1. A strontium detective story James Millen Strontium detective – Group meeting 19/10/09 Ions‽

2. Strontium detective – Group meeting 19/10/09 The scene Create a MOT Measure MOT population/density Turn off trap Do Rydberg excitation Detect ions Measure MOT again Step Rydberg laser frequency

3. Strontium detective – Group meeting 19/10/09 Rydberg excitation 460.7nm ΩpΩp ΩcΩc ~420nm  p = 2πx32MHz  c = 2πx0.3MHz for D state [1] 5s 2 1 S 0 5s5p 1 P 1 5s18d 1 D 2 or 5s18s 1 S 0

4. Integrated ion signal at each coupling laser frequency Fraction of atoms lost during Rydberg excitation Strontium detective – Group meeting 19/10/09 The crime “Ion signal” or “Ion signal height” “Loss fraction” or “Resonant loss fraction”

5. Strontium detective – Group meeting 19/10/09 Clues Coupling laser power

6. Strontium detective – Group meeting 19/10/09 Clues Probe laser power (D state only)

7. Strontium detective – Group meeting 19/10/09 Clues - timing Time evolution of the ion signal Δ t = 0 Δ t = 4μs 4μs probe and coupling pulse 4μs 3.6Vcm -1 E field pulse

8. Strontium detective – Group meeting 19/10/09 The suspects Electric field ionization [1] For 5s18d state require ~5.4kVcm -1 With our field of ~4Vcm -1 should only be able to field ionize n≈97 and above

9. Strontium detective – Group meeting 19/10/09 The suspects Blackbody ionization Gallagher (the book!) gives a rough estimate: This yields τ bb = 5μs for n = 18 and T = 900K. More recent work by some Russians [1] yields a lower rate. The lifetime of the 5s18d 1 D 2 state is 640ns [2]

10. Strontium detective – Group meeting 19/10/09 The suspects Auto-ionization (two electron excitation) [3] 5s 2 5s5p 5s18s s s p s s 5p 1/2 18s s p Δ 5s18s state is closest to the auto-ionizing resonance… over 300GHz away! However, the width of the resonance is 200GHz! So only 1.6 linewidths away

11. Strontium detective – Group meeting 19/10/09 The suspects Hot atoms Collisions with hot Rydberg atoms could cause ionization [4] The Zeeman slowed atoms have been ruled out Could a small fraction of our hot Sr atoms get excited to the Rydberg state? How do we test this?

12. Strontium detective – Group meeting 19/10/09 The suspects Collisions Rydberg atoms collide (with resonant dipole- dipole interactions) This leads to ionization, plasma formation, avalanche effect. It has also been observed to be extremely fast (sub μs) [5] [6] Tentative calculations suggest we’re exciting ~40% of our sample to the Rydberg state

13. Strontium detective – Group meeting 19/10/09 The Culprit? Electric field: Unlikely, unless population is being transferred to higher n states Black body: Looks like it’s too slow, though it can redistribute population… Auto-ionization: need to look in more detail. Is scaling with power as you’d expect? Hot atoms: pass Collisions: not seen a signature of plasma formation yet… needs more work

14. Strontium detective – Group meeting 19/10/09 References [1] “Ionization of Rydberg atoms by blackbody radiation” N. N. Bezuglov et. al. arXiv:0807.2535 (2009) [2] “Natural radiative lifetimes in the interacting 1,3 D 2 sequences in Sr” S. Svanberg et. al. Phys. Rev. A 27 947 (1983) [3] ”Sr 5p 1/2 ns 1/2 and 5p 3/2 ns 1/2 J=1 autoionizing states” T. F. Gallagher et. al. Phys. Rev. A 33 2401 (1986) [4] “Spontaneous evolution of Rydberg atoms into an ultracold plasma” P. Pillet et. al. Phys. Rev. Lett. 85 4466 (2000) [5] “Ionization due to the interaction between two Rydberg atoms” Robicheaux J. Phys. B: At. Mol. Opt. Phys. 38 S333 (2005) [6] “Autoionization of an ultracold Rydberg gas through resonant dipole coupling” M. Weidemüller et. al. arXiv:0903.3147 (2009)