Louis Harris

The Preparation and Optical Properties of Gold Blacks

The optical properties of gold smokes deposited on cellulose nitrate films under different experimental conditions have been studied. The conditions of pressure of the inert atmosphere, purity of the gas, rate of evaporation, and the distance between source and deposition surface giving the highest infra-red absorption per unit mass have been found. The thermal mass required for high infra-red absorption is small compared to the thermal masses of other receivers used for infra-red measurements. The gold “blacks” turn yellow and have lower infra-red absorption when heated above 110°C.Gold “blacks” with very high infra-red transmission (3–15μ) and low transmission at shorter wave-lengths are prepared when oxygen is present in the “inert” atmosphere.The particle size and particle distribution of the gold smokes deposited under different experimental conditions have been investigated with the electron microscope at high resolution.

A Method for the Evaporation of Alloys

We have developed a method for the evaporation of alloys in high vacuum which from the nature of the procedure used should be applicable to all alloys which are not refractory. The method has been tested on α‐brass and β‐brass and on a gold‐cadmium alloy. These alloys have components whose vapor pressures are very different and yet chemical and electron diffraction analyses on the evaporated brass alloys and chemical analysis on the evaporated cadmium‐gold alloy show that the composition of the original alloy is held to within close limits.

Preparation and Infrared Properties of Aluminum Oxide Films

A method for the preparation and mounting of thin aluminum oxide films from ordinary household aluminum foil is described. Free films covering a circular area of 3.3 cm2 and as thin as 250 A were mounted on glass rings. Infrared reflectance and transmission measurements show that there is absorption beyond 11 microns for even the thinnest films.

The Infrared Properties of Gold Smoke Deposits

An interpretation of the infrared optical properties of gold smoke deposits is presented together with new experimental results. This interpretation is based on an application of classical electromagnetic theory. The gold smoke deposits are assumed to interact with infrared radiation as if they were sheets of an essentially uniform material which is characterized by an average electrical conductivity small compared to that of ordinary metals. The electrical conductivity of gold black deposits is found to be ~10−5 times that of bulk gold, and the volume percent of gold in a gold black deposit is found to be ~0.2 percent.

The Transmittance and Reflectance of Gold Black Deposits in the 15- to 100-Micron Region

The transmittance and specular reflectance of several gold black deposits were measured from 15 to 105 μ. Differences in the conditions of preparation of the deposits are correlated with differences in their infrared properties. The large change in the absorption in the 15- to 80-μ region is associated with a condenser effect.

An Electron Microscope Study of Gold Smoke Deposits

Gold smoke deposits have been examined at high resolution with the electron microscope, and the size and distribution of the unit particles determined. The smokes were produced by evaporating gold from a hot tungsten filament in a partial atmosphere of nitrogen, and the samples were collected on thin collodion films. It was found that the size of the unit colloidal gold particles varied with the pressure of the atmosphere and the rate of evaporation. The manner of aggregation of the particles was found to be independent of the nitrogen atmosphere pressure and rate of evaporation over a relatively wide range. The aggregations, however, changed markedly when a small amount of oxygen was present during the evaporation. Measurements in the visible and infra‐red regions of the spectrum were correlated with the electron microscope investigations, and it was found that the optical properties of the deposits depend strikingly upon the manner in which the smoke particles aggregate and how the aggregates are distri...

Conductance and Relaxation Time of Electrons in Gold Blacks from Transmission and Reflection Measurements in the Far Infrared

The electrical conductivity of gold blacks is evaluated from reflection and transmission measurements in the far infrared. For sufficiently thin samples and sufficiently large wavelengths a closed expression is derived, relating the electrical conductivity of the gold black directly to the absorption and transmission coefficients. It is found that the electrical conductivity varies with wavelength, and for wavelengths larger than 105 microns this variation is attributed to a relaxation effect. The relaxation time of electrons in gold blacks is found to agree closely with that in bulk gold.

Reflection and Transmission of Radiation by Metal Films and the Influence of Nonabsorbing Backings

A set of equations is presented for the reflection and transmission of normally incident radiation by absorbing (metal) films on nonabsorbing backings.Using these equations, the results of two different methods of calculation of the reflection and transmission coefficients of a thick nonabsorbing plate are shown to be practically identical, both with and without an absorbing (metal) film on one surface of the plate. One of these methods involves the addition of intensities of the multiple reflections from the two surfaces of the plate. The other method is a rigorous calculation involving solution of the boundary condition equations (amplitude addition) for small areas followed by intensity averaging over the whole plate.In order to calculate optical constants of a metal film on a thick nonabsorbing backing, the much simpler method of intensity addition of internal reflections in the backing may usually be applied.

THE EVALUATION AND ANALYSIS OF OPTICAL AND ELECTRICAL CONSTANTS OF THIN FILMS AS FUNCTIONS OF REFLECTANCE AND TRANSMISSION DATA BY ELECTRONIC DIGITAL COMPUTATION

Whirlwind I, an electronic digital computer at M.I.T., has been used to calculate accurately the optical and electrical constants of thin metal films on nonabsorbing backings and of aluminum oxide films, and to check approximate relations which may be conveniently used in the absence of a digital computer. The conductance per square of film is found to be the principal physical property determining the behavior of conducting films toward incident infrared radiation. The variation with wavelength is related to the finite relaxation time of the free electrons and to imperfections in the films acting as condensers. In the visible wavelength region the two sets of optical constants were evaluated for different thicknesses of gold deposits and compared to optical constants previously reported.

Evaluation of the Equilibrium Constant for the N2O4 (g)=2NO2 (g) Reaction at 298.16°K from Light‐Transmission Measurements

The absorption coefficient of NO2 (g) has been found to be both temperature and pressure dependent for the temperature range 299.71° to 376.52°K and pressures from 2 to 5 cm Hg, for the 5461‐A mercury ``line. The equilibrium constants for the reaction N2O4 (g)=2NO2 (g), at the temperatures 299.71°, 303.70°, and 308.89°K, have been evaluated by combining precise transmittance measurements for the 5461‐A mercury line with the known vapor pressures of solid N2O4. ΔG°298.16 for the above change in state has been found to be 1.151±0.004 kcal/mole, which, when combined with the known value of ΔS°298.16, gives ΔH°298.16=13.640±0.100 kcal/mole.