1. The story unfolds… James Millen The story unfolds… – Group meeting 12/04/10

2. Summary The story unfolds… – Group meeting 12/04/10 Aim: to study Rydberg interactions in a cold gas, to investigate ultra-cold plasma formation. Previously: taken lots of data. Didn’t really understand it! Now: taken lots of data. Understand a non-negligible amount of it.

3. Our experiment The story unfolds… – Group meeting 12/04/10 5s 5p 5s nlnl nlnl 5p

4. Why use autoionization? The story unfolds… – Group meeting 12/04/10 We can ionize up to 100% of the Rydberg atoms We get a very high yield State and time selective

5. Method Create a MOT Measure MOT population/density Turn off trap Do Rydberg excitation Pulse 408nm & detect ions Measure MOT again Step Rydberg laser frequency The story unfolds… – Group meeting 12/04/10

6. Numbers The story unfolds… – Group meeting 12/04/10 2-5% of the atoms are lost from the MOT, dependant on Rydberg state and Rydberg laser power This loss fraction is independent of the autoionizing pulse This suggests a negligible amount of Rydberg atoms return to the MOT We can estimate we make ~ 10 3 -10 4 ions

7. Lifetime measurements – low n The story unfolds… – Group meeting 12/04/10 19D 20S 19D: τ 1 = (0.7±0.1) μs τ 2 = (2.3±1.4) μs 20S: τ 1 = (1.9±0.3) μs τ 2 = (5.3±1.0) μs Can see a longer lived tail Attribute to population ending up in high-L Rydberg states [1]

8. Aside- Ultra-cold plasma formation The story unfolds… – Group meeting 12/04/10 Gas of cold Rydberg atoms Fast ionization, some electrons leave Positive charge binds electrons. Electrons oscillate, ionizing atoms [2] See a burst of ions. Ion-Rydberg collisions create high-L Rydbergs through Stark mixing [3]

9. Lifetime measurements – high n The story unfolds… – Group meeting 12/04/10 5mW 10mW 15mW Clear multi-lifetime behaviour Power dependant lifetime (threshold?) [4] Very long lived ion tail 5mW: τ 1 = (19±2) μs 10mW: τ 1 = (37±4) μs τ 2 = (160±30) μs 15mW: τ 1 = (29±2) μs τ 2 = (220±20) μs

10. Autoionization spectra- low n The story unfolds… – Group meeting 12/04/10 Taken by varying the frequency of the 408nm laser Very wide! 19D 20S Quantum defect analysis: [5] If ν ryd = ν auto you get zero signal on resonance If ν ryd = ν auto ± ½ then you get a symmetrically split peak If ν ryd < ν auto then you get a peak to the red ν 5s19D = 2.22, ν 5p19D = 2.78 → ν ryd = ν auto - 0.56 ν 5s20s = 3.27, ν 5p20s = 3.38 → ν ryd < ν auto

11. Autoionization spectra- high n The story unfolds… – Group meeting 12/04/10 100ns 10μs 70μs Took spectra at different times after Rydberg excitation Narrows over time High-L states have longer lifetimes, hence are narrower [6]

12. Conclusion The story unfolds… – Group meeting 12/04/10 We have a good handle on our results Taking more data to fill in some gaps Working towards calculating lifetimes (Guy) Writing a paper! [1] Raithel et. al. Phys. Rev. A 69(063405) (2004) [2] Pillet et. al. Phys. Rev. Lett. 85(21) (2000) [3] Gallagher et. al.Eur. Phys. J. D 40(1) (2006) [4] Tate et. al.Phys. Rev. A 70(042713) (2004) [5] Gallagher et. al.Phys. Rev. A 29(5) (1984) [6] Gallagher et. al.Phys. Rev. A 41(1) (1990)