1. Making cold molecules from cold atoms

2. Photoassociation (AB)* Ground state A + B
Photoassociation is resonant in the photon energy
Usually achieved by adding an extra laser to a MOT – the PA laser
Very little heating involved – the molecules are ultracold
Molecules will be in low-lying rotational states
Produces molecules in highly excited vibrational states

3. Photoassociation – decay channels
Decay to free atoms
Decay to bound state
Ground state A + B
(AB)*

4. Photoassociation properties
Most efficient for high-lying vibrational states – tend to form “long-range molecules”
Extremely inefficient for tightly bound states
Decay to a deeply bound state of the molecular ground state is unlikely
Decay to two free atoms is accompanied by a large release of energy – trap loss
Li2, Na2, K2, Rb2, Cs2, Sr2 , Ca2 , LiCs, KRb, RbCs….

5. Detection methods
PRL 85, 2292 (2000)
Trap loss spectroscopy – the number of atoms in a MOT decreases when the molecules are formed. Example below: Ca2
Rev. Mod. Phys. 78, 483 (2006)
Ion detection. Ionize the molecules and count the ions produced.

6. Example: production of ultracold LiCs in the ground state
v=4
Photoassociation into v=4 (500mW, 946nm)
Spontaneous decay into ground state
Two-photon resonant ionization
PRL 101, (2008)
5000 ground state molecules per second

7. Photoassociation - advantages and limitations
Molecules are formed at ultracold temperatures
Possible to reach the ground state (though difficult)
Limited to constituents that can be laser-cooled
Production rate is usually very slow resulting in low phase-space densities

8. Magnetoassociation through a Feshbach resonance

9. Reminder about s-wave scattering
At ultracold temperatures, scattering is described by the s-wave scattering length, a
All other partial waves are suppressed by the angular momentum barrier
Toy example:
V=∞
V=V0
V=0
Zero-energy scattering state
Uppermost bound state
u(R)  exp(-R/a)
u(R)  (R – a)

10. Feshbach resonance

11. Reminder about level crossings

12. Atoms into molecules via a Feshbach resonance
B / B0
Bound molecules
Free atoms