Multimass Three-Dimensional Velocity Map Imaging from Surfaces

This paper presents a surface scattering study employing a multimass ion imaging method, in which the sample is mounted on the repeller plate of the ion optical system and ions generated at the surface are velocity mapped along the surface normal. Measurement of the ion position and arrival time at the detector using a fast imaging camera allows the direct measurement of the half-Newton spheres of multiple ions and thus the determination of the full three-dimensional velocity distributions of the species of interest. The method is illustrated by measuring the velocity distributions of multiple ions generated in secondary ion mass spectrometry (SIMS) experiments following impact by a kilo-electronvolt C₆₀⁺ primary ion beam. The implications of the results for the dynamics of secondary ion formation are discussed. The three-dimensional velocity map imaging technique developed here should be applicable to a range of surface dynamical measurements in which ions are generated or extracted in a pulsed-like fashion to enable time-of-flight detection.