Non-destructive characterization and alignment of aerodynamically focused particle beams using single particle charge detection

Abstract

We describe the first experimental measurements of aerodynamically focused particle beams using single particle image-charge detection. An aerodynamic lens produces particle beams, which at times is not aligned with the bore of the lens, thus complicating the process of aligning a particle beam with the focus of a laser beam. A key result of this work is the development of a non-optical technique for aiming a beam of particles in vacuum into the focus of a laser beam. In the present application, the laser beam is fixed in space by the geometry of a large stationary vacuum system and it is necessary to blindly aim a narrowly focused particle beam across the laser beam. Our aiming device is based on the non-destructive detection of electrically charged particles as they pass through a small metal tube that picks up the image charge of the transiting particle. Individual electrosprayed particles larger than 70 nm produce an electrical pulse that can be thresholded and counted. The duration of the detector signal provides a way to measure particle velocity and the amplitude of the signal is proportional to particle charge. The rate of particle injection into vacuum, single particle velocity and charge and particle beam shape and position can be measured with the charge detector. We show data for aiming and focusing electrosprayed polystyrene latex spheres ranging in size from 70 to 190 nm. Particle injection rates as high as 3000 per second and particle beam diameters as small as 250 μ m were achieved by using the charge detector to optimize the performance of the aerodynamic lens to focus the particles before they entered. We also describe how this detector and injector system will be implemented for real-time particle analysis for aerosol mass spectrometry and single particle X-ray diffractive imaging.