Velocity distribution measurement and two-wire field effects for electric deflection of a neutral supersonic cluster beam

The basic principle of deflection of a beam of polarizable particles by an inhomogeneous electric field is straightforward, but its application to realistic beams having spatial and velocity distributions of finite widths requires special care. We present a detailed treatment of these issues as developed for a measurement of electric polarizabilities of alkali clusters in a continuous supersonic beam. We begin by describing a general technique for extracting the beam velocity distribution from the time-of-flight profile generated by two separate 50-50 gating choppers. This method yields accurate model-free information directly from the experimental profile, without errors associated with detector delays, and with high signal throughput. We then use a simple but accurate approximation for the deflecting field variation over space, and derive analytical expressions for the deflection of beams of finite width and finite velocity spread. This allows us to evaluate the magnitude of a number of corrections to the ideal formula; the results are applicable to optimization of various electric and magnetic field deflection experiments. Finally, we demonstrate, both theoretically and experimentally, the phenomenon of local velocity variations across the beam profile which arise as a result of deflection. (C) 2002 American Institute of Physics.