Ultracold Li 2 molecules

Transcription

1 Volterra 9/27/2003 Ultracold Li 2 molecules Cheng Chin Selim Jochim Markus Bartenstein Alexander Altmeyer Gerhard Hendl Steffan Riedt Johannes Hecker Denschlag Prof. Rudolf Grimm Institut für Experimentalphysik, Universität Innsbruck, Austria

2 Outline 1. Forming and trapping ultracold molecules: Create molecules by three-body recombination. 2. Experiment setup: 3. Results: Atom molecule and molecule atom conversion Molecule-molecule scattering Measurement of molecular wavefunctions References: Exp.: S. Jochim et al., cond-mat/ Theo.: C. Chin and Rudolf Grimm, cond-mat/

3 2003, the year of Ultracold Molecules A brief review Date Mol. Mol. # Source Efficiency Reference 01/ Cs thermal gas 2% Chu, Stanford 07/ K Fermi deg. gas 50% Jin, NIST 08/ Cs BEC 12% Grimm, Innsbruck 07/2003* 87 Rb BEC 7% Rempe, MPI 08/ Li Fermi deg. gas 50% Hulet, Rice 08/2003* 6 Li Fermi deg. gas 85% Salomon, ENS 08/2003* 6 Li thermal gas 55% Grimm, Innsbruck Questions!! 09/2003* 23 Na 2 BEC ~ 5% Ketterle, MIT Why are the efficiencies higher for fermions than for bosons? *not yet unpublished What is the atom-molecule conversion mechanism? What do we know about these long-range molecules?

4 Boson or Fermion? We tried both! Boson: 133 Cesium Fermion: 6 Lithium molecules unknown molecules Starting from a pure atom sample Burst of atoms* atoms atoms Experiment: Cs 2 J. Herbig, et. al., Science (301) 1510 Li 2 S. Jochim et. al., cond-mat/ Theory: *Cs 2 P. Julienne, private communication Li 2 C. Chin and R. Grimm, c-m/

5 Experimental setup Resonantor set up at Brewster s angle 130X enhancement of a 2W YAG laser : r /2 =2.3kHz, a /2 =1.7 MHz Deep dipole trap efficiently collects ~ 10 7 cold 6 Li atoms Mosk et al. Opt. Lett 26, (2001)

6 How do we form 6 Li 2 molecules? 50% - 50% mixture of 6 Li atoms in the lowest two ground states Optimal formation of molecules by 3-body recombination at 690G (Feshbach resonance occurs at 850G) binding energy/ h [MHz] Feshbach creation resonance G Energy [GHz] 850 G m f1 =1/2;m f2 =-1/2> continuum ν=36,l=0,s=0,f=2,m F =0> magnetic field [G] magnetic field [G] dissociation ν=36,l=0,s=0;f=0,m F =0> 1200G L=0,S=1,I=1;F=2,m F =0> L=0,S=1,I=1;F=1,m F =0> L=0,S=1,I=1;F=0,m F =0> Feshbach resonance magnetic field [G] scattering length [a 0 ]

7 Thermalization of atoms and molecules After forced evaporation : N=2.5X10 6, T=2.5 K Quickly switch the magnetic field to 690G and wait... Detection method atom number 2x10 6 1x N+2M N+2M (total particle #) : Ramp the field to 1200G to dissociate the molecules N 2M time [s] N(atom#): Do not ramp the field 2M (2x molecule #): Deduced from N+2M and N

8 Preparation of a pure molecular sample Stern-Gerlach separation: At lower fields, at > mol. Magnetic field gradient pushes the atoms out of the trap atom number 1.5x x x N+2M trap depth: 19µK B-field: 568G N magnetic field gradient [G/cm]

9 Endothermic dissociation of molecules. Starting from a pure sample of molecules particle atom number 4x10 5 4x10 5 3x10 5 2x10 5 1x10 5 N+2M N+2M N N Feshbach 2M 2Mresonance magnetic field [G] magnetic field (G)

10 What are the conversion mechanisms? In a pure atom sample: Li+Li +Li Li 2 +Li In a pure molecule sample: Li 2 +Li 2 Li 2 +Li+Li Atomic states Molecular state Molecule formation E Molecule dissociation atom/molecule fraction phase-space density 1.0 atom fraction molecule p.s.d. molecule fraction 15 atom p.s.d. T 15 molecule p.s.d. T binding energy E/k [µk] temperature T [µk]

11 Molecular collisions and mol.- at. conversion Starting from pure molecules, we observe Molecule collision loss < 3X10-13 cm 3 /s at B=690G Molecule collision loss of 5X10-11 cm 3 /s at B=546G atom number (10 4 ) 30 2N mol (690G) Possible explanation At large scattering length a r 0, size of the molecule is 10 N (690G) Can we measure the size at of the molecule? d ~ a The inelastic collision rate scales as 3 2N (546G) mol ~ (r 0 /d) 3 ~ a -3 Petrov, et al., cond-mat/ time (s)

12 Microwave spectroscopy of cold molecules Experiment: Prepare an atom-molecule mixture. Drive microwave transition to an upper hyperfine state. Theory: (molecule size r a) (r)~r - 1 ē r/ r (p)~(1+p 2 / 2 r 2 ) - 1 (p) 2 scales the transition rate to continuum with K.E.= p 2 /2m. transition x rate [a 0 ] (a.u.) a> c> a> b> 0.0 m> atom fractional loss fractional loss G microwave: GHz 0.0 E ~ h 2 /m r 2 Experiment microwave r 3 molecule ~ E -3/2 Theory r -2 E b Magnetic E b ~ rfield -2 Magnetic E/Efield b [G] Molecule size r p ~ h/ r Magnetic field B-643G [G] 620G 0.2 microwave: GHz Magnetic field B-620G [G]

13 Conclusion and outlook Creation of cold molecules with an efficiency > 50% Thermal equilibrium of atoms and molecules is reached.. Create molecules in different internal state? Observation of inelastic collisions among molecules Molecules are stable near Feshbach resonance! Li 2 -Li 2 Feshbach resonance or photoassociation of Li 4. Evaporative cooling of molecules? Toward quantum degeneracy and atomic BCS transition.