Robert C. Plumb

Theory of Formation of Very Thin Oxide Films on Metals

The growth of oxide on initially oxide‐free evaporated aluminum films at room temperature and atmospheric pressure has been studied in parallel experiments, using elliptically polarized light to measure the film thickness and vibrating capacitor measurements to measure changes in the Volta potential. The film thickness reaches 21 A in 106 sec and varies essentially logarithmically with time in the time range from 102 to 106 sec. The Volta potential is almost constant during the initial stages of growth and then varies approximately logarithmically with time. The observed change of Volta potential, interpreted as resulting from changes in the Galvani potential, is approximately 350 mV in 106 sec of oxidation.A logical extension of the Mott theory is presented which includes (1) the effect of electronic space charge resulting from equilibration of electrons between the metal and oxide and (2) the electric field in the oxide, both in its role as a barrier to electron penetration of the oxide and in its contr...

Raman and Infrared Spectra and Structures of the Aluminate Ions

The Raman and infrared spectra of the aluminate ions in alkaline solutions have been measured. Two ionic species, one predominant from pH 8 to pH 12 and the other predominant above pH 13, may be identified from the spectra. At intermediate pHs both forms are present. The selection rules and the polarization of the lines of the low‐pH species are consistent with either a square‐planar Al(OH)4− structure or a polymeric structure in which each metal ion is surrounded by six OH− ions and the perturbations on the Oh symmetry are not great enough to be resolved. The ion does not appear to be tetrahedral Al(OH)4−, as has been previously thought. Pronounced aging effects observed with the low‐pH anion support a polymeric structure rather than the square‐planar Al(OH)4−. The spectra show that the high‐pH species is AlO2− with D∞h symmetry. The observation of two distinct species and measurements of the pH ranges over which they are stable, confirm the results of earlier measurements by some of these authors using...

Ascent of Sap in Trees

Experimental results concerning the ascent of sap in the xylem are usually interpreted in terms of gradients of hydrostatic pressure in the xylem conduits. In this report an alternative model is proposed that is equally consistent with the experimental results: under static conditions the water column is supported by a gradient in the chemical activity of the water, and the hydrostatic pressure is constant throughout. Observations that support the new model are cited, and experiments are suggested that would permit a choice between the two models.

Analysis of Elliptically Polarized Light

Exact equations for the analysis of elliptically polarized light with a nonexact quarter-wave compensator are developed. Procedures are described for calibrating a quarter-wave compensator.

Specific Surface Area of Evaporated Aluminum

The variations in the specific surface area of evaporated aluminum due to substrate temperature, film thickness, and angle of incidence have been determined experimentally. Increasing the substrate temperature from 0° to 100°C decreases the specific surface area by a factor as large as 9. Increasing the film thickness from 3000 to 12 000A increases the specific surface area by a factor as large as 17. The specific surface area increases with increasing angle of incidence and has been observed to change in the range of 1.0 to 25.2. Atomistic models for the growth of rough films from vapor are considered. One model which has been proposed in which asperities grow from islands which form during the initial stages of condensation is rejected on the basis of experiment. A second model in which asperities grow from crystallites having preferred crystallographic orientations is found to be adequate to explain the observations of this and certain other investigations.

Some Observations on the Use of Elliptically Polarized Light to Study Metal Surfaces

Measurements and calculations using elliptically polarized light to determine the thickness and index of refraction of barium stearate films on a variety of substrates are described. Attention is focused especially upon problems associated with the study of very thin films, that is, films which are less than 100 A thick.Measurements of the optical constants of metal substrates are presented. An apparent dependence of the index of refraction n and extinction coefficient κ and independence of the absorption coefficient k upon the angle of incidence is observed and is discussed.Several effects, including accuracy and sensitivity, of the angle of incidence in studying film growth are noted. It is shown that there exists a characteristic angle of incidence at which the growth of a film produces a negligible change in the ellipticity, and it is shown how advantage may be taken of this to facilitate measurements of rapidly changing surfaces.It is demonstrated that extraneous films are satisfactorily accounted for in measuring thin films by means of the Drude equations through their effects upon the apparent optical constants of the substrate. Advantage may be taken of this fact to extend the thickness range over which the Drude linear equations are applicable by a computational procedure.The problem of the anomalous index of refraction of very thin films as determined by the Drude equations is considered. It is concluded that the anomalies revealed by the Drude equations are associated with the system under study and are not caused by any inherent limitation in the Drude equations. It appears that at the interface between a dielectric layer and metal system there is an absorbing region which produces the anomaly in the refractive index calculated by the Drude equations and for which explicit allowance must be made in order to make accurate measurements of the thickness of very thin films.

Effective insect fogging - The origin of sea breezes - Hot air balloons

Three applications of the changes in pressure or volume of a gas when its temperature is changed.

Knowing some thermodynamics can save a life

Nature makes extraordinarily good use of physical chemistry principles to keep the body functioning isothermally.