Experimental Determination of the Imaging Properties of a 200 kV Electron Microscope

Abstract

Optical transforms from a through focal series of images of amorphous films of Ge were used to measure the spatial frequencies of maximum and minimum phase contrast of a specific 200 kV lEOL electron microscope. This information was used to determine precise values for the spherical aberration coefficient and defect of focus. Under the appropriate conditions of lens excitation the spherical aberration coefficient was found to be as low as 0·94 mm. Other image defects revealed with great precision were associated with astigmatism, beam divergence and specimen drift in the microscope stage. Quantitative examples illustrating these effects are discussed.

An Electron Microscope Study of Defect-free YBa2Cu3O7?x (x » 0.15) with Superconductivity at 93 K

“…Fig. 2a shows the [001] orientation of a bulky particle at high resolution, using the top-entry JEOL 200CX microscope, whose characteristics have been described by Glanvill et al (1986). Simple squares form this lattice image, and three crystal thickness regimes appear.…”

“…It is well known that mixed-phase YBa2Cu307_x may become superconducting in the temperature range 50-106 K, the precise temperature depending upon the preparation conditions (WU et al 1987;Syono et al 1987;Strobel et al 1987;David et al 1987;Gopalakrishnan et al 1987). Much interest has centred upon the identification of the phase which has the major contribution to the superconductivity, and to its preparation as a pure phase.…”

The Anatomy of an Image

“…We describe elsewhere (Glanvill et al 1985) how we use optical diffractograms of a through focus series of amorphous films of C, Si and Ge to determine the spatial frequencies of maximum and minimum phase contrast, and thence the spherical aberration coefficient C s of the JEOL 200 kV electron microscope. In the particular configuration we used, with a tilting stage holder placed very low in the objective lens and with a double-gap condenser, the measured spherical aberration is 0·94 mm.…”

A practical direct-space refinement cycle for structure images

“…Unfortunately, these simulations, which have great asthetic appeal, bear no resemblance to the experimental results. The simulations contain no damping functions, apart from the use of isotropic room-temperature B factors, and of the 200CX transmission function determined by Glanville, Moodie, Whitfield & Wilson (1986). Given the complexity of the averaged structure, and the extreme sensitivity of the simulations to tunnel-ion location, this failure is hardly surprising since an X-ray-determined unit cell is likely to be both over-symmetrized and overdensely occupied.…”