The spectral intensities of over 39,000 lines for 70 elements were determined and published in tabular form. The new tables provide spectrochemists with much-needed quantitative intensity values for those elements most commonly encountered in their analyses (page 59).

established to gather and index all published information on atomic transition probabilities. An exhaustive survey was made of the literature, and a primary reference file of approximately 600 references was catalogued. Selected bibliographies and tables of available data are now in preparation. A wall-stabilized high-current arc source was constructed and used to study transition probabilities of atomic hydrogen and oxygen. This apparatus will also be used to measure transition probabilities of a large number of other elements. A study of the hydrogen line profiles indicates that a measurement of these profiles can be used to calculate a temperature for the arc plasma that is reliable to about ± 2 percent.

A set of tables containing spectral intensities for 39,000 lines of 70 elements, as observed in a copper matrix in a d-c arc, was completed and published. Studies of the intensity data indicate that they may be converted to approximate transition probabilities. These data are not of the precision obtainable by the methods previously mentioned, but the vast number of approximate values available will be useful in many areas.

Atomic Energy Levels. Research continues on the very complex spectra of the rare earth elements. New computer and automation techniques were applied to these spectra with considerable success. A number of energy levels were found in the spectrum of cerium; none had previously

been known in this spectrum. Substantial progress was made in the analyses of the spectra of thorium, praseodymium, ytterbium, bromine, and dysprosium.

The work on spectrum analysis has been aided greatly by theoretical prediction of the positions of energy levels in low, even configurations. Extensive computations were made on the first spectra of hafnium and tantalum and the third spectrum of praseodymium.

Collision Cross Sections. The measurement and calculation of lowenergy collision cross sections was continued, with efforts concentrated on construction of apparatus and refinement of instrumentation. Special emphasis is being given to the development of sources of low-energy monoenergetic electron beams for use in measurement of elastic and inelastic collision cross sections.

Theoretical studies of the use of refined wave functions in the calculation of electron scattering and photodetachment are continuing. Several highvacuum instruments for measuring electron collision cross sections are essentially completed.

The photodetachment of electrons from carbon negative ions was observed and studied. Careful measurements of detachment near threshold for the process lead to a value for the electron affinity of carbon of 1.25±.03 ev (28.6 kcal). Values of upper limits for the photodetachment cross sections of several of the important atmospheric negative ions were determined at a wavelength of 4000 A.

Studies involving very precise measurements of the dependence of drift velocities of argon ions in the parent gas on the electric field have been completed. The results raise serious doubts concerning the validity of current theory describing the motions of charged particles in gases.

A data center was established to gather and index all published information on collision cross sections. A complete file of reprints of papers on low-energy electron cross sections was collected. A code has been worked out for tabulating the large number (over 800) of references on punch cards. About one-half the papers have been coded. The data collection will be extended to cover other atomic cross sections.

Standard Wavelengths. The wavelength of the resonant line emitted by an atomic beam of mercury 198 was measured relative to the standard wavelength emitted by the krypton-86 isotope, which was adopted in 1960 as the new international standard of length. The line emitted by the mercury-198 beam is nearly ten times as sharp as the krypton standard line.

Several atomic beam devices are under development that show promise of producing wavelength standards potentially superior to the present international standard (see 2.1.1., p. 21).

Infrared Spectroscopy of Gases. During the past year a great deal of materials research was conducted on the infrared spectra and vibrationrotation energy levels of various compounds. This research was sponsored

work reconfirmed these values and explained disparities in previously published data. Because of the close relation between rutile and several ferroelectric titanates, the temperature dependence of the dielectric constant was studied between 1.6 and 1,060 °K. In both the c- and a-directions, the dielectric constant changes only by a factor of 2 or 3 and no anomalies occur. Lorentz corrections, polarizabilities, and effective charges were calculated. It appears that the polarizabilities in rutile are within 10 percent of the critical values for the ferroelectric catastrophe over the entire temperature range.

A considerable amount of data was taken on the electrical, magnetic, and optical properties of rutile. There is now strong evidence that one of the major defects in reduced rutile is an interstitial Ti-ion.

Titanium sesquioxide with excess oxygen has been shown to be a p-type semiconductor below the transition point at 480 °K, while the conduction is metallic in character above that temperature.

A theoretical study was made of the electrical conductivity, Hall coefficient, and thermoelectric power of decomposing oxides as a function of the oxygen vapor pressure at high temperatures. As a result of this analysis, methods were selected to derive intrinsic parameters such as energy gap and effective masses from measurements of the pressure dependence of the transport properties.

Investigations of electron spin resonance absorption in oxides were initiated in collaboration with the mineral products laboratory (see p. 84). Electron Scattering. A single scattering experiment tied together three previously unrelated topics (diffraction, characteristic-loss scattering, and plural scattering theory) and provided new insight into the theoretical model of electron scattering in solids. The measurements represent the first detailed experimental example of a theory of electron scattering proposed in 1921 by Wentzel, and support a model of inelastic scattering proposed by U. Fano of NBS in 1956. They show that electrons elastically scattering from aluminum foils have the same angular distribution for all foil thicknesses, indicating that the elastic scattering process is a type of diffraction. Moreover, the inelastically scattered electrons undergo repeated collisions of a type involving long-range interactions with the electrons of the target material.

Culminating several years of research effort. the first time-resolved photographs of a pulsed cadmium atomic beam (density ~1011 particles/cm3, equivalent to 10-5 mm Hg pressure) was achieved by means of an electronoptical stroboscopic method. The purpose of this work, which is sponsored by the Office of Naval Research, is to develop a method to measure the vectorvelocity distributions of gas molecules reflected from solid surfaces and to use these data to compute the coefficients of thermal accommodation and viscous slip. Low-density collimated pulses of gas atoms (or molecules), moving rectilinearly in a narrow velocity range (around 300 m/sec), are photographed by an electron optical schlieren technique. The electron

beam is pulsed stroboscopically, thus providing a picture of the instantaneous distribution of atoms at a known time after the formation of the atomic beam pulse. The time delay between the atomic beam pulse and the electron beam pulse is varied so that the velocity distribution in the atomic beam may be studied.

Atomic Constants.

Plans were developed and equipment is being assembled for a new precision measurement of the cyclotron frequency of the electron. This experiment will provide a sensitive check on the predictions of quantum electrodynamics concerning the anomalous magnetic moment of the electron.

Atomic Standards of Frequency. The construction of several prototype frequency standards based on hyperfine resonances in rubidium vapor was completed, and the performance of the standards is being evaluated by systematically intercomparing them with the primary standards at the Bureau's Boulder Laboratories. Plans to use a miniaturized version of the clock for a test of relativity theory were discontinued as a result of measurements made elsewhere using the newly discovered Mossbauer effect.

A rubidium vapor maser with an oscillation parameter of about 0.6 was developed. Further improvements being attempted at Columbia University are expected to achieve unity oscillation.