Density functional theory calculations on Ybusing the Hartree Fock system and the analogous Dirac-Hartree-Fock methods predict that Ybis stable in the Py,z state. The electron affinity obtained after the correlation energy functionals used are calibrated against Caand Lu is 2f 1 mHartree (54+ 27 meV).The calculations indicate that the Pilz state is unlikely to be bound due to spin-orbit coupling reducing its binding energy.It had long been thought that atoms with closed subshells such &s Ca, Sr, Ba and Ra (whose closed valence subshells are n s ' , n = 4, 5, 6 and 7), could not bind an extra electron to form a stable negative ion. With the predictions of stable Ca-, Sr-, Ba-and Ra-from a variety of theoretical methods by Froese Fischer et a/ (1987, to be referred to as FFLV), Vosko et al (i989, to be referred to as VLM), Froese Fischer (1989) and Kim and Greene (1989), and the subsequent observation by Pegg e l a/ (1987) and Garwan et a/ (1990) of the first three, this view was forced to be abandoned. As emphasized by VLM, the crucial new ingredient in the theoretical work was the extra electron going into an n.p orbital instead of following the usual trend of occupying the same orbital as the neutral atom with one higher atomic number. (In these cases this trend would place the extra electron in the (n -1)d orbital. The deviation from this pattern was observed by Feigerle et a/ (1981) who found that Sc-and Yhave the configurations [Ar]4s24p1 and [Kr]5s25p1, respectively.) To date, this np occupancy has only been confirmed for Ca-where the electron affinity (EA = 43 & 7 meV = 1.6 zk 0.25 mHartree) and configuration ([Ar]4s24p' 'PO) have been determined experimentally by Pegg et al (1987). A natural question that arises is whether there are any other closed-subshell atoms that have stable negative ions. Kim and Greene (1989) thought that the most plausible candidates were Zn-, Cdand Hg-, but concluded that the 'Po states of these ions are unlikely to be stable and suggested that the heavier alkaline earths may be the only closed-subshell atoms whose negative ions are stable. They did not consider Yb, however; a n atom with closed subshells (i.e. [Xe]4f146sZ) near the end of the lanthanide series and thus easily t Supported in part by the Natural Sciences and Engineering Resemh Council of Canada and the Ontario Centre for Large Scde Computation.

A scanning interference electron microscope (SIEM) capable of observing magnetic induction distribution with high sensitivity and spatial resolution has been developed. The SIEM uses a pair of fine coherent scanning probes and detects their relative phase change by magnetic induction, giving raster images of microscopic magnetic distributions. Its performance has been demonstrated by observing magnetic induction distributed near the edge of a recorded magnetic storage medium. Obtained images are compared with corresponding images taken in the scanning Lorentz electron microscope mode using the same microscope, and the differences between them are discussed.

A new method of micromagnetometry, electron phase counting (EPC), using a scanning transmission electron microscope with differential phase contrast and interference imaging modes has been proposed and tested. By counting the relative phase changes occurring in a pair of mutually coherent fine probes as they are moved from an observation point to a nearby reference point, integrated magnetic flux density at the observation point can be quantified without recourse to instrumental signal intensity calibration. Its feasibility for practical micromagnetometry has been confirmed by the successful application of two different EPC procedures to a NiFe alloy film.

The performance of a compact high-current circular microtron is investigated by means of computer simulation. The microtron investigated here is operated with the Russian second type of acceleration and has an electron gun, composed of a cathode and an anode, located outside the accelerating cavity. The simulation results enable us to see the possibility of stable acceleration, to estimate the capture coefficient, and to predict the initial conditions necessary for stable acceleration.

We propose a new type of monochromatic electron beam source that incorporates the resonant tunneling effect for ultrahigh-resolution electron microscopes. One set of barrier and quantum well layers on a very sharp GaAs tip surface and a triangular vacuum level barrier created by applying a negative voltage to the sharp tip form a double-barrier structure which enables the resonant tunneling effect to occur. Computer simulations indicated large peaks of electron transmittance through the vacuum/GaAs/AlAs/GaAs double-barrier system. A monochromatic electron beam of about 30 meV in full width at half-maximum is expected at room temperature.