measurements show that the generally accepted optical absorption coefficients in the wavelength range 850 to 1100 A are in serious error. Further measurements in xenon and krypton have resolved peaks due to inner shell "d" electrons.

Some of these energy levels had not previously been observed. Other measurements with 20 keV electrons have shown inner shell excitations in aluminium and beryllium, permitting measurements of cross sections for these processes as well as x-ray absorption coefficients. In this very soft-100 eV x-ray region few similar measurements exist.

Electron Scattering From Liquid Alloys.-The unique measurements on electron scattering from liquid metals have been extended to alloys. These measurements, besides showing changes in the band structure of bismuth upon melting, have shown that in indium-bismuth and indium-aluminum alloys the excitation energies change continuously with composition.

Field Emission Energy Distribution Measurements.-The first precision measurements of field emission energy distribution from known perfect single crystal planes of tungsten have been completed. These measurements yield a value for the work function

A micrograph produced in one of the Bureau's field-ion emission studies. The pattern shows a clean surface of tungsten. Atoms appear as bright spots of light.

of the 110 plane of tungsten which is anomalously high. This result is significant in that it cannot be reconciled with the currently accepted theories of electron emission.

Photodetachment of Negative Ions.-The absolute cross section for the photodetachment of electrons from I has been redetermined to be about a factor of two larger than the previously accepted value. The difference is ascribed primarily to one systematic error whose absolute limit can now be measured for the first time in the new NBS photodetachment apparatus. The absolute cross section and its variation with wavelength in the threshold region for the photodetachment of electrons from SHand SD- have also been measured. Preliminary analysis of the data indicates no isotope effect in marked contrast to similar experiments with OH- and OD- performed recently at NBS.

Plasma Spectroscopy.-The wall-stabilized arc was used to measure a number of transition probabilities in Sulfur II. Measurements were also made of the line broadening in Sulfur II, and the results provided part of the motivation for a re-examination of the theory of Stark broadening for positive ions. Shock tube measurements of some ultraviolet oscillator strengths in Carbon II were also completed; they agree nicely with some recent calculations on Carbon I and II performed at NBS.

High Resolution Bremsstrahlung Measurements.-Investigations of 1 to 4 MeV bremsstrahlung from intermediate and thin targets are in progress. A new experimental tool is the lithium drifted germanium detector, whose high resolution permits careful mapping of the upper end of the x-ray spectrum. Ge detectors ranging from 14 to 10 cm3 are being used and the total absolute efficiency of the detectors as a function of energy is being measured with NBS radioactivity standards. Emphasis is being placed on absolute calibration of the bremsstrahlung flux density to permit greater accuracy in the measurement of nuclear and atomic cross sections.

Energy Required to Create an Electron-Hole Pair. The relative energy required to create an electron-hole pair by ionizing radiation, e, was measured in silicon in the interval from 4.2 to 77 °K. These measurements indicated that decreased less than 1 percent in this range. The slight temperature variation of E is in agreement with the predictions of the Schockley theory. Work done elsewhere had indicated that ɛ was as high at 5.2 eV at 20 °K.

Theory of Thick-Target Bremsstrahlung Production.—The bremsstrahlung emerging from thick targets has been calculated for aluminum and tungsten absorbers with electron energies be

tween 100 keV and 10 MeV. The dimensions of such thick targets are comparable with the range of the incident electrons, and the multiple scattering of the electrons and bremsstrahlung photons is important. The available theory of bremsstrahlung cross section, when combined with Monte Carlo transport calculations, is adequate to account for the available experimental results.

Energy Transfer. The intensity distribution of the CH emission spectrum produced in the reaction of oxygen-atoms with acetylene has been studied. The rotational energy initially carried by CH is distributed in a highly non-equilibrium fashion and is effectively redistributed to a room-temperature thermal distribution by collisions with inert gases. This rotational energy relaxation process has been interpreted in terms of a "two-state model," one state corresponding to the initial, high-temperature distribution, and the other to room temperature. The efficiency for rotational energy transfer has been deduced.

Spectrum of Helium.-Experimental and theoretical studies on the vacuum ultraviolet emission spectrum of helium have provided information on the transient species He2. The 600 A bands, which show structure similar to vibrational transitions, are not due to emission for electronically excited He2 but come from emission during the collision of an excited and a normal helium atom. The structure is merely a reflection of the nodes and antinodes in the wave-function in the continuum region above an attractive potential well. The spectra permit deductions concerning the shape and depth of the well.

Fluorescence Appearance Energies.-"Fluorescence Appearance Energies" is a new technique in which photons selected by a vacuum monochromator are absorbed by simple molecules (H2O, N2O, NO2) contained in a cell. The fluorescence of fragments is measured normal to the incident beam. The threshold energies for the appearance of the fluorescence due to OH (A2+), NO (A2+), NO (B2) have been measured for the first time. The fluorescence yields have been measured over the wavelength interval from the appearance threshold down to the LiF cutoff at 1050 Å. These fluorescence yield measurements, in the case of N2O photolysis, have also revealed thresholds for the production of N (2D) and N2 (B3rg).

Matrix Isolation Characterization of Free Radicals.-The thermodynamic properties of NCN have been calculated from the measurement of fundamental vibration frequencies. The carbon nitrogen stretching force constant approaches a value characteristic of a doubly bonded species. The force constants and thermodynamic properties of CNN have been similarly derived.

Measurements on HNF have also led to values for the thermodynamic constants of this radical.

Radio Materials.-Measurement emphasis in this area is on methods of measuring dielectrics at elevated temperatures. The effect of sample shape on spin wave resonances has been studied in polycrystalline materials, and related studies on single crystals are in progress. An apparatus for studying dielectric properties of materials at 2mm wavelength has been completed. Large, highly pure crystals of sodium chloride and calcite have been grown.

Quantum Electronics.-Experimental studies of the Raman and Brillouin effects produced by coherent radiation have continued. Progress has also been made in the redetermination of the fine structure constant by the study of ionized helium, and in the evaluation of an absorption Stark voltmeter.

Radio Plasma.-During the past year work based upon the brush cathode plasma developed at NBS has been completed. Various diagnostic tools have been compared, and their ranges of agreement ascertained. The recombination coefficient of helium was measured. Some new millimeter wave interferometers for diagnosis were built. Considerable experimental and theoretical work upon the noise emission maxima of plasma in a magnetic field (the so-called Landauer effect) was done.

Solid State Properties

The experimental determination of magnetic hyperfine fields is important in understanding the microscopic mechanisms of magnetic behavior. To supplement nuclear magnetic resonance studies of the 4d transition metals, a study of the 4f transition metals was begun. Europium was selected as the first rare-earth metal to be studied. This element becomes antiferromagnetic at 87 °K and possesses a helical (screw type) spin structure.

Thermodynamic and Transport Properties

Electromotive Force Series for Halides and Oxides. Electromotive force series for the elements in solid and molten halides and oxides were developed for a number of temperatures from 25 to 3000 °C. In these series the elements are arranged in decreasing order of reducing power, i.e., each element has a positive electrical potential when placed in contact with the element below it in the series. Since these series give the relative stability of elements or metals in solid and molten salts, they have wide utility in interpreting industrial processes, such as electrochemical production of energy, electroplating, or electrorefining. The fluoride,

chloride, bromide, and iodide series cover 69, 96, 90, and 82 elements, respectively.

Membrane Reference Electrodes for Molten Salts.-A potentiometric method utilizing glass membrane reference electrodes was developed for determining the composition of molten salts with respect to cations for which reversible electrodes can be constructed. The method is capable of measuring the concentration of heavy metal ions with an accuracy of 1 percent even in very dilute solution as long as a small amount of sodium ion is present in the melt.

Heat Capacity at Low Temperature.-Measured results were made of the heat capacity of niobium-doped SrTiO3 between 0.03 and 1 K. Results demonstrated the removal of an anomaly below 0.38 °K by application of magnetic field, thus confirming the superconducting nature of the transition. The "thermodynamic critical field" (calculated from T. and the y term from the normal-state specific heat measurements) is in good agreement with that obtained by magnetic moment measurements in specimens of the same carrier concentration.

Transport Properties of Compressed Gases.-The transport properties of dilute gases at pressures around 1 atmosphere are independent of pressure or density. However, when a gas is compressed this is no longer true. Heretofore it was believed that the density dependence of the transport properties of gases could be represented by a power series similar to that of the thermodynamic properties. It was discovered that this is not the case and that actually the transport properties of gases depend logarithmatically on the density. This development should affect significantly the procedures for correlation of experimental data and the prediction of transport properties of compressed gases.

Excitation Energy Transfer.-A new detector was developed for the transfer of excitation energy between identical chromophors. It was used to study the transfer in a system where dipole-dipole coupling between the chromophors is expected to be very small. An anomalously large transfer distance of 45 Å was determined, which cannot be explained by present theories. Measurements were also completed on intramolecular excitation energy transfer on two molecules where the chromophors are at opposite ends of a rigid steroid bridge. These two molecules have the unique property that there is complete energy transfer to the lowest energy state in the singlet system but no energy transfer in the triplet system.

Phase Transition Measurement Technique.-A novel technique