The story unfolds…

James Millen

The story unfolds… – Group meeting 12/04/10

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.

Our experiment

The story unfolds… – Group meeting 12/04/10

5s 5s5p

5snl nl 5p

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

Method

Create aMOT

Measure MOTpopulation/densi

ty

Turn off trap

Do Rydbergexcitation

Pulse 408nm & detect ions

Measure MOTagain

Step Rydberglaser frequency

The story unfolds… – Group meeting 12/04/10

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 ~ 103-104 ions

Lifetime measurements – low n

The story unfolds… – Group meeting 12/04/10

19D

20S

19D: τ1 = (0.7±0.1) μsτ2 = (2.3±1.4) μs

20S: τ1 = (1.9±0.3) μsτ2 = (5.3±1.0) μs

• Can see a longer lived tail

• Attribute to population ending up in high-L Rydberg states [1]

Aside- Ultra-cold plasma formation

The story unfolds… – Group meeting 12/04/10

Gas of coldRydberg 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]

Lifetime measurements – high n

The story unfolds… – Group meeting 12/04/10

5mW10m

W

15mW

• Clear multi-lifetime behaviour

• Power dependant lifetime (threshold?) [4]

• Very long lived ion tail

5mW: τ1 = (19±2) μs

10mW: τ1 = (37±4) μs τ2 = (160±30) μs

15mW: τ1 = (29±2) μs τ2 = (220±20) μs

Autoionization spectra- low n

The story unfolds… – Group meeting 12/04/10

• Taken by varying the frequency of the 408nm laser

• Very wide!

19D

20SQuantum defect

analysis: [5]

• If νryd= νauto you get zero signal on resonance

• If νryd = νauto± ½ then you get a symmetrically split peak

• If νryd< νauto then you get a peak to the red

ν5s19D = 2.22, ν5p19D = 2.78 → νryd = νauto- 0.56

ν5s20s = 3.27, ν5p20s = 3.38 → νryd < νauto

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]

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)