Progress Towards Formation and Spectroscopy of Ultracold Ground-state Rb 2 Molecules in an Optical Trap H.K. Pechkis, M. Bellos, J. RayMajumder, R. Carollo, E.E. Eyler, P.L. Gould, and W.C. Stwalley Physics Department, University of Connecticut Supported by the National Science Foundation

1.Photoassociative formation of ultracold molecules. 2.Efficient production of high-v Rb 2 molecules. 3.Optical trapping of Rb and ground-state Rb 2. 4.Future plans: low-v levels, dipolar KRb molecules. 5.Recent progress elsewhere. Outline

Optically trapped ultracold molecules Applications: High precision spectroscopy Molecular quantum degeneracy Ultracold chemistry: collisions and reactions: Vibrational quenching: Rb 2 ( v )  Rb 2 ( v– 1), etc. Eventually, true reactions. For heteronuclear/polar molecules: Exchange reactions: KRb + K  K 2 + Rb. Dipole interactions, novel phases. Quantum computing.

Making cold molecules hot molecules cold molecules hot molecules cold molecules cold atoms Very cold molecules Laser cooling Buffer gas cooling Electrostatic slowing Mechanical slowing Photoassociation Magnetoassociation (Feshbach resonances) ?

Formation and detection of high-v ground-state Rb 2 Steps: 1) PA to form ultracold Rb 2 *. 2) Radiative stabilization into the singlet ground state, X 1 Σ g +. 3) Detection using REMPI through the 2 1 Σ u + state. PA 597

Experimental Scheme

X 1 Σ g + ground-state vibrational levels agree well with calculations using potentials from Seto, et al.* But Franck-Condon factors for decay do not— they peak near v"=119. Levels with v″>117 can be photodissociated by the PA laser, but why are so strong? Y. Huang et al., J. Phys. B: At. Mol. Opt. Phys. 39, S857-S869. *J. Y. Seto, R.J. Le Roy, J. Verges, and C. Amiot, J. Chem. Phys. 113, 3067 (2000). Detection Spectra for singlet Rb 2 : 2 1  u + (v') ← X (v")

Resonantly coupled vibrational wave functions can enhance molecule formation Resonant E bind = 9.39 cm -1 Non-resonant E bind = cm -1 Singlet character: blue Triplet character: red Full description: H. K. Pechkis, et al., Phys. Rev. A 76, (2007).

Configuration for PA in an optical trap 10.6  m CO 2 laser wavelength is far from most resonances (“QUEST” = Quasi-electrostatic trap). Near-harmonic trap relies on dc polarizabilities. Configuration allows a 1-D optical lattice with minimal effort. 70  m waist gives a trap depth of ~400  K (atoms) and 700  K (molecules). L 2 L 1 W 1 W 2 L 3 CO 2, 10.6  m, W PA, ~795 nm

Loading Rb atoms from a MOT Compression phase increases MOT density, lowers temperature. Absorption imaging allows size and density determination for cold, dense Rb atom samples. 5S 1/2 5P 3/ Trap Repump Image of optical trap, 49  m width, with 4 x 10 6 atoms at density n = 6 x cm -3. Typical loading efficiency: 10-15%

Effects of compression/molasses phase Without compression With compression n~4.8x10 11 cm -3 n~1.23x10 12 cm -3 MOT Temperature Tem perat ure: 131  K Temperature: 54  K QUEST Density Time (  s)

Absorption imaging of dense cold atoms n~4.8x10 11 cm -3 n n~1.23x10 12 cm -3 MOT (at right): n = 1.2 x QUEST (below) 30 ms after turning off MOT: N = 2 x 10 6 n = 5 x 10 11

Sequence for PA in the optical dipole trap PA rates are enhanced by high density, low T. May need to turn off CO 2 laser to avoid ac Stark shifts. Collision rates can be measured by varying detection pulse timing:

Initial results for PA in the QUEST QUEST, PA laser on resonance N = 1.4x10 6, n = 1.7x10 11 cm -3 N = 9.5x10 5, n = 7.6x10 10 cm -3 This is steady-state PA, with optical trap always on. Atom number reduced by 35%, peak density by 55%. PA spectrum (MOT trap loss) QUEST, PA off-resonant

Coming Next 1.Measure collisional vibrational quenching rates with fully state- resolved detection. Compare optical trap loss rates for atoms (at right), molecules, and molecules + atoms. 2.Production of low- v Rb 2 by Raman transfer. This scheme efficiently exploits singlet-triplet mixing but requires two “dump” lasers A similar scheme using spon- taneous decay can populate v =0 with 3.5% efficiency N (10 6 ) Time (s)   900 ms

Recent results on Cs 2 from Orsay, Freiburg n~4.8x10 11 cm -3 n n~1.23x10 12 cm -3 P. Staanum, S.D. Kraft, J. Lange, R. Wester, and M. Weidemuller, Phys. Rev. Lett. 96, (2006). Freiburg: For the a 3  u + state, both at high v (32-47) and low v (4-6), Both groups used partially state-selective detection of molecules to measure vibrational collisional losses in a CO 2 laser QUEST. Orsay: For X 1  g +, v ~ 103, F Cs =3, Similar rates for low-v (7-9) and for a 3  u + (v = 33-47). Cs, F=3 Cs, F=4 Cs 2 Cs 2 + Cs, F=3 N. Zahzam, T. Vogt, M. Mudrich, D. Comparat, and P. Pillet, Phys. Rev. Lett. 96, (2006).

New CsRb results from Yale (DeMille group) n~4.8x10 11 cm -3 n n~1.23x10 12 cm -3 From Eric R. Hudson, Nathan B. Gilfoy, S. Kotochigova, Jeremy M. Sage, and D. DeMille, Phys. Rev. Lett. 100, (2008). High- v molecules in the state are produced by PA in a QUEST. Vibrational quenching is determined from trap loss rates.

Coming Soon: optically trapped KRb See the next talk for some impressive new KRb results from JILA! Raman transfer (L2, L3) can produce low-v ground-state KRb, starting from high-v levels of the X or a states. Several schemes are possible—can be evaluated based on transition moments, convenience of wavelengths, etc. We have already extensively studied PA and the spectra of high-v ground-state molecules formed in a MOT.

Conclusions In a MOT, Rb 2 is efficiently produced in high-v levels of the X 1  g + state via PA and radiative decay. State-selective detection is achieved. Resonant coupling of the 0 u + states can greatly enhance X state formation in high- v levels. A CO 2 laser optical dipole trap has been loaded Rb atoms with n~10 12 cm -3 and T ~ 50  K. Preliminary experiments demonstrate efficient PA in the optical trap. Next: Detect X 1  g + molecules in the optical trap, measure vibrational quenching rates. Results elsewhere for Cs 2 and RbCs are somewhat puzzling. Soon: Dipolar KRb molecules; Raman transfer to low-v levels.