Research
MOT Atom Trapping

Trapping Neutral Mercury

We are setting up a magneto-optical trap for neutral mercury. Mercury has some features that make it interesting in the experiments presented here.

Experiments

  1. It does not have fine- or hyperfine structure in the groundstate. Therefore, no repumping laser is required and trapping should be straight forward.
  2. It has two meta-stable states, one of which is the lower level of a cycling transition, i.e. a magneto-optical trap can be set up to also trap mercury on that transition. The lifetimes of these two meta-stable states are fairly long. In fact they are so long that they have never been measured. Calculations show lifetimes in the order of 5 seconds.
Picture of our Hg magneto-optical trap taken with an UV enhanced EMCCD-Camera
Picture of our Hg magneto-optical trap taken with an UV enhanced EMCCD-Camera

The project has the following goals:

  1. Measure the lifetimes of the meta-stable states and evaluate their potential in atomic clocks.
  2. Perform photo-association spectroscopy and determine the long-range potential energy surfaces of mercury dimer. Mercury dimer is important for the test of new methods of ab-initio calculations due to the relativistic effects and correlations present. These measurements will result in important input for these models.
  3. Form ultra-cold, i.e. translationally, vibrationally and rotationally cold Hg dimers by pumping Raman transitions in the dimer.

Surprisingly many important experiments have been performed using Mercury. This is especially true for atomic physics. Examples are:

  • 1911: Super-conductivity (Kammerlingh-Onnes)
  • 1912: Resonance fluorescence (Wood)
  • 1912/26: Quenching of resonance fluorescence
  • 1913: Experiments by Franck and Hertz
  • 1920/27 Stepwise excitation
  • 1923: Hanle Effect
  • 1915/23: Pressure broadening
  • 1924: Collision induced reactions
  • 1950: Optical pumping (Kastler)
  • 1976: 1st EPR Experiment w/ lasers (Fry)

Therefore, besides hydrogen mercury is probably the most important element in the history of atomic physics. Also, in modern times mercury is the basis of fascinating experiments.

Mercury as a trapping species

Mercury offers the opportunity of exciting experiments in cooling and trapping. These include:

  1. Neutral Mercury as a possible new time standard
  2. Formation of ultra-cold molecules by photo-association
  3. Bose-Einstein condensation in a meta-stable state
Some properties of mercury relevant to cooling and trapping
vapor pressure 1.68 10-8 at 208 K
average velocity 146.6 m/s
most likely velocity 129.9 m/s
cooling transition 3P1 <- 1S0 3D3 <- 3P2
cooling wavelength 253.7 nm 365.0 nm
average number of photons for cooling 17 000 24 000
life time of upper cooling state 125 ns 7.8 ns
Doppler temperature 30 microK 490 microK
Doppler limited speed 7.8 mm/s 5.5 mm/s
Saturation Intensity 10 mW/cm 55 mW/cm

Level scheme of mercury

Level scheme of Mercury with relevant transitions shown.

Relative position of the transition for the various bosonic and fermionic isotopes. The line height reflects the natural abundance of the various isotopes.

A particular difficulty in trapping mercury is the generation of continuous wave UV radiation at the trapping frequency of mercury, i.e. the well-known 253.7 nm transition. The laser system consists of an ECDL emitting 1014nm. The infrared light is then amplified in an Yb-doped fiber. Frequency doubling is achieved in two consecutive enhancement cavities using LBO and BBO as the nonlinear medium.

Magneto-optical trap

Our magneto optical trap is relatively standard with a background pressure of 5 x 10-10 mbar. The Laser radiation is split in three branches of identical intensity and aligned via retro-reflectors in the typical 'crossing (see image of principle).' configuration of MOTs. The Anti-Helmholtz coils are water cooled. And we use a UV enhanced EMCCD camera to image the cloud of cold Hg atoms.

Special attention was given to the Hg reservoir. It consists of a copper double walled tube and is held at a temperature of -40°C using a compressor cooler. A two-stage Peltier element then enables cooling of the reservoir to temperatures between -40°C and -72°C.

MOT: principle and setup

Principle of the Magneto-Optical Trap

Setup

In principle a magneto-optical trap consists of six laser beams, two opposing for each direction of space. The laser beams are red detuned which leads to a velocity dependent force. In combination with a three dimensional magnetic field, leading to a position dependent force, atoms can be cooled and trapped. To simplify the setup, we use three laser beams which are reflected back through the magnetic field.