Greetings! You have found yourself at the homepage of the Bohn Group, run (in a loose sense) by John Bohn at the University of Colorado. This group is part of the JILA AMO Physics Center, which you can learn about here:

What's New

Item: We all know that the electron has a magnetic dipole moment, but did you know that it probably has an electric dipole moment, too? If it exists, this dipole is really small, so to measure it you'd have to apply a gigantic electric field. And the best way to do this, as far as anybody has figured out, is to put the electron inside a polar molecule. This idea has spawned novel experiments like the one by Eric Cornell at JILA. If this experiment measures the electron's electric dipole moment, then the Standard Model of Physics(TM) needs to be repaired or extended.

A big uncertainty in interpreting these experiments is knowing just how big the field is inside the molecule, or more properly, the "effective electric field" that accounts for molecular structure. Recently Ed Meyer has made big progress in estimating the size of this field. Whereas previous estimates rely on high-powered, relativistic, many-body calculations of molecular structure, Ed was able to accomplish the same feat using stone knives and bearskins.

 

Item: John Bohn recently gave a public lecture on "The Physics of Baseball at a Mile High." Slides from this talk are available here (pdf, 2.2MB).

 

Rotons have ripples dep't: A Bose-Einstein condensate composed of dipolar particles woud have a smooth density profile under "ordinary" circumstances. But now Ryan Wilson has shown that, near the threshold of instability, these condensates can exhibit ripples when they are perturbed. These ripples are connected to a roton-like excitation in the condensate, in analogy with the roton mode in superfluid helium. Read more about it here.

Figure. Stabiltiy diagram for dipolar Bose-Einstein condensates (BEC's). D is an effective dipole strength, while λ is trap aspect ratio. The main thing here are the inserts, which show iso-density surfaces of condensates. (a) is caused by poking a small hole in the center of the BEC, say with a focused laser. (b) is caused by spinning the BEC into a vortex state.

 

Item: With the Colorado Rockies in the World Series in 2007, the physics of baseball at high altitude takes on a new significance. To combat the effects of thinner atmosphere in Denver, the Rockies have stored baseballs at 50% relative humidity since 2002. Did this help them win the pennant? Ed Meyer addresses the aerodynamics of this issue here.

Figure. Proof-positive that baseballs stored in a humid environment are larger than those stored in a dry environment. Ball diameters (d) and masses (m) are given as a ratio to their values when they are held at 30% relative humidity.

 

Item: A paramagnetic molecule like OH can be confined in a magnetic trap. However, this molecule can also be acted on by an electric field. With the two fields each pulling in different directions, who can make sense of what's going to happen to the molecule? Manuel Lara can, and his theory of the magneto-electrostatic trap helps to understand the experiments reported here.

Update 6/08: A more detailed theory of these traps, and the losses they produce, can be found here.

Figure. Potential energy surface for the ZEeman-STark (ZEST) trap, which acts on and confines an OH molecule. It would also make an interesting design for southwestern dinnerware.

 

Item: Suppose you had a Bose-Einstein condensate (BEC) composed of particles that had dipole moments. If this BEC is confined in a pancake-shaped trap, then its shape (see figure below) strikingly resembles a red blood cell. For details, see this paper by Shai Ronen and Daniele Bortolotti.

Figure. An iso-density surface of a Bose-Einstein condensate composed of dipolar particles. The particles are assumed to be aligned with their dipole moments parallel to the symmetry axis of the figure. This shape reminds you of a red blood cell, doesn't it? Or, maybe one of those fruit cookies where they leave a space in the middle for the fruit. Or perhaps the Aus'zogene, a German pastry. (Thanks to S. Yelin for this last observation.)

 

Item: Atoms are fairly easy to cool to microKelvin temperatures, but many interesting ground state molecules (such as OH) are difficult to cool below even 10 milliKelvin. Would it help to immerse the OH molecules in a bath of cold atoms, say of rubidium? Manuel Lara answers this question here. (The answer is "no.")

Figure. Scattering cross sections for Rb-OH cold collisions. The elastic (solid) and inelastic (dashed) cross sections are comparable. This means that every time an OH molecule hits a Rb atom, the odds are good that the molecule would be ejected from a magnetic trap. Thus it is unlikely that you could sympathetically cool OH in a magnetic trap of Rb.

 

Item: New techniques in slowing molecules make it possible to measure molecular spectra with crazy precision. Jun Ye's group has recently re-evaluated the Zeeman effect in the OH molecule at low field. Ed Meyer's theory explains the result, here .

Figure. This is the change in the transition frequency between different parity eigenstates of the ground state of OH, as a function of magnetic field. Your first thought is that parity states don't care about magnetic fields, and that therefore all these transition frequencies should be constant. Uh-uh, not so -- there are subtle differences.

 

This page was last updated on July 20, 1969.