Deceleration and trapping

1. Deceleration and trapping

2. Production Deceleration Trapping Cooling Electric / magnetic / light Sympathetic Cooling Supersonic Expansion Switched Fields 1K, ~600m/s 100 mK, 0m/s 100 mK, 0m/s <1mK, 0m/s A route to ultracold molecules

3. Supersonic expansion Thermal energy of gas converted into forward kinetic energy For details, see: “Atomic and molecular beam methods”, edited G.Scoles et. al, Oxford University Press (1988)

4. Seeding a supersonic expansion Ca Target Pulsed CaF beam Skimmer Pulsed Valve Pulsed YAG (10mJ, 10ns) 2% SF6 in 4 bar Ar • Direct seeding for molecules with high vapour pressure • Laser ablation • Electric discharge • Photo-dissociation

5. An electric field gradient exerts a force At high fields, the Stark shift becomes linear in E and meff is then a constant

6. Using the Stark shift to decelerate molecules

7. Stark deceleration

8. Longitudinal focussing in a Stark decelerator Switch off when molecule is here • Molecules that are ahead decelerate more • Molecules that are behind decelerate less • The packet of molecules is bunched Known as: Longitudinal focussing Bunching Phase-stability

9. Stark deceleration and phase-stability Zero deceleration – maximum phase-space acceptance Maximum deceleration – zero phase-space acceptance

10. Strong-field seekers Weak-field seekers , but Transverse focussing The beam blows up in the transverse direction The beam is focussed in the transverse direction Use a weak-field seeking state if possible (good for OH, NH3, CO, LiH…)

11. Decelerator for weak-field-seekers

12. A deceleration experiment Detector Decelerator Skimmer Supersonic source

13. Examples – decelerating LiH (Imperial) and OH (Berlin) LiH OH

14. Trapping weak-field seekers

15. Example – trapping NH3 (Berlin) Nature 406, 491 (2000)

16. Strong-field-seekers can be trapped too Results from Berlin group

17. A storage ring for molecules Nature 411, 174 (2001)

18. An even better storage ring for molecules Results from Berlin group – storing ammonia in a ring

19. Decelerating and trapping on a chip Results from Berlin group – deceleration and trapping of metastable CO on a chip S.A. Meek, H. Conrad and G. Meijer, Science 324, 1699 (2009)

20. Decelerating using Rydberg states... Phys. Rev. Lett. 103, 123001 (2009)

21. Decelerating molecular oxygen Phys. Rev. A 77, 051401 (R) (2008) Decelerating using pulsed magnetic fields…

22. Decelerating using light fields Idea : Conceptually similar to electrostatic deceleration – use light forces instead To avoid spontaneous emission, light is far-off-resonance Need high intensity pulsed lasers Counter-propagating pulsed laser beams – form an optical lattice Introduce small frequency difference between the two beams – the lattice moves Molecules can ‘surf’ the waves of the lattice – either accelerated or decelerated Chirp the frequency difference to keep the lattice speed the same as the molecule speed

23. Optical Stark decelerator – results from UCL Benzene Nature Physics 2, 465 (2006) J. Phys. B 39, S1097 (2006) Phys. Chem. Chem. Phys. 8, 2985 (2006)

24. Deceleration - Advantages and limitations • Widely applicable (though considerable development still needed) • Results in cold POLAR molecules – good for most experiments • Molecules are in the ground state – good for most experiments • Need a favourable dipole-moment to mass ratio • Molecules are cold, but not ultracold (second stage cooling schemes…)

25. Sympathetic cooling Trapped atoms, T = 10 – 100 mK Trapped molecules, T = 10-100mK What happens in the soup? Need to know the ratio of elastic to inelastic rates Elastic collisions – Molecules reach ultracold temperatures Inelastic collisions – Trap loss, quantum chemistry