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Determination of fundamental constants
using laser cooled molecular ions
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Jeroen Koelemeij,PostDoc
C. W. Chou, D. B. Hume, J. C. J. Koelemeij, D. J. Wineland, and T. Rosenband,PRL 104, 070802 (2010)
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Jeroen Koelemeij,PostDoc
Mohammad Ali Haddad, PhD student
Working on molecular ions
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Outline
• The proton-to-electron mass ratio μ
• Measuring vibrations in HD+
• Ion trap• Laser Cooling Atoms• Cooling molecules• Measurements• Results
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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
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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
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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
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Measuring
Movement disturbs measurement
• Vacuum• Cooling
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Ion trap
• Ions are trapped using electric fields
• Impossible to trap them with static fields
• RF-fields create a harmonic pseudopotential
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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
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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
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Cooling of molecules
• No direct laser cooling• Use sympathetic cooling • Energy transfer from HD+
to atomic ions due to Coulomb interactions
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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)
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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
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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
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Measuring
• Dissociating HD+
• Laser dissociates molecules from excited state
• Change in response of the ion cloud if HD+ was in excited state
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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)
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No measurement data yet
• Still setting up 780nm laser• Stabilization, frequency
calibration• Ion trap can still be
improved
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But we do have trapped ions
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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/