REPORT FOR ANALYTICAL CHEMISTS Announcing the

New (S&S) EA System for

ELECTROPHORESIS

EA-4 Power Control Supply Designed especial ly for electropho­resis. Continuously variable voltage Ο to 500 V. Stable: Supplies constant volt­age. (Ripple less than ± 0 .1%. Unit regulates to ± 0.1%.) Also can supply constant current over entire range. No va r i ance in m A w i t h change in load ± 90%. Double scale meter shows V and mA. Exclusive built-in t imer with automatic shut-off. Four chambers — simultaneous operat ion (7 tests per chamber) . Constant cur rent contro l over entire electrophoretic range.

EA-I Electrophoresis Chamber High impact polystyrene; water cooling jacket. Domed see-through l id. Safety interlock. Platinum electrodes run en­tire chamber length. Polarity reversing swi tch. Simple, accurate method of attaching sample strip with flexible holders in integral part of chamber unit . This system offers features and advan­tages never before found in electro­phoresis equipment. The design is su­perb—and the system was precision built by scientists expressly for scien­tists. Our free brochure will give you a fu l l descr ipt ion complete wi th addi­t ional p ictures.

_ _ F R E E BROCHURE_ MAIL COUPON TODAY!

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3 0 A · ANALYTICAL CHEMISTRY

observable systematic variation in intensities that might be attributed to successively heavier elements S, Se, and Te. In addition, it was dis­covered that the LEED patterns from the (001) surfaces of the crys­tals (produced by cleavage with a knife edge) showed symmetry and dimensions of the unit mesh at the surface to be the same as that pre­dicted from a knowledge of the bulk atomic structure; hence, there is no rearrangement of the surfaces as has been determined for other semiconductors. Studies such as these have been extremely impor­tant in the "rebirth" of low energy electron diffraction, because of the importance of applications of semi­conductors in the burgeoning field of solid-state devices (6).

The wavelengths used in low en­ergy electron diffraction studies are in the same range as those for dif­fraction of x-rays. The basic dif­ference between the two techniques is that x-rays are highly penetrat­ing whereas low energy electrons are not. MacRae (3) has fully dis­cussed the differences between low energy electron diffraction and x-ray diffraction.

EPITAXIAL STUDIES. Reflection high energy electron diffraction (HEED) is, in many respects, com­plementary to low energy electron diffraction, and is frequently used for studies of epitaxial films of only a few monolayers thickness. For example, this technique has recent­ly been used to study the epitaxy of evaporated Cu films on Cu crystals (9). However, if the surface to be studied is rough, high energy elec­tron diffraction techniques might be severely limited. Both tech­niques are used in many laborato­ries.

For general studies of epitaxy, low energy electron diffraction might be expected to provide in­formation of the following type (10) : " (1) The substrate surface— whether contaminated or not—some indication of surface topography, e.g., facets or steps, (2) The nature of the deposit at less than a mono­layer coverage, e.g., the formation of two dimensional alloys, random deposit; (3) The crystal structure of the deposit at greater than mono­layer coverage; (4) The relative orientations of deposit and sub­

strate; (5) Some indication of the average size and shape of the nuclei of the deposit: (6) The effect of substrate temperature on the above ; (7) The effect of evaporation rate; (8) The effect of a third compo­nent, e.g., an active gas introduced at various points during the evapo­ration." However, it is pointed out (10) that neither LEED nor HEED can provide information directly re­lating to the role of surface imper­fections on the mode of growth, and the effect of microgeometry on growth must be studied by parallel work with high resolution electron microscopy.

The work by Taylor (10) is an excellent example of the use of low energy electron diffraction for epi­taxial growth studies. In this case a tungsten crystal substrate was positioned normal to the incident low energy electron beam and cop­per of 99.99% purity was evaporat­ed from a Knudsen cell within the diffraction chamber. The system permitted continuous study of the adsorption of copper, from partial alloying and formation of well-oriented uniformly thin copper (111) at room temperature up to a total flux of 20 atomic layers, at which point there was very little sign of any ordered copper.

The design of the basic compo­nents of the LEED apparatus is constantly being improved. An ar­ticle in 1965 by Β. Ε. Dicker (11) describes a universal motion speci­men manipulator for use with an ultra-high vacuum system; the ma­nipulator is designed with the pri­mary requirements of locating and maintaining the specimen at the focus of an electron beam, provid­ing for rotation and tilting of the specimen so that its face is perpen­dicular to the beam, and for the avoidance of magnetic materials and fields that might deflect the low energy electrons. Develop­mental theory for the design of the electron beam optics, with a view to obtaining the most intense beam possible, has been published by Helmer at Varian Associates (12). A technique for preparation of spherically shaped grids has been published (13), as has a decription of a double grid repeller system for improvement of electron resolution (14)- An arrangement for alterna-