Magnetic time-of-flight photoelectron spectrometer for mass-selected negative cluster ions

Design considerations and initial results are presented for a new type of time-of-flight photoelectron spectrometer which is particularly suited to the study of cold metal and semiconductor cluster anions prepared in a supersonic molecular beam. The desired cluster is extracted from the molecular beam, mass-selected after an initial time-of-flight, and decelerated as it enters the photoelectron spectrometer. Photoelectrons ejected from the cluster by an ArF excimer laser are collected with >98% efficiency in an intense pulsed magnetic field of carefully controlled divergence. This divergent field parallelizes the photoelectron trajectories and maps smoothly onto a low, uniform magnetic field which guides the electrons along a 234-cm flight tube leading to a microchannel-plate detector. The strong magnetic fields and simple, open design provide excellent rejection of stray photoelectrons in a clean, ultrahigh-vacuum environment. The UPS spectrum of Si₂₀^- is given as an example.