Design considerations and performance characteristics of a dual mode time-of-flight mass spectrometer system for surface reactivity studies

Abstract

We have designed a reflectron type time-of-flight mass spectrometer (TOFMS) system with both SIMS and electron impact (EI) ion optics. The system was designed to study the interaction of solid surfaces with reactive species initially in the gas phase. In the EI mode, the main TOFMS assembly, which consists of the flight tubes, reflectron, and a channel plate detector, is fitted with the EI ion source. The entire TOFMS is mounted in a rotatable cage permitting 270° of rotation about the sample center. Reactive species are supplied by a pulsed molecular beam source. The EI source has an unobstructed view of the target throughout most of the 2π solid angle defined by the plane of the sample. Angular and time-resolved detection of scattered beam species and desorbed reaction products to a limiting partial pressure of ∼10 -13 Pa is possible in this configuration. In the SIMS detection configuration, the main TOFMS assembly is refitted with SIMS extraction optics and repositioned on the sample axis. The pulsed molecular beam source is again used to supply reactive species. By controlling the timing relationship between the arrival of the pulsed molecular beam and the pulsed ion beam used to sputter the target, time-resolved detection of transient surface species is possible. A 50% mass resolution of ∼1400 has been demonstrated in both operating modes. The feasibility of time-resolved detection in both operating modes has also been demonstrated.