, M. Repp, R. Cˆot´e, O. Dulieu,R. Wester and M. Weidem¨uller,
New J. Phys. , 2009, , 055034. ?? ||