DEVELOPMENT OF A NOVEL DUAL TIME-OF-FLIGHT IMAGING MASS SPECTROMETER: PRINCIPAL, REALIZATION, AND OPTIMAL PERFORMANCE

Abstract

A novel dual time-of-flight imaging analyzer has been developed for studies of gas phase reactions and the scattering or desorption of ions and molecules from surfaces. The analyzer is capable of experimentally selecting a two-dimensional slice of particles from a three-dimensional flux without the necessity for deconvolution of the resulting velocity images by the Abel transform. The analyzer operates through ionization of the scattered species and their subsequent flight through a field-free region. This initial flight allows a dispersion according to the species natural velocity distribution. The second time of flight deflects the ions through a right angle and through a flight tube allowing dispersion according to mass or charge before detection. The analyzer offers two modes of operation — the first of these produces a mass spectrum of the desorbing species, the second produces a two-dimensional velocity map of the desorbing species. Trial results using an effusive beam source and acetone as a test gas have demonstrated the operation of the analyzer. The operation of the analyzer has been simulated and optimized to reduce ion flight aberrations. A set of orthogonal two-dimensional polynomial functions have been derived to reduce residual aberrations across a broad range of operating conditions. An upper limit to the temporal resolution of the analyzer has been established and a set of working parameters for low distortion electron beam ionization have been presented.