Signals from molecular beams using a surface ionization detector

Abstract

The response of a surface ionization detector with a Re-heated filament was examined as a function of arrival rate of CsCl molecules. The source of CsCl was an effusion cell. The surface ionization detector was moved kinematically on a track toward the CsCl source. The intensity was dependent on the vapor pressure of CsCl (dependent on effusion cell temperature), on the inverse square root of absolute temperature of the effusion cell, and on the inverse square of the distance between the source and the detector. For small arrival rates of CsCl with high-Re-filament temperatures (CsCl dissociated on the Re surface), the ion current of Cs+ from the detector was found to be linearly dependent on the arrival rate over a range of magnitude of 160. For larger arrival rates, deviations from linearity were observed which may be explained by desorption of neutral CsCl(ad) molecules [incomplete dissociation of CsCl(ad)] due to the presence of large amounts of Cl(ad). The equilibrium between Cl(ad) and Cl2(ad) also is significant since the formation of Cl2(ad) provides a sink for Cl(ad). In these experiments negative changes in the work function of Re should be insignificant.