Suspending Charged Particles

Why study particles?
Small particles have interesting and potentially useful chemical and physical properties. Our goal has been to isolate and probe small particles that are suspended in a vacuum chamber. This enables us to investigate an array of fundamental processes - heating with a laser beam, cooling through collisions with surrounding buffer gas, coalescence of two particles, and emission of light from fluorescent nanoparticles.

We have suspended particles of soot, CdSe nanoparticles, and polystyrene particles and used lasers to probe their properties. Currently we are investigating fluorescent nanodiamonds (NDs) and dye-sensitised metal-oxide nano particles.
How to store and investigate small particles
Charged particles can be trapped in a quadrupole trap (Paul trap), which consists of two hyperbolic endcap electrodes and a ring electrode (shown at left). The electrical potentials on the electrodes are oscillated rapidly such that a charged particle at the trap centre (the red dot) experiences an effective restoring force which is proportional to its distance from the trap centre. The charged particle can be levitated indefinitely!
Our electrodynamic trap
Our quadrupole ion trap, shown at right, consists of 8 rod electrodes (which together serve as the ring electrode) and two end cap electrodes. This arrangement, which is adapted from a design by Gerlich and coworkers, enables excellent access to the centre of the trap for probe laser beams.

Introduction of the particles into the trap is accomplished using laser induced acoustic desorption. Typically, several drops of an aqueous suspension of the particles are deposited on a silicon wafer. After drying, the wafer is inserted into the vacuum chamber and held just above the quadrupole trap. A pulse of light from a Nd:YAG laser intercepts the rear of the wafer causing an acoustic shock that propelled NDs from the wafer into the quadrupole trap. NDs with an appropriate mass to charge ratio were trapped between the end-cap electrodes and thenceforth could be suspended indefinitely. To facilitate the deceleration and capture of NDs, the chamber is filled with ~30 mTorr of N2 buffer gas.
Trapped Nanodiamonds
Nanodiamonds can be doped with colour centres that lead to the absorption and emission of visible light. The most extensively investigated color centre is associated with a substitutional nitrogen atom situated adjacent to a lattice vacancy with single negative charge (NV- centre). We have proved nanodiamonds confined in the quadrupole ion trap using pulsed and continuous laser light. At low laser intensities the diamonds emit light at wavelengths that are characteristic of single dopant nitrogen atoms situated adjacent to a vacancy (NV and NV- centers) or two nitrogen atoms adjacent a vacancy (H3 center).