tions of the first excited electronic state of CO. Knowledge of the magnitude of this potential maximum is of interest because of the great importance of the value of the dissociation energy of CO in thermochemical studies.

Alkali Molecular Spectra.-Several lines of the argon-ion laser excite definite vibrational-rotational levels in an upper electronic state of Na2, Rb2 and NaK, causing a relatively simple fluorescence spectrum. Using a Fabry-Perot interferometer crossed with a spectrograph, this spectrum was investigated and molecular constants, potential curves and Franck-Condon-factors were determined. This method

allows a high accuracy because single lines can be resolved. Since the high laser intensity creates a high population in the upper molecular states, one can measure the lifetime and angular-momentum coupling condition of these states with level-crossing techniques.

Solid State Properties

Electron Paramagnetic Resonance.-A technique was developed for the measurement of fast paramagnetic relaxation times which employs the Optical Faraday Rotation. This technique was used to measure the relaxation time of paramagnetic Eu2+ in CaF2:Eu2+ at cryogenic temperatures.

Theoretical Magnetism.-It was shown that any theory of magnetism based on a one-particle distribution function, such as the Weiss molecular field theory, is inconsistent if it assumes no correlation between spins. The consistency condition, a direct consequence of equilibrium fluctuations, was invoked to provide a measure of two-spin correlations, thereby implying a two-particle distribution function compatible with the one-particle Weiss distribution. The resulting theory is exact at high temperatures for evaluating such inherently two-particle quantities as heat capacity.

Various effective field techniques for the study of magnetism, appropriate for the Ising interaction, were examined with an infinite chain used as the basic cluster. These theories were found to offer no substantial improvement over the analogous theories using very small clusters.

Nuclear Magnetic Resonance.-A new parametric nuclear spin saturation technique has been developed using 27Al nuclei in ruby. With the proper orientation of the c-axis with respect to the external magnetic field and rf magnetic field, magnetic Am=±2 nuclear spin transitions were observed. The theoretical calculation of this effect gave excellent agreement with experiment, and indicated that Am=±3 and ±4 magnetic transitions should be observable.

Field-Electron Studies of Adsorbed Atoms.-When atoms are adsorbed on a metal surface, the highest electron energy level of the atom is shifted upward and broadened into a band which may be several electron volts wide. Recently, it was predicted that the distribution in energy of electrons field-emitted from a metal through single atoms on the metal surface would reveal information about the shift and broadening of the atomic energy level. Experiments conducted with single strontium atoms on an oxygen-coated tungsten surface verified the theoretical prediction and established the feasibility of obtaining information about the interaction of atoms and surfaces with this technique.

Electronic Characterization of High-Purity Metals.-A strong correlation between low temperature electrical resistance and chemical purity has been shown for many metals. To exploit this fact an eddy-current apparatus has been developed and refined to allow rapid determination of the residual resistance ratio, which is the ratio of the electrical resistance at the ice point to the resistance at liquid helium temperatures. This contactless method works with bulk specimens and avoids both thin wires and soldered contacts. The method is especially useful for production control or inhomogeneity testing. A large number of metals have been tested for the NBS Standard Reference Material program, including Ag, Al, Au, Cd, Cu, Mo, Pt, Sn, W, Zn, and several noble metal alloys.

Characteristics of Slush Hydrogen.-Activities in the physical characterization of liquid-solid mixtures of hydrogen (slush hydrogen) have continued. A densitometer has been constructed which provides a traceable calibration for slush density instruments used as secondary or transfer standards. At present, the most successful density measuring instruments utilize nuclear radiation attenuation techniques. A parametric study of the performance of a centrifugal pump operating with slush hydrogen was made. It was found that both fresh and aged hydrogen mixtures could be successfully pumped and that performance and cavitation characteristics are the same when the difference in fluid density is considered.

Standard Reference Thermocouple Tables.-Printed and punched card tables resulting from NBS research and analysis are available in 1 K intervals from 4 to 280 K for ISA type E, K, and T low-temperature thermocouple materials. These tables have been smoothly joined at 273.15 K to previous NBS high-temperature thermocouple tables.

Properties of Solid Parahydrogen.-Dielectric-constant measurements along the melting line of solid hydrogen at pressures between 18 and 320 atm and temperatures between 14.4 and 22 K have led to the first calculation of the molecular polarizability (with an estimated precision of a few parts per 10,000) of this substance. New measurements of the melting pressure were also obtained leading to accurate densities for the melting solid. Knowledge of these properties is required for the design of efficient aerospace systems utilizing slush hydrogen (liquid-solid mixtures).

Thermodynamic and Transport Properties

Specific Heats of Gaseous and Liquid Oxygen.-The specific heats, C, and Csaturation, of gaseous and liquid oxygen have been measured from the triple point to room temperature at pressures to 350 atm.

V

They were combined with earlier PVT measurements to produce what is believed to be the most accurate and comprehensive thermodynamic functions for this substance in the cryogenic temperature range. Accurate thermodynamic functions are required by the aerospace and gas separation industries.

Refractive Index of Gaseous and Liquid Hydrogen. The refractive index of liquid hydrogen has been measured with great precision over a very wide range of temperature and density. Data were obtained at temperatures between 15 and 300 K and at pressures up to 233 atm using a Fabry-Perot interferometer. The density and temperature dependence of the Lorentz-Lorenz function was also analyzed. The measurements are useful for the determination of fluid densities and mass flow rates, and for providing new information on the interactions between molecules.

High-Speed Thermodynamic Measurements System. The first phase of the High-Speed (millisecond time resolution) Thermodynamic Measurements System developed for research at high temperatures (1800 K to melting points) has become operational. This unique system provides simultaneous dynamic measurement of specific heat, electrical resistivity, and hemispherical total emittance of electrical conductors. Recording of the data is completely automated; up to 2000 data words can be measured and stored in digital form (with 0.01 percent resolution) in the core memory during an experiment of subsecond duration. This information is retrieved immediately after the experiment and processed by a remote-control, time-sharing computer. Data on molybdenum in the range 1800-2800 K have been obtained.

Quantum Mechanical Virial Coefficients of Helium.-The velocity of sound in helium from 2 to 20 K obtained at NBS has been analyzed and preliminary values of the second virial coefficient in this range have been obtained. These values are better than previous ones and are of importance to improvement of the International Temperature Scale. It was possible to obtain these improved values because accurate theoretical computations of the second virial for the Lennard-Jones potential have yielded an appropriate form for the temperature dependence of the second virial. The calculations covered the complete temperature range from near absolute zero to the classical region. By separating the virial into Boltzmann and exchange terms, the suppression of statistical effects with rising temperature could be examined in detail. The theoretical results have been compared with experiments over the whole range leading to improvement of the intermolecular potential function for helium.

PVT Measurements in the Glassy State. As part of a continuing study of the influence of pressure, temperature, and volume history on the physical properties of glass-forming liquids and polymers, specific volume measurements to 1000 bar along various isothermal and isobaric paths into the glassy region on poly (vinyl acetate) have been completed. These studies have helped to resolve several controversial points about the formation of glassy state, and the applicability of reversible thermodynamics to the glassy state.

Theory of Transport Phenomena. Such phenomena as frequency dependence of sound velocity, nonlocal effects near the critical point, and nonlinear dependence of viscosity on the rate of shear are not adequately explained by classical fluid dynamical theory, but require a generalization of it. One generalization, which describes noninstantaneous, nonlocal, and nonlinear responses of flows to the thermodynamic forces (gradients), has been derived by statistical mechanical methods. The conservation equations are unchanged in form, but new expressions are given for the pressure tensor and heat current vector. The new expressions are determined by microscopic quantities (e.g., interparticle potential), and are the sum of reversible and irreversible parts. The reversible parts are the average fluxes in a local equilibrium ensemble that includes nonlocal effects. The expression for the irreversible parts are the analog of the classical transport relations, and are linear combinations of integrals over space and time of the thermodynamic forces multiplied by the correlations of fluxes averaged in a local equilibrium ensemble.

Theory of Critical Phenomena.-A fluctuation theory of critical phenomena in fluids has been formulated and applied in several instances. The theory generalizes the Ornstein-Zernike theory of density fluctuations to fluctuations of energy, pressure and similar functions. It introduces an important new mathematical object, the critical eigenvector, which has several significant roles near the critical point. The difference in molecular distribution functions in the liquid and gas phase just below the critical point is proportional to this eigenvector, and it is the most probable fluctuation in the molecular distribution functions just above the critical point. It is known from experiments that the specific heat at constant volume, while aparently infinite at the critical point, approaches infinity much more slowly than the specific heat at constant pressure. This fact can be explained if the molecular distribution functions have a certain asymptotic behavior, which can be understood from the role of the critical eigenvector as the most probable fluctuation.

The Intermolecular Potential and Experimental Data.-A method to evaluate quantitatively the relationship between model intermo