Murtadha A. Khakoo

An attempt to observe Mott scattering optically

The authors report an attempt to make an optical observation of Mott scattering, involving the first studies of inelastic collisions between polarized electrons and a heavy noble gas. Polarization fractions of light emitted by the 5p 56p(5/2)3(3D3) state of Xe following impact excitation in an axial collision geometry were measured as a function of the incident electron energy, and compared with distorted-wave Born calculations. The theoretical and experimental results agree qualitatively in the energy range where cascade contributions to the photon signal are small. The authors failed to measure non-zero values of the linear polarization fraction eta 1 which would have constituted unambiguous evidence for Mott scattering and/or the importance of higher-order excitation processes in this system.

CLEAN ULTRAHIGH VACUUM SYSTEM WITH SINGLE-STRUCTURE DIFFUSION PUMPS

A practical example of the use of single‐structure diffusion pumping groups in an experiment requiring hydrocarbon‐free, ultrahigh vacuum is presented. With the use of proper vacuum procedures and careful foreline design and operation, but with no exotic hardware or liquid nitrogen, we have demonstrated that such a pump could provide vacuum that was ‘‘clean,’’ i.e., which had no residual pump‐oil contamination to a partial pressure below 10−8 mbar. Their use is thus compatible, at least in peripheral vacuum chambers, with standard ‘‘UHV‐surface science’’ processes. We discuss our vacuum system in detail, and present a review of the literature on the production of clean vacua with diffusion pumps.