OPTIMAL LONG-TERM IMAGING OF A CHARGED MICROPARTICLE AT THE CENTER OF A PAUL TRAP IN AN ATMOSPHERE NEAR STANDARD TEMPERATURE AND PRESSURE - EXPERIMENT AND STOCHASTIC-MODEL

The thermal rms fluctuation in position of a levitated microparticle in a Paul trap in N2 near standard temperature and pressure is calculated numerically from a stochastic equation of motion and is measured experimentally by long term imaging. The calculations reveal loci of minima within the lowest stability region and enhanced fluctuations as the first instability is approached. Pseudopotential results deviate below our calculations progressively with increased drive potential. This disparity grows by more than an order of magnitude near instabilities, although the pseudopotential approach provides a good asymptotic approximation for small drive potentials. Long term imaging experiments are performed in a trap modified to eliminate stray static fields at its ac ''null'' point. These experiments are well-described by the stochastic model and suggest the use of such a trap as a stage for long term fluorescence and Raman microphotography experiments on individual microparticles in a gaseous environment.