Determination of fundamental constants

using laser cooled molecular ions

 

Jeroen Koelemeij,PostDoc

C. W. Chou, D. B. Hume, J. C. J. Koelemeij, D. J. Wineland, and T. Rosenband,PRL 104, 070802 (2010)

 

Jeroen Koelemeij,PostDoc

Mohammad Ali Haddad, PhD student

Working on molecular ions

Outline

• The proton-to-electron mass ratio μ

• Measuring vibrations in HD+

• Ion trap• Laser Cooling Atoms• Cooling molecules• Measurements• Results

μ• In general, value of

physical constants can depend on units

• Dimensionless constants do not depend on the stick you measure with

• Two dimensionless parameters needed for gross structure of atoms and molecules

• Fine structure constant α• The proton-to-electron

mass ratio μ

μ Vibrations and rotations in molecules and vibrations in

crystal lattices depend on μ• Properties of matter like specific heat capacity, thermal

conductivity depend on these motions• μ=mp/me=1836.1526724718(80)• Want an even more accurate value• Compare results of different techniques

HD+

Study vibrations of simplest molecule, like H2

+

HD+ has a permanent electric dipole which allows for a vibrational transition within an electronic state

QED Calculations on vibrational transitions limited by knowledge of μ

Vary μ to fit calculations to experimental data

ftheory(μ)=fexp

Measuring

Movement disturbs measurement

• Vacuum• Cooling

Ion trap

• Ions are trapped using electric fields

• Impossible to trap them with static fields

• RF-fields create a harmonic pseudopotential

Laser Cooling

• Atoms accelerate or decelerate when absorbing light

• Need absorption only when would decelerate an atom

• When atom is moving opposite to laser beam

• Absorb light only at resonance frequency

• Doppler effect• Laser detuned below

resonance

Cooling cycle

• Need many photon absorptions

• Requires a two-level system

• Molecules can decay to many rovibrational levels

• They do not end up in the same initial state

Cooling of molecules

• No direct laser cooling• Use sympathetic cooling • Energy transfer from HD+

to atomic ions due to Coulomb interactions

Cooling of molecules

• Frequency of radial harmonic motion ωr in RF-pseudo-potential scales with 1/m

• Potential energy scales with (m ωr

2)∝1/m• Use ions with small mass

for greater Coulomb-interaction

• Be+

B. Roth, J. Koelemeij et al., PRA 74, 040501 (2006)

Laser cooling

• Beryllium ions• Tuning to the right

wavelength• We need 313 nm• Frequency doubling of 626

nm dye laser • 532 nm solid state laser to

pump the dye

Measuring

• Measure the absorption frequency

• Detect absorption of probe laser while changing its frequency

• Transition to other vibrational state

• Low spontaneous emission rate

• Fluorescence is very weak

Measuring

• Dissociating HD+

• Laser dissociates molecules from excited  state

• Change in response of the ion cloud if HD+ was in excited state

Measuring

• Be+ is visible because of cooling laser

• Fluorescence depends on temperature

• Drive HD+ harmonic motion in trap

• Temperature depends on amount of HD+

• Fluorescence depends on amount of HD+

B. Roth, J. Koelemeij et al., PRA 74, 040501 (2006)

No measurement data yet

 • Still setting up 780nm laser• Stabilization, frequency

calibration• Ion trap can still be

improved

But we do have trapped ions

 

Sources used for pictures:

http://www.informaworld.com/ampp/image?path=/713172969/713548228/F0001.png

http://gottatopic.com/how-long-is-a-meter/

http://www.andybrain.com/sciencelab/2008/04/26/learn-about-volume-and-space-with-ice-water/