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/