Experiments with Stark-decelerated and trapped polar molecules

Steven Hoekstra

Molecular Physics Department ( Gerard Meijer)Fritz-Haber-Institutder Max-Planck-Gesellschaft

Berlin, Germany

Introduction & Motivation: What to do withStark-decelerated and trapped polar molecules

Using the long interaction time:

- High precision measurements / spectroscopy- Understanding trap loss mechanisms: Blackbody radiation- Measurement of the lifetime of metastable CO

Using the low energy:

- Collision experiments- Reflection of OH molecules by a magnetic mirror- Dipole-dipole interactions, ultracold collisions- Towards a higher phase space density: Magnetic trapping and accumulation of NH molecules

Introduction & Motivation: What to do withStark-decelerated and trapped polar molecules

Using the long interaction time:

- High precision measurements / spectroscopy- Understanding trap loss mechanisms: Blackbody radiation- Measurement of the lifetime of metastable CO

Using the low energy:

- Collision experiments Science 313, 1617 (2006)

- Reflection of OH molecules by a magnetic mirror NJP 10, 053018 (2008)

- Dipole-dipole interactions, ultracold collisions- Towards a higher phase space density: Magnetic trapping and accumulation of NH molecules

The effect of blackbody radiation on trapped polar molecules

Goal of the experiment:

Demonstrate the effect of BBR on neutral polar molecules by exploiting the difference between OH and OD

The Experimental setup

Controlling the velocity of OH moleculesusing the Stark decelerator

OH and ODhave similar stark shifts, can even be

decelerated and trapped together

The slow molecules are brought to a standstill at the center of an electrostatic trap,then the trap is switched on

7 kV -15 kV15 kV

-15 kV -15 kV15 kV

Room temperatureblackbody radiation

spectrum

All deceleratedmolecules areinitially in the J=3/2, f state.

Losses due to 1. background gas* 0.17 s-1

2. blackbody radiation OH: 0.36 s-1

OD: 0.14 s-1

*Joop J. Gilijamse, Steven Hoekstra, Sebastiaan Y. T. van de Meerakker, Gerrit C. Groenenboom, Gerard Meijer, Science,313,1617 (2006)

Conclusion:Optical pumping by blackbody radiation is a feature shared by all polar molecules and fundamentally limits the time that these molecules can be kept in a single quantum state in a trap.

Steven Hoekstra, Joop J. Gilijamse, Boris Sartakov, Nicolas Vanhaecke, Ludwig Scharfenberg, Sebastiaan Y. T. van de Meerakker, and Gerard Meijer, PRL98,133001 (2007)

The radiative lifetime of metastable CO (a3π, v=0)

Decelerate and trap CO (a3π), then directly measure the radiative lifetime by observing trap decay

P

Σ

τ~3 ms

Time-of-flight profile of the trapping of a decaying molecule:

a3P1, v=0, J=1 : 2.63 ± 0.03 ms a3P2, v=0, J=2 : 143 ± 4 ms

ratio of the lifetimes: 54.4 ± 1.6known ratio from spectroscopy: 54.66 ± 0.01

Conclusion: the radiative lifetime of metastable CO (a3π) has been measured with unprecedented precision by deceleration and trapping

Joop J. Gilijamse, Steven Hoekstra, Samuel A. Meek, Markus Metsala, Sebastiaan Y. T. van de Meerakker, Gerard Meijer and GerritC.Groenenboom, JCP127,221102 (2007)

Introduction and Motivation: What to do withStark-decelerated and trapped polar molecules

Using the long interaction time:

- High precision measurements / spectroscopy- Understanding trap loss mechanisms: Blackbody radiation- Measurement of the lifetime of metastable CO JCP127,221102 (2007)

Using the low energy:

- Collision experiments Science 313, 1617 (2006)

- Reflection of OH molecules by a magnetic mirror NJP 10, 053018 (2008)

- Dipole-dipole interactions, ultracold collisions- Towards a higher phase space density: Magnetic trapping and accumulation of NH molecules

PRL98,133001 (2007)

NH trapping and accumulation in a magnetic trap

Current densities: 107-108 cm-3

Current temperatures: 50-500 mK (~0.05-0.5 cm-1)Current trapping times: a few seconds (BBR &backgr.gas)

Experiment repetition rate: 10 Hz

Increase of phase-space density by simply reloading the next decelerated packet of molecules into the trap

is impossible due to Liouville’s Theorem…

Groundstate

Metastable state(t of a few seconds)

neglible Stark shift

• Stark shift @ 100 kV/cm:1.67 cm-1

• produced in a supersonic expansion by photolysis of HN3

• can be decelerated using Stark decelerator and trapped electrostatically

v=0, j=2

v=0, j=1

The NH molecule

-

Excitation:584 nm

Background freedetection:336 nm

t ~ 450 ns

Deceleration of metastable NH

Electrostatic trapping of metastable NH

Lower limit for the radiative life time of the NH(a1D,v’=0,J‘=2) : t≥ 2.7 seconds

S. Hoekstra et al,Phys. Rev. A 063408 (2007)

The slow molecules are brought to a standstill at the center of an electrostatic trap,then the trap is switched on

7 kV -15 kV15 kV

-15 kV -15 kV15 kV

Groundstate

Metastable state(t> 2.7 seconds)

Back to the NH molecule …

Stark Decelerationand trapping

…. but Zeeman shift!

(10 Hz)

Magnetic trap, continuously on, accumulates cold molecules, increases phase-space density

Excitation:584 nm

Detection:336 nm

t ~ 450 ns

Cold source of groundstate NH,with negligible Stark shift

… also has a Zeeman shift …

Accumulation of groundstate NH

Magnetic trapping: 2x16 windings, anti-hemholz, 1000 Ampere, 28 kW cooling power300-600 Gauss/cm

Deceleration:special electrodes for improved deceleration from 150 m/s to standstill, that fit inside a magnetic trap

Transfer laser:60 mJ pulsed 584 nm fromRDL-PDA combination or cwlaser using a power-build-up cavity

Magnetically trapped NH (a1D)• Metastable NH

molecules are very efficiently stopped using new electrodes

• Magnetically trapped metastable NH molecules

• Not all stopped molecules are trapped:

- expect 50% max - trap only 7 mK deep for metastable molecules!

• Currently working on detection of trapped groundstate NH molecules

Blackbody radiation

Metastable CO lifetime

Magnetic trapping of NH (a1D)

Conclusions

Basv/dMeerakker

JoopGilijamse Peter

Zieger

Gerard Meijer

LudwigScharfenberg

MarkusMetsala

Technical support: Sandy Gewinner, HenrikHaak

Theoretical support: Boris Sartakov, GerritGroenenboom

StevenHoekstra

BBR: Nicolas Vanhaecke CO: Samuel Meek