TECHNOLOGY
H. J. Trurnit, Chemical Corps Medical Laboratories, adjusts valve to control air flow through reaction chamber of recording ellipsometer. Vapor adsorption on films is measured with photo cell at near end of optical bench and continuously recorded
N e w Look into the Invisible Continuous thickness measurements of layers now possible with recording ellipsometer HY DO rain droplets run d o w n a windshield without wetting it and later cover the same surface in a homogeneous liquid film? H o w is it possible that a few drops of a certain material added to a gallon of poor machine oil will transform it into an excellent lubricant? More complete answers to these questions may result soon from research on adsorption properties of surfaces, both solid and liquid. A useful research tool, recently developed at Army Chemical Center Medical Laboratories, is the recording ellipsometer which permits continuous measurement of adsorbed layers. In it, H. J. Trurnit has combined optical measurement of light reflected through a surface film with a photoelectric recording system. Chemical Center scientists developed this technique to follow surface reactions whether in a vacuum, at various pressures, or on solids submerged in a liquid. They have used it extensively in the study of biological enzyme reactions. Continuous records of such reactions have proved particularly useful to interpret results following completion of laboratory experiments. Standard Instrument. A standard ellipsometer is used in the optical system. P l a n e polarized light is passed through the film and becomes elliptically polarized when reflected. Elliptical characteristics d e p e n d upon optical constants of metal reflector surface, re1680
adsorbed
fractive index of film, w a v e length and angle of incidence of light source, and thickness of film on the metal. Variations of film thickness alter t h e ratio of major a n d minor axes of t h e ellipse and their position in space. By measurement of this alteration—called p a r a m eter of t h e ellipse—Trurnit determined film thicknesses for several reactions. A compensator placed in the p a t h of reflected light reconverts it from elliptical to plane polarized light. Azimuth of reconverted light, which is dependent upon parameter* of the ellipse, is measured using an analyzer. T h e analyzer is rotated with reference to the compensator until light passing through them reaches a minimum intensity. With this technique film thickness may be measured before a surface film is placed in a reacting medium and again after reaction to determine extent of reaction. Early optical methods did not permit continuous observation and recording of the progress of a surface reaction. Trurnit eliminates this h a n d i c a p by placing a highly sensitive photo cell in the path of reflected light to convert intensity into a voltage potential. Potential is then related to film thickness, empirically, using a standard reference material calibrated under comparable conditions. In this manner, h e records progress of a surface reaction continuously and observes reaction rates closely. CHEMICAL
Advantages Are Many. Army researchers claim several advantages for this technique. First, it is not necessary to remove t h e reflecting slide from reaction medium to m e a s u r e film thickness changes. Sensitivity of photo cells used makes possible m e a s u r e m e n t of thickness changes of only a few angstroms. Empirical calibration against a standard material cancels out factors introduced by light passage through t h e reacting medium. Not all surface reactions m a y b e studied with an ellipsometer, h o w ever. It may be employed with only a small number of superimposed monomolecular films. Reaction m e d i u m must b e transparent. Films studied must also b e transparent, b u t this does not normally lead to difficulty as nearly all films of molecular thickness are free of colloidal particles. Calibrating m a terial—barium stéarate is often u s e d must b e transparent, too. Chemical Center scientists are enthusiastic about potentialities of t h e ellipsometer because of its ability to measure film thicknesses t o a two h u n d r e d millionth of an inch. T h e y predict its use in research will benefit both industry and the military.
Survey Chemists Develop Analyses f o r Uranium, Thorium Over 200,000 samples of radioactive material have b e e n analyzed for uranium under investigations of U. S. Geological Survey. Moreover, samples h a v e also been analyzed for thorium. Fluorimetric as well as colorimetric methods are among those developed for uranium analysis by Survey chemists. Fluorimetric methods are widely employed by chemists, says Geological Survey, because these methods are exceedingly sensitive, as little as one tenbillionth of a gram of uranium being readily detectable. I n addition, fluorimetric methods are a d a p t a b l e to rapid analysis of large n u m b e r of samples and are particularly applicable to analysis of low grade materials. A mesityl oxide extraction m e t h o d for the analysis of thorium, developed by Survey chemists, is said to b e particularly useful and is gaining wide acceptance b y analysts. Analytical m e t h o d s developed b y Geological Survey are results of investigations on behalf of the Atomic Energy Commission a n d earlier for t h e Manhattan District Project. T o m a k e these methods available to the public, they have been compiled as Geological Survey Bulletin 1006. Copies are available from Superintendents of Documents, Government Printing Office, Washington 25, D. C., for $1.00. AND
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