Bieske Laser Spectroscopy Group

Particle Trap and Particle Spectroscopy

         

An ideal quadrupole trap consists of two hyperbolic endcap electrodes and a ring electrode. Oscillating voltages applied to the electrodes generate an effective harmonic potential near the trap centre that serves to constrain the charged particle. allowing it to be levitated indefinitely. The actual trap (shown below) consists of two end cap electrodes and 8 rod electrodes (which together approximate the ring electrode of the ideal trap).

                 

A particle's oscillation frequency in the trap depends on its mass-to-charge ratio, allowing the trap to be used as a mass spectrometer. In practice, the oscillation frequency is ascertained by monitoring light scattered by the particle as it moves in and out of a probe laser beam.  By sensing the scattered light with a photomultiplier tube and Fourier transforming the signal, the particle’s oscillation frequencies are obtained.

Experiments have been conducted in which the laser-induced fluorescence of stored particles, such as dye-doped spherical polystyrene particles, have been obtained. The emission spectrum (shown below) is dominated by morphology-dependent resonances (MDR), that appear as sharp peaks.  By analysing the spectrum one can deduce the radius and refractive index of the sphere.