New shift seen in cyclotrons
Jul 1, 2004
A subtle change in the motion of charged particles in magnetic fields has been discovered by physicists in the US. David Pritchard and colleagues at the Massachusetts Institute of Technology have shown that the resulting shift in the cyclotron frequency of the particle can be used to measure its quantum state (Nature 430 58). The effect could be used to measure the dipole moments of molecular ions, test fundamental symmetries, and weigh individual chemical bonds.
Cyclotrons are routinely employed to compare the masses of molecules and study chemical reaction rates. The cyclotron frequency is given by the simple formula qB/m, where q and m are the charge and mass of the particle and B is the magnetic field. However, Pritchard and co-workers have discovered that this formula needs to be tweaked for particles that can be polarized. This happens because an electric dipole moment is induced in the particle and the two ends of the dipole move at slightly different speeds. The end result is that the cyclotron frequency is shifted by a small amount.
The MIT team found that this frequency shift was about 1 part in 109 for a CO+ ion. Moreover, since the polarizability of the ion depends on its internal quantum state, the cyclotron frequency changes whenever the ion jumps to a different internal state. By measuring these changes, Pritchard and co-workers were able to calculate the dipole moment of the molecule. They were also able to determine the quantum state of the molecule by simply measuring its cyclotron frequency.
"This is a very general effect that had not been recognized before, and it needs to be taken into account in various experiments," team member James Thompson told PhysicsWeb.
These include fundamental tests of CPT (charge-parity-time reversal) symmetry and experiments that use molecular ions to search for an electron dipole moment.