A strontium detective story

James Millen

Strontium detective – Group meeting 19/10/09

Ions‽

Strontium detective – Group meeting 19/10/09

The scene

Create aMOT

Measure MOTpopulation/densi

ty

Turn off trap

Do Rydbergexcitation

Detect ionsMeasure

MOTagain

Step Rydberglaser frequency

Strontium detective – Group meeting 19/10/09

Rydberg excitation

460.7nm

Ωp

Ωc~420n

m

p= 2πx32MHz

c= 2πx0.3MHzfor D state [1]

5s2 1S0

5s5p 1P1

5s18d 1D2

or 5s18s 1S0

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”

Strontium detective – Group meeting 19/10/09

Clues

Coupling laser power

Strontium detective – Group meeting 19/10/09

Clues

Probe laser power (D state only)

Strontium detective – Group meeting 19/10/09

Clues - timing

Time evolution

of the ion signal

Δt = 0 Δt = 4μs

4μs probe andcoupling pulse4μs 3.6Vcm-1

E field pulse

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

Strontium detective – Group meeting 19/10/09

The suspects

Blackbody ionization

Gallagher (the book!) gives a rough estimate:

This yields τbb = 5μs for n = 18 and T = 900K. More recent work by some Russians [1] yields a lower rate.

The lifetime of the 5s18d 1D2 state is 640ns [2]

Strontium detective – Group meeting 19/10/09

The suspects

Auto-ionization (two electron excitation) [3]

5s2

5s5p

5s18s

s

sp

ss

5p1/2 18ss

pΔ 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

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?

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

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

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 1,3D2

sequences in Sr”S. Svanberg et. al. Phys. Rev. A 27 947 (1983)

[3] ”Sr 5p1/2ns1/2 and 5p3/2ns1/2 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)